                            Storage Devices

  Wilko Bulte

   <wilko@FreeBSD.org>

   $FreeBSD:
   doc/en_US.ISO8859-1/articles/storage-devices/article.sgml,v 1.16
   2010/07/31 11:06:18 pgj Exp $

   FreeBSD is a registered trademark of the FreeBSD Foundation.

   Many of the designations used by manufacturers and sellers to
   distinguish their products are claimed as trademarks. Where those
   designations appear in this document, and the FreeBSD Project was
   aware of the trademark claim, the designations have been followed
   by the "(TM)" or the "(R)" symbol.

   This article talks about storage devices with FreeBSD.

     --------------------------------------------------------------

1 Using ESDI hard disks

   Copyright (c) 1995, Wilko Bulte <wilko@FreeBSD.org>. 24 September
   1995.

   ESDI is an acronym that means Enhanced Small Device Interface. It
   is loosely based on the good old ST506/412 interface originally
   devised by Seagate Technology, the makers of the first affordable
   5.25" Winchester disk.

   The acronym says Enhanced, and rightly so. In the first place the
   speed of the interface is higher, 10 or 15 Mbits/second instead of
   the 5 Mbits/second of ST412 interfaced drives. Secondly some
   higher level commands are added, making the ESDI interface
   somewhat "smarter" to the operating system driver writers. It is
   by no means as smart as SCSI by the way. ESDI is standardized by
   ANSI.

   Capacities of the drives are boosted by putting more sectors on
   each track. Typical is 35 sectors per track, high capacity drives
   I have seen were up to 54 sectors/track.

   Although ESDI has been largely obsoleted by IDE and SCSI
   interfaces, the availability of free or cheap surplus drives makes
   them ideal for low (or now) budget systems.

     --------------------------------------------------------------

  1.1 Concepts of ESDI

    1.1.1 Physical connections

   The ESDI interface uses two cables connected to each drive. One
   cable is a 34 pin flat cable edge connector that carries the
   command and status signals from the controller to the drive and
   vice-versa. The command cable is daisy chained between all the
   drives. So, it forms a bus onto which all drives are connected.

   The second cable is a 20 pin flat cable edge connector that
   carries the data to and from the drive. This cable is radially
   connected, so each drive has its own direct connection to the
   controller.

   To the best of my knowledge PC ESDI controllers are limited to
   using a maximum of 2 drives per controller. This is compatibility
   feature(?) left over from the WD1003 standard that reserves only a
   single bit for device addressing.

     --------------------------------------------------------------

    1.1.2 Device addressing

   On each command cable a maximum of 7 devices and 1 controller can
   be present. To enable the controller to uniquely identify which
   drive it addresses, each ESDI device is equipped with jumpers or
   switches to select the devices address.

   On PC type controllers the first drive is set to address 0, the
   second disk to address 1. Always make sure you set each disk to an
   unique address! So, on a PC with its two drives/controller maximum
   the first drive is drive 0, the second is drive 1.

     --------------------------------------------------------------

    1.1.3 Termination

   The daisy chained command cable (the 34 pin cable remember?) needs
   to be terminated at the last drive on the chain. For this purpose
   ESDI drives come with a termination resistor network that can be
   removed or disabled by a jumper when it is not used.

   So, one and only one drive, the one at the farthest end of the
   command cable has its terminator installed/enabled. The controller
   automatically terminates the other end of the cable. Please note
   that this implies that the controller must be at one end of the
   cable and not in the middle.

     --------------------------------------------------------------

  1.2 Using ESDI disks with FreeBSD

   Why is ESDI such a pain to get working in the first place?

   People who tried ESDI disks with FreeBSD are known to have
   developed a profound sense of frustration. A combination of
   factors works against you to produce effects that are hard to
   understand when you have never seen them before.

   This has also led to the popular legend ESDI and FreeBSD is a
   plain NO-GO. The following sections try to list all the pitfalls
   and solutions.

     --------------------------------------------------------------

    1.2.1 ESDI speed variants

   As briefly mentioned before, ESDI comes in two speed flavors. The
   older drives and controllers use a 10 Mbits/second data transfer
   rate. Newer stuff uses 15 Mbits/second.

   It is not hard to imagine that 15 Mbits/second drive cause
   problems on controllers laid out for 10 Mbits/second. As always,
   consult your controller and drive documentation to see if things
   match.

     --------------------------------------------------------------

    1.2.2 Stay on track

   Mainstream ESDI drives use 34 to 36 sectors per track. Most
   (older) controllers cannot handle more than this number of
   sectors. Newer, higher capacity, drives use higher numbers of
   sectors per track. For instance, I own a 670 MB drive that has 54
   sectors per track.

   In my case, the controller could not handle this number of
   sectors. It proved to work well except that it only used 35
   sectors on each track. This meant losing a lot of disk space.

   Once again, check the documentation of your hardware for more
   info. Going out-of-spec like in the example might or might not
   work. Give it a try or get another more capable controller.

     --------------------------------------------------------------

    1.2.3 Hard or soft sectoring

   Most ESDI drives allow hard or soft sectoring to be selected using
   a jumper. Hard sectoring means that the drive will produce a
   sector pulse on the start of each new sector. The controller uses
   this pulse to tell when it should start to write or read.

   Hard sectoring allows a selection of sector size (normally 256,
   512 or 1024 bytes per formatted sector). FreeBSD uses 512 byte
   sectors. The number of sectors per track also varies while still
   using the same number of bytes per formatted sector. The number of
   unformatted bytes per sector varies, dependent on your controller
   it needs more or less overhead bytes to work correctly. Pushing
   more sectors on a track of course gives you more usable space, but
   might give problems if your controller needs more bytes than the
   drive offers.

   In case of soft sectoring, the controller itself determines where
   to start/stop reading or writing. For ESDI hard sectoring is the
   default (at least on everything I came across). I never felt the
   urge to try soft sectoring.

   In general, experiment with sector settings before you install
   FreeBSD because you need to re-run the low-level format after each
   change.

     --------------------------------------------------------------

    1.2.4 Low level formatting

   ESDI drives need to be low level formatted before they are usable.
   A reformat is needed whenever you figgle with the number of
   sectors/track jumpers or the physical orientation of the drive
   (horizontal, vertical). So, first think, then format. The format
   time must not be underestimated, for big disks it can take hours.

   After a low level format, a surface scan is done to find and flag
   bad sectors. Most disks have a manufacturer bad block list listed
   on a piece of paper or adhesive sticker. In addition, on most
   disks the list is also written onto the disk. Please use the
   manufacturer's list. It is much easier to remap a defect now than
   after FreeBSD is installed.

   Stay away from low-level formatters that mark all sectors of a
   track as bad as soon as they find one bad sector. Not only does
   this waste space, it also and more importantly causes you grief
   with bad144 (see the section on bad144).

     --------------------------------------------------------------

    1.2.5 Translations

   Translations, although not exclusively a ESDI-only problem, might
   give you real trouble. Translations come in multiple flavors. Most
   of them have in common that they attempt to work around the
   limitations posed upon disk geometries by the original IBM PC/AT
   design (thanks IBM!).

   First of all there is the (in)famous 1024 cylinder limit. For a
   system to be able to boot, the stuff (whatever operating system)
   must be in the first 1024 cylinders of a disk. Only 10 bits are
   available to encode the cylinder number. For the number of sectors
   the limit is 64 (0-63). When you combine the 1024 cylinder limit
   with the 16 head limit (also a design feature) you max out at
   fairly limited disk sizes.

   To work around this problem, the manufacturers of ESDI PC
   controllers added a BIOS prom extension on their boards. This BIOS
   extension handles disk I/O for booting (and for some operating
   systems all disk I/O) by using translation. For instance, a big
   drive might be presented to the system as having 32 heads and 64
   sectors/track. The result is that the number of cylinders is
   reduced to something below 1024 and is therefore usable by the
   system without problems. It is noteworthy to know that FreeBSD
   does not use the BIOS after its kernel has started. More on this
   later.

   A second reason for translations is the fact that most older
   system BIOSes could only handle drives with 17 sectors per track
   (the old ST412 standard). Newer system BIOSes usually have a
   user-defined drive type (in most cases this is drive type 47).

     Warning: Whatever you do to translations after reading this
     document, keep in mind that if you have multiple operating
     systems on the same disk, all must use the same translation

   While on the subject of translations, I have seen one controller
   type (but there are probably more like this) offer the option to
   logically split a drive in multiple partitions as a BIOS option. I
   had select 1 drive == 1 partition because this controller wrote
   this info onto the disk. On power-up it read the info and
   presented itself to the system based on the info from the disk.

     --------------------------------------------------------------

    1.2.6 Spare sectoring

   Most ESDI controllers offer the possibility to remap bad sectors.
   During/after the low-level format of the disk bad sectors are
   marked as such, and a replacement sector is put in place
   (logically of course) of the bad one.

   In most cases the remapping is done by using N-1 sectors on each
   track for actual data storage, and sector N itself is the spare
   sector. N is the total number of sectors physically available on
   the track. The idea behind this is that the operating system sees
   a "perfect" disk without bad sectors. In the case of FreeBSD this
   concept is not usable.

   The problem is that the translation from bad to good is performed
   by the BIOS of the ESDI controller. FreeBSD, being a true 32 bit
   operating system, does not use the BIOS after it has been booted.
   Instead, it has device drivers that talk directly to the hardware.

   So: do not use spare sectoring, bad block remapping or whatever it
   may be called by the controller manufacturer when you want to use
   the disk for FreeBSD.

     --------------------------------------------------------------

    1.2.7 Bad block handling

   The preceding section leaves us with a problem. The controller's
   bad block handling is not usable and still FreeBSD's filesystems
   assume perfect media without any flaws. To solve this problem,
   FreeBSD use the bad144 tool. Bad144 (named after a Digital
   Equipment standard for bad block handling) scans a FreeBSD slice
   for bad blocks. Having found these bad blocks, it writes a table
   with the offending block numbers to the end of the FreeBSD slice.

   When the disk is in operation, the disk accesses are checked
   against the table read from the disk. Whenever a block number is
   requested that is in the bad144 list, a replacement block (also
   from the end of the FreeBSD slice) is used. In this way, the
   bad144 replacement scheme presents "perfect" media to the FreeBSD
   filesystems.

   There are a number of potential pitfalls associated with the use
   of bad144. First of all, the slice cannot have more than 126 bad
   sectors. If your drive has a high number of bad sectors, you might
   need to divide it into multiple FreeBSD slices each containing
   less than 126 bad sectors. Stay away from low-level format
   programs that mark every sector of a track as bad when they find a
   flaw on the track. As you can imagine, the 126 limit is quickly
   reached when the low-level format is done this way.

   Second, if the slice contains the root filesystem, the slice
   should be within the 1024 cylinder BIOS limit. During the boot
   process the bad144 list is read using the BIOS and this only
   succeeds when the list is within the 1024 cylinder limit.

     Note: The restriction is not that only the root filesystem must
     be within the 1024 cylinder limit, but rather the entire slice
     that contains the root filesystem.

     --------------------------------------------------------------

    1.2.8 Kernel configuration

   ESDI disks are handled by the same wddriver as IDE and ST412 MFM
   disks. The wd driver should work for all WD1003 compatible
   interfaces.

   Most hardware is jumperable for one of two different I/O address
   ranges and IRQ lines. This allows you to have two wd type
   controllers in one system.

   When your hardware allows non-standard strappings, you can use
   these with FreeBSD as long as you enter the correct info into the
   kernel config file. An example from the kernel config file (they
   live in /sys/i386/conf BTW).

 # First WD compatible controller
 controller      wdc0    at isa? port "IO_WD1" bio irq 14 vector wdintr
 disk            wd0     at wdc0 drive 0
 disk            wd1     at wdc0 drive 1
 # Second WD compatible controller
 controller      wdc1    at isa? port "IO_WD2" bio irq 15 vector wdintr
 disk            wd2     at wdc1 drive 0
 disk            wd3     at wdc1 drive 1

     --------------------------------------------------------------

  1.3 Particulars on ESDI hardware

    1.3.1 Adaptec 2320 controllers

   I successfully installed FreeBSD onto a ESDI disk controlled by a
   ACB-2320. No other operating system was present on the disk.

   To do so I low level formatted the disk using NEFMT.EXE (ftpable
   from www.adaptec.com) and answered NO to the question whether the
   disk should be formatted with a spare sector on each track. The
   BIOS on the ACD-2320 was disabled. I used the free configurable
   option in the system BIOS to allow the BIOS to boot it.

   Before using NEFMT.EXE I tried to format the disk using the
   ACB-2320 BIOS built-in formatter. This proved to be a show
   stopper, because it did not give me an option to disable spare
   sectoring. With spare sectoring enabled the FreeBSD installation
   process broke down on the bad144 run.

   Please check carefully which ACB-232xy variant you have. The x is
   either 0 or 2, indicating a controller without or with a floppy
   controller on board.

   The y is more interesting. It can either be a blank, a A-8 or a D.
   A blank indicates a plain 10 Mbits/second controller. An A-8
   indicates a 15 Mbits/second controller capable of handling 52
   sectors/track. A D means a 15 Mbits/second controller that can
   also handle drives with > 36 sectors/track (also 52?).

   All variations should be capable of using 1:1 interleaving. Use
   1:1, FreeBSD is fast enough to handle it.

     --------------------------------------------------------------

    1.3.2 Western Digital WD1007 controllers

   I successfully installed FreeBSD onto a ESDI disk controlled by a
   WD1007 controller. To be precise, it was a WD1007-WA2. Other
   variations of the WD1007 do exist.

   To get it to work, I had to disable the sector translation and the
   WD1007's onboard BIOS. This implied I could not use the low-level
   formatter built into this BIOS. Instead, I grabbed WDFMT.EXE from
   www.wdc.com Running this formatted my drive just fine.

     --------------------------------------------------------------

    1.3.3 Ultrastor U14F controllers

   According to multiple reports from the net, Ultrastor ESDI boards
   work OK with FreeBSD. I lack any further info on particular
   settings.

     --------------------------------------------------------------

  1.4 Further reading

   If you intend to do some serious ESDI hacking, you might want to
   have the official standard at hand:

   The latest ANSI X3T10 committee document is: Enhanced Small Device
   Interface (ESDI) [X3.170-1990/X3.170a-1991] [X3T10/792D Rev 11]

   On Usenet the newsgroup comp.periphs is a noteworthy place to look
   for more info.

   The World Wide Web (WWW) also proves to be a very handy info
   source: For info on Adaptec ESDI controllers see
   http://www.adaptec.com/. For info on Western Digital controllers
   see http://www.wdc.com/.

     --------------------------------------------------------------

  1.5 Thanks to...

   Andrew Gordon for sending me an Adaptec 2320 controller and ESDI
   disk for testing.

     --------------------------------------------------------------

2 What is SCSI?

   Copyright (c) 1995, Wilko Bulte <wilko@FreeBSD.org>. July 6, 1996.

   SCSI is an acronym for Small Computer Systems Interface. It is an
   ANSI standard that has become one of the leading I/O buses in the
   computer industry. The foundation of the SCSI standard was laid by
   Shugart Associates (the same guys that gave the world the first
   mini floppy disks) when they introduced the SASI bus (Shugart
   Associates Standard Interface).

   After some time an industry effort was started to come to a more
   strict standard allowing devices from different vendors to work
   together. This effort was recognized in the ANSI SCSI-1 standard.
   The SCSI-1 standard (approximately 1985) is rapidly becoming
   obsolete. The current standard is SCSI-2 (see Further reading),
   with SCSI-3 on the drawing boards.

   In addition to a physical interconnection standard, SCSI defines a
   logical (command set) standard to which disk devices must adhere.
   This standard is called the Common Command Set (CCS) and was
   developed more or less in parallel with ANSI SCSI-1. SCSI-2
   includes the (revised) CCS as part of the standard itself. The
   commands are dependent on the type of device at hand. It does not
   make much sense of course to define a Write command for a scanner.

   The SCSI bus is a parallel bus, which comes in a number of
   variants. The oldest and most used is an 8 bit wide bus, with
   single-ended signals, carried on 50 wires. (If you do not know
   what single-ended means, do not worry, that is what this document
   is all about.) Modern designs also use 16 bit wide buses, with
   differential signals. This allows transfer speeds of
   20Mbytes/second, on cables lengths of up to 25 meters. SCSI-2
   allows a maximum bus width of 32 bits, using an additional cable.
   Quickly emerging are Ultra SCSI (also called Fast-20) and Ultra2
   (also called Fast-40). Fast-20 is 20 million transfers per second
   (20 Mbytes/sec on a 8 bit bus), Fast-40 is 40 million transfers
   per second (40 Mbytes/sec on a 8 bit bus). Most hard drives sold
   today are single-ended Ultra SCSI (8 or 16 bits).

   Of course the SCSI bus not only has data lines, but also a number
   of control signals. A very elaborate protocol is part of the
   standard to allow multiple devices to share the bus in an
   efficient manner. In SCSI-2, the data is always checked using a
   separate parity line. In pre-SCSI-2 designs parity was optional.

   In SCSI-3 even faster bus types are introduced, along with a
   serial SCSI busses that reduces the cabling overhead and allows a
   higher maximum bus length. You might see names like SSA and fibre
   channel in this context. None of the serial buses are currently in
   widespread use (especially not in the typical FreeBSD
   environment). For this reason the serial bus types are not
   discussed any further.

   As you could have guessed from the description above, SCSI devices
   are intelligent. They have to be to adhere to the SCSI standard
   (which is over 2 inches thick BTW). So, for a hard disk drive for
   instance you do not specify a head/cylinder/sector to address a
   particular block, but simply the number of the block you want.
   Elaborate caching schemes, automatic bad block replacement etc are
   all made possible by this "intelligent device" approach.

   On a SCSI bus, each possible pair of devices can communicate.
   Whether their function allows this is another matter, but the
   standard does not restrict it. To avoid signal contention, the 2
   devices have to arbitrate for the bus before using it.

   The philosophy of SCSI is to have a standard that allows
   older-standard devices to work with newer-standard ones. So, an
   old SCSI-1 device should normally work on a SCSI-2 bus. I say
   Normally, because it is not absolutely sure that the
   implementation of an old device follows the (old) standard closely
   enough to be acceptable on a new bus. Modern devices are usually
   more well-behaved, because the standardization has become more
   strict and is better adhered to by the device manufacturers.

   Generally speaking, the chances of getting a working set of
   devices on a single bus is better when all the devices are SCSI-2
   or newer. This implies that you do not have to dump all your old
   stuff when you get that shiny 80GB disk: I own a system on which a
   pre-SCSI-1 disk, a SCSI-2 QIC tape unit, a SCSI-1 helical scan
   tape unit and 2 SCSI-1 disks work together quite happily. From a
   performance standpoint you might want to separate your older and
   newer (=faster) devices however. This is especially advantageous
   if you have an Ultra160 host adapter where you should separate
   your U160 devices from the Fast and Wide SCSI-2 devices.

     --------------------------------------------------------------

  2.1 Components of SCSI

   As said before, SCSI devices are smart. The idea is to put the
   knowledge about intimate hardware details onto the SCSI device
   itself. In this way, the host system does not have to worry about
   things like how many heads a hard disks has, or how many tracks
   there are on a specific tape device. If you are curious, the
   standard specifies commands with which you can query your devices
   on their hardware particulars. FreeBSD uses this capability during
   boot to check out what devices are connected and whether they need
   any special treatment.

   The advantage of intelligent devices is obvious: the device
   drivers on the host can be made in a much more generic fashion,
   there is no longer a need to change (and qualify!) drivers for
   every odd new device that is introduced.

   For cabling and connectors there is a golden rule: get good stuff.
   With bus speeds going up all the time you will save yourself a lot
   of grief by using good material.

   So, gold plated connectors, shielded cabling, sturdy connector
   hoods with strain reliefs etc are the way to go. Second golden
   rule: do no use cables longer than necessary. I once spent 3 days
   hunting down a problem with a flaky machine only to discover that
   shortening the SCSI bus by 1 meter solved the problem. And the
   original bus length was well within the SCSI specification.

     --------------------------------------------------------------

  2.2 SCSI bus types

   From an electrical point of view, there are two incompatible bus
   types: single-ended and differential. This means that there are
   two different main groups of SCSI devices and controllers, which
   cannot be mixed on the same bus. It is possible however to use
   special converter hardware to transform a single-ended bus into a
   differential one (and vice versa). The differences between the bus
   types are explained in the next sections.

   In lots of SCSI related documentation there is a sort of jargon in
   use to abbreviate the different bus types. A small list:

     * FWD: Fast Wide Differential

     * FND: Fast Narrow Differential

     * SE: Single Ended

     * FN: Fast Narrow

     * etc.

   With a minor amount of imagination one can usually imagine what is
   meant.

   Wide is a bit ambiguous, it can indicate 16 or 32 bit buses. As
   far as I know, the 32 bit variant is not (yet) in use, so wide
   normally means 16 bit.

   Fast means that the timing on the bus is somewhat different, so
   that on a narrow (8 bit) bus 10 Mbytes/sec are possible instead of
   5 Mbytes/sec for "slow" SCSI. As discussed before, bus speeds of
   20 and 40 million transfers/second are also emerging (Fast-20 ==
   Ultra SCSI and Fast-40 == Ultra2 SCSI).

     Note: The data lines > 8 are only used for data transfers and
     device addressing. The transfers of commands and status messages
     etc are only performed on the lowest 8 data lines. The standard
     allows narrow devices to operate on a wide bus. The usable bus
     width is negotiated between the devices. You have to watch your
     device addressing closely when mixing wide and narrow.

     --------------------------------------------------------------

    2.2.1 Single ended buses

   A single-ended SCSI bus uses signals that are either 5 Volts or 0
   Volts (indeed, TTL levels) and are relative to a COMMON ground
   reference. A singled ended 8 bit SCSI bus has approximately 25
   ground lines, who are all tied to a single "rail" on all devices.
   A standard single ended bus has a maximum length of 6 meters. If
   the same bus is used with fast-SCSI devices, the maximum length
   allowed drops to 3 meters. Fast-SCSI means that instead of
   5Mbytes/sec the bus allows 10Mbytes/sec transfers.

   Fast-20 (Ultra SCSI) and Fast-40 allow for 20 and 40 million
   transfers/second respectively. So, F20 is 20 Mbytes/second on a 8
   bit bus, 40 Mbytes/second on a 16 bit bus etc. For F20 the max bus
   length is 1.5 meters, for F40 it becomes 0.75 meters. Be aware
   that F20 is pushing the limits quite a bit, so you will quickly
   find out if your SCSI bus is electrically sound.

     Note: If some devices on your bus use "fast" to communicate your
     bus must adhere to the length restrictions for fast buses!

   It is obvious that with the newer fast-SCSI devices the bus length
   can become a real bottleneck. This is why the differential SCSI
   bus was introduced in the SCSI-2 standard.

   For connector pinning and connector types please refer to the
   SCSI-2 standard (see Further reading) itself, connectors etc are
   listed there in painstaking detail.

   Beware of devices using non-standard cabling. For instance Apple
   uses a 25pin D-type connecter (like the one on serial ports and
   parallel printers). Considering that the official SCSI bus needs
   50 pins you can imagine the use of this connector needs some
   "creative cabling". The reduction of the number of ground wires
   they used is a bad idea, you better stick to 50 pins cabling in
   accordance with the SCSI standard. For Fast-20 and 40 do not even
   think about buses like this.

     --------------------------------------------------------------

    2.2.2 Differential buses

   A differential SCSI bus has a maximum length of 25 meters. Quite a
   difference from the 3 meters for a single-ended fast-SCSI bus. The
   idea behind differential signals is that each bus signal has its
   own return wire. So, each signal is carried on a (preferably
   twisted) pair of wires. The voltage difference between these two
   wires determines whether the signal is asserted or de-asserted. To
   a certain extent the voltage difference between ground and the
   signal wire pair is not relevant (do not try 10 kVolts though).

   It is beyond the scope of this document to explain why this
   differential idea is so much better. Just accept that electrically
   seen the use of differential signals gives a much better noise
   margin. You will normally find differential buses in use for
   inter-cabinet connections. Because of the lower cost single ended
   is mostly used for shorter buses like inside cabinets.

   There is nothing that stops you from using differential stuff with
   FreeBSD, as long as you use a controller that has device driver
   support in FreeBSD. As an example, Adaptec marketed the AHA1740 as
   a single ended board, whereas the AHA1744 was differential. The
   software interface to the host is identical for both.

     --------------------------------------------------------------

    2.2.3 Terminators

   Terminators in SCSI terminology are resistor networks that are
   used to get a correct impedance matching. Impedance matching is
   important to get clean signals on the bus, without reflections or
   ringing. If you once made a long distance telephone call on a bad
   line you probably know what reflections are. With 20Mbytes/sec
   traveling over your SCSI bus, you do not want signals echoing
   back.

   Terminators come in various incarnations, with more or less
   sophisticated designs. Of course, there are internal and external
   variants. Many SCSI devices come with a number of sockets in which
   a number of resistor networks can (must be!) installed. If you
   remove terminators from a device, carefully store them. You will
   need them when you ever decide to reconfigure your SCSI bus. There
   is enough variation in even these simple tiny things to make
   finding the exact replacement a frustrating business. There are
   also SCSI devices that have a single jumper to enable or disable a
   built-in terminator. There are special terminators you can stick
   onto a flat cable bus. Others look like external connectors, or a
   connector hood without a cable. So, lots of choice as you can see.

   There is much debate going on if and when you should switch from
   simple resistor (passive) terminators to active terminators.
   Active terminators contain slightly more elaborate circuit to give
   cleaner bus signals. The general consensus seems to be that the
   usefulness of active termination increases when you have long
   buses and/or fast devices. If you ever have problems with your
   SCSI buses you might consider trying an active terminator. Try to
   borrow one first, they reputedly are quite expensive.

   Please keep in mind that terminators for differential and
   single-ended buses are not identical. You should not mix the two
   variants.

   OK, and now where should you install your terminators? This is by
   far the most misunderstood part of SCSI. And it is by far the
   simplest. The rule is: every single line on the SCSI bus has 2
   (two) terminators, one at each end of the bus. So, two and not one
   or three or whatever. Do yourself a favor and stick to this rule.
   It will save you endless grief, because wrong termination has the
   potential to introduce highly mysterious bugs. (Note the
   "potential" here; the nastiest part is that it may or may not
   work.)

   A common pitfall is to have an internal (flat) cable in a machine
   and also an external cable attached to the controller. It seems
   almost everybody forgets to remove the terminators from the
   controller. The terminator must now be on the last external
   device, and not on the controller! In general, every
   reconfiguration of a SCSI bus must pay attention to this.

     Note: Termination is to be done on a per-line basis. This means
     if you have both narrow and wide buses connected to the same
     host adapter, you need to enable termination on the higher 8
     bits of the bus on the adapter (as well as the last devices on
     each bus, of course).

   What I did myself is remove all terminators from my SCSI devices
   and controllers. I own a couple of external terminators, for both
   the Centronics-type external cabling and for the internal flat
   cable connectors. This makes reconfiguration much easier.

   On modern devices, sometimes integrated terminators are used.
   These things are special purpose integrated circuits that can be
   enabled or disabled with a control pin. It is not necessary to
   physically remove them from a device. You may find them on newer
   host adapters, sometimes they are software configurable, using
   some sort of setup tool. Some will even auto-detect the cables
   attached to the connectors and automatically set up the
   termination as necessary. At any rate, consult your documentation!

     --------------------------------------------------------------

    2.2.4 Terminator power

   The terminators discussed in the previous chapter need power to
   operate properly. On the SCSI bus, a line is dedicated to this
   purpose. So, simple huh?

   Not so. Each device can provide its own terminator power to the
   terminator sockets it has on-device. But if you have external
   terminators, or when the device supplying the terminator power to
   the SCSI bus line is switched off you are in trouble.

   The idea is that initiators (these are devices that initiate
   actions on the bus, a discussion follows) must supply terminator
   power. All SCSI devices are allowed (but not required) to supply
   terminator power.

   To allow for un-powered devices on a bus, the terminator power
   must be supplied to the bus via a diode. This prevents the
   backflow of current to un-powered devices.

   To prevent all kinds of nastiness, the terminator power is usually
   fused. As you can imagine, fuses might blow. This can, but does
   not have to, lead to a non functional bus. If multiple devices
   supply terminator power, a single blown fuse will not put you out
   of business. A single supplier with a blown fuse certainly will.
   Clever external terminators sometimes have a LED indication that
   shows whether terminator power is present.

   In newer designs auto-restoring fuses that "reset" themselves
   after some time are sometimes used.

     --------------------------------------------------------------

    2.2.5 Device addressing

   Because the SCSI bus is, ehh, a bus there must be a way to
   distinguish or address the different devices connected to it.

   This is done by means of the SCSI or target ID. Each device has a
   unique target ID. You can select the ID to which a device must
   respond using a set of jumpers, or a dip switch, or something
   similar. Some SCSI host adapters let you change the target ID from
   the boot menu. (Yet some others will not let you change the ID
   from 7.) Consult the documentation of your device for more
   information.

   Beware of multiple devices configured to use the same ID. Chaos
   normally reigns in this case. A pitfall is that one of the devices
   sharing the same ID sometimes even manages to answer to I/O
   requests!

   For an 8 bit bus, a maximum of 8 targets is possible. The maximum
   is 8 because the selection is done bitwise using the 8 data lines
   on the bus. For wide buses this increases to the number of data
   lines (usually 16).

     Note: A narrow SCSI device can not communicate with a SCSI
     device with a target ID larger than 7. This means it is
     generally not a good idea to move your SCSI host adapter's
     target ID to something higher than 7 (or your CDROM will stop
     working).

   The higher the SCSI target ID, the higher the priority the devices
   has. When it comes to arbitration between devices that want to use
   the bus at the same time, the device that has the highest SCSI ID
   will win. This also means that the SCSI host adapter usually uses
   target ID 7. Note however that the lower 8 IDs have higher
   priorities than the higher 8 IDs on a wide-SCSI bus. Thus, the
   order of target IDs is: [7 6 .. 1 0 15 14 .. 9 8] on a wide-SCSI
   system. (If you are wondering why the lower 8 have higher
   priority, read the previous paragraph for a hint.)

   For a further subdivision, the standard allows for Logical Units
   or LUNs for short. A single target ID may have multiple LUNs. For
   example, a tape device including a tape changer may have LUN 0 for
   the tape device itself, and LUN 1 for the tape changer. In this
   way, the host system can address each of the functional units of
   the tape changer as desired.

     --------------------------------------------------------------

    2.2.6 Bus layout

   SCSI buses are linear. So, not shaped like Y-junctions, star
   topologies, rings, cobwebs or whatever else people might want to
   invent. One of the most common mistakes is for people with
   wide-SCSI host adapters to connect devices on all three connecters
   (external connector, internal wide connector, internal narrow
   connector). Do not do that. It may appear to work if you are
   really lucky, but I can almost guarantee that your system will
   stop functioning at the most unfortunate moment (this is also
   known as "Murphy's law").

   You might notice that the terminator issue discussed earlier
   becomes rather hairy if your bus is not linear. Also, if you have
   more connectors than devices on your internal SCSI cable, make
   sure you attach devices on connectors on both ends instead of
   using the connectors in the middle and let one or both ends
   dangle. This will screw up the termination of the bus.

   The electrical characteristics, its noise margins and ultimately
   the reliability of it all are tightly related to linear bus rule.

   Stick to the linear bus rule!

     --------------------------------------------------------------

  2.3 Using SCSI with FreeBSD

    2.3.1 About translations, BIOSes and magic...

   As stated before, you should first make sure that you have a
   electrically sound bus.

   When you want to use a SCSI disk on your PC as boot disk, you must
   aware of some quirks related to PC BIOSes. The PC BIOS in its
   first incarnation used a low level physical interface to the hard
   disk. So, you had to tell the BIOS (using a setup tool or a BIOS
   built-in setup) how your disk physically looked like. This
   involved stating number of heads, number of cylinders, number of
   sectors per track, obscure things like precompensation and reduced
   write current cylinder etc.

   One might be inclined to think that since SCSI disks are smart you
   can forget about this. Alas, the arcane setup issue is still
   present today. The system BIOS needs to know how to access your
   SCSI disk with the head/cyl/sector method in order to load the
   FreeBSD kernel during boot.

   The SCSI host adapter or SCSI controller you have put in your
   AT/EISA/PCI/whatever bus to connect your disk therefore has its
   own on-board BIOS. During system startup, the SCSI BIOS takes over
   the hard disk interface routines from the system BIOS. To fool the
   system BIOS, the system setup is normally set to No hard disk
   present. Obvious, is it not?

   The SCSI BIOS itself presents to the system a so called translated
   drive. This means that a fake drive table is constructed that
   allows the PC to boot the drive. This translation is often (but
   not always) done using a pseudo drive with 64 heads and 32 sectors
   per track. By varying the number of cylinders, the SCSI BIOS
   adapts to the actual drive size. It is useful to note that 32 * 64
   / 2 = the size of your drive in megabytes. The division by 2 is to
   get from disk blocks that are normally 512 bytes in size to
   Kbytes.

   Right. All is well now?! No, it is not. The system BIOS has
   another quirk you might run into. The number of cylinders of a
   bootable hard disk cannot be greater than 1024. Using the
   translation above, this is a show-stopper for disks greater than 1
   GB. With disk capacities going up all the time this is causing
   problems.

   Fortunately, the solution is simple: just use another translation,
   e.g. with 128 heads instead of 32. In most cases new SCSI BIOS
   versions are available to upgrade older SCSI host adapters. Some
   newer adapters have an option, in the form of a jumper or software
   setup selection, to switch the translation the SCSI BIOS uses.

   It is very important that all operating systems on the disk use
   the same translation to get the right idea about where to find the
   relevant partitions. So, when installing FreeBSD you must answer
   any questions about heads/cylinders etc using the translated
   values your host adapter uses.

   Failing to observe the translation issue might lead to un-bootable
   systems or operating systems overwriting each others partitions.
   Using fdisk you should be able to see all partitions.

   You might have heard some talk of "lying" devices? Older FreeBSD
   kernels used to report the geometry of SCSI disks when booting. An
   example from one of my systems:

 aha0 targ 0 lun 0: <MICROP 1588-15MB1057404HSP4>
 da0: 636MB (1303250 total sec), 1632 cyl, 15 head, 53 sec, bytes/sec 512

   Newer kernels usually do not report this information. e.g.

 (bt0:0:0): "SEAGATE ST41651 7574" type 0 fixed SCSI 2
 da0(bt0:0:0): Direct-Access 1350MB (2766300 512 byte sectors)

   Why has this changed?

   This info is retrieved from the SCSI disk itself. Newer disks
   often use a technique called zone bit recording. The idea is that
   on the outer cylinders of the drive there is more space so more
   sectors per track can be put on them. This results in disks that
   have more tracks on outer cylinders than on the inner cylinders
   and, last but not least, have more capacity. You can imagine that
   the value reported by the drive when inquiring about the geometry
   now becomes suspect at best, and nearly always misleading. When
   asked for a geometry, it is nearly always better to supply the
   geometry used by the BIOS, or if the BIOS is never going to know
   about this disk, (e.g. it is not a booting disk) to supply a
   fictitious geometry that is convenient.

     --------------------------------------------------------------

    2.3.2 SCSI subsystem design

   FreeBSD uses a layered SCSI subsystem. For each different
   controller card a device driver is written. This driver knows all
   the intimate details about the hardware it controls. The driver
   has a interface to the upper layers of the SCSI subsystem through
   which it receives its commands and reports back any status.

   On top of the card drivers there are a number of more generic
   drivers for a class of devices. More specific: a driver for tape
   devices (abbreviation: sa, for serial access), magnetic disks (da,
   for direct access), CDROMs (cd) etc. In case you are wondering
   where you can find this stuff, it all lives in /sys/cam/scsi. See
   the man pages in section 4 for more details.

   The multi level design allows a decoupling of low-level bit
   banging and more high level stuff. Adding support for another
   piece of hardware is a much more manageable problem.

     --------------------------------------------------------------

    2.3.3 Kernel configuration

   Dependent on your hardware, the kernel configuration file must
   contain one or more lines describing your host adapter(s). This
   includes I/O addresses, interrupts etc. Consult the manual page
   for your adapter driver to get more info. Apart from that, check
   out /sys/i386/conf/LINT for an overview of a kernel config file.
   LINT contains every possible option you can dream of. It does not
   imply LINT will actually get you to a working kernel at all.

   Although it is probably stating the obvious: the kernel config
   file should reflect your actual hardware setup. So, interrupts,
   I/O addresses etc must match the kernel config file. During system
   boot messages will be displayed to indicate whether the configured
   hardware was actually found.

     Note: Note that most of the EISA/PCI drivers (namely ahb, ahc,
     ncr and amd will automatically obtain the correct parameters
     from the host adapters themselves at boot time; thus, you just
     need to write, for instance, controller ahc0.

   An example loosely based on the FreeBSD 2.2.5-Release kernel
   config file LINT with some added comments (between []):

 # SCSI host adapters: `aha', `ahb', `aic', `bt', `nca'
 #
 # aha: Adaptec 154x
 # ahb: Adaptec 174x
 # ahc: Adaptec 274x/284x/294x
 # aic: Adaptec 152x and sound cards using the Adaptec AIC-6360 (slow!)
 # amd: AMD 53c974 based SCSI cards (e.g., Tekram DC-390 and 390T)
 # bt: Most Buslogic controllers
 # nca: ProAudioSpectrum cards using the NCR 5380 or Trantor T130
 # ncr: NCR/Symbios 53c810/815/825/875 etc based SCSI cards
 # uha: UltraStore 14F and 34F
 # sea: Seagate ST01/02 8 bit controller (slow!)
 # wds: Western Digital WD7000 controller (no scatter/gather!).
 #

 [For an Adaptec AHA274x/284x/294x/394x etc controller]
 controller      ahc0

 [For an NCR/Symbios 53c875 based controller]
 controller      ncr0

 [For an Ultrastor adapter]
 controller      uha0    at isa? port "IO_UHA0" bio irq ? drq 5 vector uhaintr

 # Map SCSI buses to specific SCSI adapters
 controller      scbus0  at ahc0
 controller      scbus2  at ncr0
 controller      scbus1  at uha0

 # The actual SCSI devices
 disk da0 at scbus0 target 0 unit 0      [SCSI disk 0 is at scbus 0, LUN 0]
 disk da1 at scbus0 target 1             [implicit LUN 0 if omitted]
 disk da2 at scbus1 target 3             [SCSI disk on the uha0]
 disk da3 at scbus2 target 4             [SCSI disk on the ncr0]
 tape sa1 at scbus0 target 6             [SCSI tape at target 6]
 device cd0 at scbus?                    [the first ever CDROM found, no wiring]

   The example above tells the kernel to look for a ahc (Adaptec
   274x) controller, then for an NCR/Symbios board, and so on. The
   lines following the controller specifications tell the kernel to
   configure specific devices but only attach them when they match
   the target ID and LUN specified on the corresponding bus.

   Wired down devices get "first shot" at the unit numbers so the
   first non "wired down" device, is allocated the unit number one
   greater than the highest "wired down" unit number for that kind of
   device. So, if you had a SCSI tape at target ID 2 it would be
   configured as sa2, as the tape at target ID 6 is wired down to
   unit number 1.

     Note: Wired down devices need not be found to get their unit
     number. The unit number for a wired down device is reserved for
     that device, even if it is turned off at boot time. This allows
     the device to be turned on and brought on-line at a later time,
     without rebooting. Notice that a device's unit number has no
     relationship with its target ID on the SCSI bus.

   Below is another example of a kernel config file as used by
   FreeBSD version < 2.0.5. The difference with the first example is
   that devices are not "wired down". "Wired down" means that you
   specify which SCSI target belongs to which device.

   A kernel built to the config file below will attach the first SCSI
   disk it finds to da0, the second disk to da1 etc. If you ever
   removed or added a disk, all other devices of the same type (disk
   in this case) would "move around". This implies you have to change
   /etc/fstab each time.

   Although the old style still works, you are strongly recommended
   to use this new feature. It will save you a lot of grief whenever
   you shift your hardware around on the SCSI buses. So, when you
   re-use your old trusty config file after upgrading from a
   pre-FreeBSD2.0.5.R system check this out.

 [driver for Adaptec 174x]
 controller      ahb0 at isa? bio irq 11 vector ahbintr

 [for Adaptec 154x]
 controller      aha0    at isa? port "IO_AHA0" bio irq 11 drq 5 vector ahaintr

 [for Seagate ST01/02]
 controller      sea0    at isa? bio irq 5 iomem 0xc8000 iosiz 0x2000 vector seaintr

 controller      scbus0

 device          da0     [support for 4 SCSI harddisks, da0 up da3]
 device          sa0     [support for 2 SCSI tapes]

 [for the CDROM]
 device          cd0     #Only need one of these, the code dynamically grows

   Both examples support SCSI disks. If during boot more devices of a
   specific type (e.g. da disks) are found than are configured in the
   booting kernel, the system will simply allocate more devices,
   incrementing the unit number starting at the last number "wired
   down". If there are no "wired down" devices then counting starts
   at unit 0.

   Use man 4 scsi to check for the latest info on the SCSI subsystem.
   For more detailed info on host adapter drivers use e.g., man 4 ahc
   for info on the Adaptec 294x driver.

     --------------------------------------------------------------

    2.3.4 Tuning your SCSI kernel setup

   Experience has shown that some devices are slow to respond to
   INQUIRY commands after a SCSI bus reset (which happens at boot
   time). An INQUIRY command is sent by the kernel on boot to see
   what kind of device (disk, tape, CDROM etc.) is connected to a
   specific target ID. This process is called device probing by the
   way.

   To work around the "slow response" problem, FreeBSD allows a
   tunable delay time before the SCSI devices are probed following a
   SCSI bus reset. You can set this delay time in your kernel
   configuration file using a line like:

 options         SCSI_DELAY=15         #Be pessimistic about Joe SCSI device

   This line sets the delay time to 15 seconds. On my own system I
   had to use 3 seconds minimum to get my trusty old CDROM drive to
   be recognized. Start with a high value (say 30 seconds or so) when
   you have problems with device recognition. If this helps, tune it
   back until it just stays working.

     --------------------------------------------------------------

    2.3.5 Rogue SCSI devices

   Although the SCSI standard tries to be complete and concise, it is
   a complex standard and implementing things correctly is no easy
   task. Some vendors do a better job then others.

   This is exactly where the "rogue" devices come into view. Rogues
   are devices that are recognized by the FreeBSD kernel as behaving
   slightly (...) non-standard. Rogue devices are reported by the
   kernel when booting. An example for two of my cartridge tape
   units:

 Feb 25 21:03:34 yedi /kernel: ahb0 targ 5 lun 0: <TANDBERG TDC 3600       -06:>
 Feb 25 21:03:34 yedi /kernel: sa0: Tandberg tdc3600 is a known rogue

 Mar 29 21:16:37 yedi /kernel: aha0 targ 5 lun 0: <ARCHIVE VIPER 150  21247-005>
 Mar 29 21:16:37 yedi /kernel: sa1: Archive  Viper 150 is a known rogue

   For instance, there are devices that respond to all LUNs on a
   certain target ID, even if they are actually only one device. It
   is easy to see that the kernel might be fooled into believing that
   there are 8 LUNs at that particular target ID. The confusion this
   causes is left as an exercise to the reader.

   The SCSI subsystem of FreeBSD recognizes devices with bad habits
   by looking at the INQUIRY response they send when probed. Because
   the INQUIRY response also includes the version number of the
   device firmware, it is even possible that for different firmware
   versions different workarounds are used. See e.g.
   /sys/cam/scsi/scsi_sa.c and /sys/cam/scsi/scsi_all.c for more info
   on how this is done.

   This scheme works fine, but keep in mind that it of course only
   works for devices that are known to be weird. If you are the first
   to connect your bogus Mumbletech SCSI CDROM you might be the one
   that has to define which workaround is needed.

   After you got your Mumbletech working, please send the required
   workaround to the FreeBSD development team for inclusion in the
   next release of FreeBSD. Other Mumbletech owners will be grateful
   to you.

     --------------------------------------------------------------

    2.3.6 Multiple LUN devices

   In some cases you come across devices that use multiple logical
   units (LUNs) on a single SCSI ID. In most cases FreeBSD only
   probes devices for LUN 0. An example are so called bridge boards
   that connect 2 non-SCSI hard disks to a SCSI bus (e.g. an Emulex
   MD21 found in old Sun systems).

   This means that any devices with LUNs != 0 are not normally found
   during device probe on system boot. To work around this problem
   you must add an appropriate entry in /sys/cam/scsi and rebuild
   your kernel.

   Look for a struct that is initialized like below: (FIXME: which
   file? Do these entries still exist in this form now that we use
   CAM?)

 {
         T_DIRECT, T_FIXED, "MAXTOR", "XT-4170S", "B5A",
         "mx1", SC_ONE_LU
 }

   For your Mumbletech BRIDGE2000 that has more than one LUN, acts as
   a SCSI disk and has firmware revision 123 you would add something
   like:

 {
         T_DIRECT, T_FIXED, "MUMBLETECH", "BRIDGE2000", "123",
         "da", SC_MORE_LUS
 }

   The kernel on boot scans the inquiry data it receives against the
   table and acts accordingly. See the source for more info.

     --------------------------------------------------------------

    2.3.7 Tagged command queuing

   Modern SCSI devices, particularly magnetic disks, support what is
   called tagged command queuing (TCQ).

   In a nutshell, TCQ allows the device to have multiple I/O requests
   outstanding at the same time. Because the device is intelligent,
   it can optimize its operations (like head positioning) based on
   its own request queue. On SCSI devices like RAID (Redundant Array
   of Independent Disks) arrays the TCQ function is indispensable to
   take advantage of the device's inherent parallelism.

   Each I/O request is uniquely identified by a "tag" (hence the name
   tagged command queuing) and this tag is used by FreeBSD to see
   which I/O in the device drivers queue is reported as complete by
   the device.

   It should be noted however that TCQ requires device driver support
   and that some devices implemented it "not quite right" in their
   firmware. This problem bit me once, and it leads to highly
   mysterious problems. In such cases, try to disable TCQ.

     --------------------------------------------------------------

    2.3.8 Bus-master host adapters

   Most, but not all, SCSI host adapters are bus mastering
   controllers. This means that they can do I/O on their own without
   putting load onto the host CPU for data movement.

   This is of course an advantage for a multitasking operating system
   like FreeBSD. It must be noted however that there might be some
   rough edges.

   For instance an Adaptec 1542 controller can be set to use
   different transfer speeds on the host bus (ISA or AT in this
   case). The controller is settable to different rates because not
   all motherboards can handle the higher speeds. Problems like
   hang-ups, bad data etc might be the result of using a higher data
   transfer rate then your motherboard can stomach.

   The solution is of course obvious: switch to a lower data transfer
   rate and try if that works better.

   In the case of a Adaptec 1542, there is an option that can be put
   into the kernel config file to allow dynamic determination of the
   right, read: fastest feasible, transfer rate. This option is
   disabled by default:

 options        "TUNE_1542"             #dynamic tune of bus DMA speed

   Check the manual pages for the host adapter that you use. Or
   better still, use the ultimate documentation (read: driver
   source).

     --------------------------------------------------------------

  2.4 Tracking down problems

   The following list is an attempt to give a guideline for the most
   common SCSI problems and their solutions. It is by no means
   complete.

     * Check for loose connectors and cables.

     * Check and double check the location and number of your
       terminators.

     * Check if your bus has at least one supplier of terminator
       power (especially with external terminators.

     * Check if no double target IDs are used.

     * Check if all devices to be used are powered up.

     * Make a minimal bus config with as little devices as possible.

     * If possible, configure your host adapter to use slow bus
       speeds.

     * Disable tagged command queuing to make things as simple as
       possible (for a NCR host adapter based system see man
       ncrcontrol)

     * If you can compile a kernel, make one with the SCSIDEBUG
       option, and try accessing the device with debugging turned on
       for that device. If your device does not even probe at
       startup, you may have to define the address of the device that
       is failing, and the desired debug level in
       /sys/cam/cam_debug.h. If it probes but just does not work, you
       can use the camcontrol(8) command to dynamically set a debug
       level to it in a running kernel (if CAMDEBUG is defined). This
       will give you copious debugging output with which to confuse
       the gurus. See man camcontrol for more exact information. Also
       look at man 4 pass.

     --------------------------------------------------------------

  2.5 Further reading

   If you intend to do some serious SCSI hacking, you might want to
   have the official standard at hand:

   Approved American National Standards can be purchased from ANSI at

       13th Floor
       11 West 42nd Street
       New York
       NY 10036
       Sales Dept: (212) 642-4900
     

   You can also buy many ANSI standards and most committee draft
   documents from Global Engineering Documents,

       15 Inverness Way East
       Englewood
       CO, 80112-5704
       Phone: (800) 854-7179
       Outside USA and Canada: (303) 792-2181
       Fax: (303) 792- 2192
     

   Many X3T10 draft documents are available electronically on the
   SCSI BBS (719-574-0424) and on the ncrinfo.ncr.com anonymous FTP
   site.

   Latest X3T10 committee documents are:

     * AT Attachment (ATA or IDE) [X3.221-1994] (Approved)

     * ATA Extensions (ATA-2) [X3T10/948D Rev 2i]

     * Enhanced Small Device Interface (ESDI)
       [X3.170-1990/X3.170a-1991] (Approved)

     * Small Computer System Interface -- 2 (SCSI-2) [X3.131-1994]
       (Approved)

     * SCSI-2 Common Access Method Transport and SCSI Interface
       Module (CAM) [X3T10/792D Rev 11]

   Other publications that might provide you with additional
   information are:

     * "SCSI: Understanding the Small Computer System Interface",
       written by NCR Corporation. Available from: Prentice Hall,
       Englewood Cliffs, NJ, 07632 Phone: (201) 767-5937 ISBN
       0-13-796855-8

     * "Basics of SCSI", a SCSI tutorial written by Ancot Corporation
       Contact Ancot for availability information at: Phone: (415)
       322-5322 Fax: (415) 322-0455

     * "SCSI Interconnection Guide Book", an AMP publication (dated
       4/93, Catalog 65237) that lists the various SCSI connectors
       and suggests cabling schemes. Available from AMP at (800)
       522-6752 or (717) 564-0100

     * "Fast Track to SCSI", A Product Guide written by Fujitsu.
       Available from: Prentice Hall, Englewood Cliffs, NJ, 07632
       Phone: (201) 767-5937 ISBN 0-13-307000-X

     * "The SCSI Bench Reference", "The SCSI Encyclopedia", and the
       "SCSI Tutor", ENDL Publications, 14426 Black Walnut Court,
       Saratoga CA, 95070 Phone: (408) 867-6642

     * "Zadian SCSI Navigator" (quick ref. book) and "Discover the
       Power of SCSI" (First book along with a one-hour video and
       tutorial book), Zadian Software, Suite 214, 1210 S. Bascom
       Ave., San Jose, CA 92128, (408) 293-0800

   On Usenet the newsgroups comp.periphs.scsi and comp.periphs are
   noteworthy places to look for more info. You can also find the
   SCSI-FAQ there, which is posted periodically.

   Most major SCSI device and host adapter suppliers operate FTP
   sites and/or BBS systems. They may be valuable sources of
   information about the devices you own.

     --------------------------------------------------------------

3 * Disk/tape controllers

  3.1 * SCSI

     --------------------------------------------------------------

  3.2 * IDE

     --------------------------------------------------------------

  3.3 * Floppy

     --------------------------------------------------------------

4 Hard drives

  4.1 SCSI hard drives

   Contributed by Satoshi Asami <asami@FreeBSD.org>. 17 February
   1998.

   As mentioned in the SCSI section, virtually all SCSI hard drives
   sold today are SCSI-2 compliant and thus will work fine as long as
   you connect them to a supported SCSI host adapter. Most problems
   people encounter are either due to badly designed cabling (cable
   too long, star topology, etc.), insufficient termination, or
   defective parts. Please refer to the SCSI section first if your
   SCSI hard drive is not working. However, there are a couple of
   things you may want to take into account before you purchase SCSI
   hard drives for your system.

     --------------------------------------------------------------

    4.1.1 Rotational speed

   Rotational speeds of SCSI drives sold today range from around
   4,500RPM to 15,000RPM. Most of them are either 7,200RPM or
   10,000RPM, with 15,000RPM becoming affordable (June 2002). Even
   though the 10,000RPM drives can generally transfer data faster,
   they run considerably hotter than their 7,200RPM counterparts. A
   large fraction of today's disk drive malfunctions are
   heat-related. If you do not have very good cooling in your PC
   case, you may want to stick with 7,200RPM or slower drives.

   Note that newer drives, with higher areal recording densities, can
   deliver much more bits per rotation than older ones. Today's
   top-of-line 7,200RPM drives can sustain a throughput comparable to
   10,000RPM drives of one or two model generations ago. The number
   to find on the spec sheet for bandwidth is "internal data (or
   transfer) rate". It is usually in megabits/sec so divide it by 8
   and you will get the rough approximation of how much megabytes/sec
   you can get out of the drive.

   (If you are a speed maniac and want a 15,000RPM drive for your
   cute little PC, be my guest; however, those drives become
   extremely hot. Do not even think about it if you do not have a fan
   blowing air directly at the drive or a properly ventilated disk
   enclosure.)

   Obviously, the latest 15,000RPM drives and 10,000RPM drives can
   deliver more data than the latest 7,200RPM drives, so if absolute
   bandwidth is the necessity for your applications, you have little
   choice but to get the faster drives. Also, if you need low
   latency, faster drives are better; not only do they usually have
   lower average seek times, but also the rotational delay is one
   place where slow-spinning drives can never beat a faster one. (The
   average rotational latency is half the time it takes to rotate the
   drive once; thus, it is 2 milliseconds for 15,000RPM, 3ms for
   10,000RPM drives, 4.2ms for 7,200RPM drives and 5.6ms for 5,400RPM
   drives.) Latency is seek time plus rotational delay. Make sure you
   understand whether you need low latency or more accesses per
   second, though; in the latter case (e.g., news servers), it may
   not be optimal to purchase one big fast drive. You can achieve
   similar or even better results by using the ccd (concatenated
   disk) driver to create a striped disk array out of multiple slower
   drives for comparable overall cost.

   Make sure you have adequate air flow around the drive, especially
   if you are going to use a fast-spinning drive. You generally need
   at least 1/2'' (1.25cm) of spacing above and below a drive.
   Understand how the air flows through your PC case. Most cases have
   the power supply suck the air out of the back. See where the air
   flows in, and put the drive where it will have the largest volume
   of cool air flowing around it. You may need to seal some unwanted
   holes or add a new fan for effective cooling.

   Another consideration is noise. Many 10,000 or faster drives
   generate a high-pitched whine which is quite unpleasant to most
   people. That, plus the extra fans often required for cooling, may
   make 10,000 or faster drives unsuitable for some office and home
   environments.

     --------------------------------------------------------------

    4.1.2 Form factor

   Most SCSI drives sold today are of 3.5'' form factor. They come in
   two different heights; 1.6'' ("half-height") or 1''
   ("low-profile"). The half-height drive is the same height as a
   CDROM drive. However, do not forget the spacing rule mentioned in
   the previous section. If you have three standard 3.5'' drive bays,
   you will not be able to put three half-height drives in there
   (without frying them, that is).

     --------------------------------------------------------------

    4.1.3 Interface

   The majority of SCSI hard drives sold today are Ultra, Ultra-wide,
   or Ultra160 SCSI. As of this writing (June 2002), the first
   Ultra320 host adapters and devices become available. The maximum
   bandwidth of Ultra SCSI is 20MB/sec, and Ultra-wide SCSI is
   40MB/sec. Ultra160 can transfer 160MB/sec and Ultra320 can
   transfer 320MB/sec. There is no difference in max cable length
   between Ultra and Ultra-wide; however, the more devices you have
   on the same bus, the sooner you will start having bus integrity
   problems. Unless you have a well-designed disk enclosure, it is
   not easy to make more than 5 or 6 Ultra SCSI drives work on a
   single bus.

   On the other hand, if you need to connect many drives, going for
   Fast-wide SCSI may not be a bad idea. That will have the same max
   bandwidth as Ultra (narrow) SCSI, while electronically it is much
   easier to get it "right". My advice would be: if you want to
   connect many disks, get wide or Ultra160 SCSI drives; they usually
   cost a little more but it may save you down the road. (Besides, if
   you can not afford the cost difference, you should not be building
   a disk array.)

   There are two variant of wide SCSI drives; 68-pin and 80-pin SCA
   (Single Connector Attach). The SCA drives do not have a separate
   4-pin power connector, and also read the SCSI ID settings through
   the 80-pin connector. If you are really serious about building a
   large storage system, get SCA drives and a good SCA enclosure
   (dual power supply with at least one extra fan). They are more
   electronically sound than 68-pin counterparts because there is no
   "stub" of the SCSI bus inside the disk canister as in arrays built
   from 68-pin drives. They are easier to install too (you just need
   to screw the drive in the canister, instead of trying to squeeze
   in your fingers in a tight place to hook up all the little cables
   (like the SCSI ID and disk activity LED lines).

     --------------------------------------------------------------

  4.2 * IDE hard drives

     --------------------------------------------------------------

5 Tape drives

   Contributed by Jonathan M. Bresler <jmb@FreeBSD.org>. 2 July 1996.

     --------------------------------------------------------------

  5.1 General tape access commands

   mt(1) provides generic access to the tape drives. Some of the more
   common commands are rewind, erase, and status. See the mt(1)
   manual page for a detailed description.

     --------------------------------------------------------------

  5.2 Controller Interfaces

   There are several different interfaces that support tape drives.
   The interfaces are SCSI, IDE, Floppy and Parallel Port. A wide
   variety of tape drives are available for these interfaces.
   Controllers are discussed in Disk/tape controllers.

     --------------------------------------------------------------

  5.3 SCSI drives

   The st(4) driver provides support for 8mm (Exabyte), 4mm (DAT:
   Digital Audio Tape), QIC (Quarter-Inch Cartridge), DLT (Digital
   Linear Tape), QIC Mini cartridge and 9-track (remember the big
   reels that you see spinning in Hollywood computer rooms) tape
   drives. See the st(4) manual page for a detailed description.

   The drives listed below are currently being used by members of the
   FreeBSD community. They are not the only drives that will work
   with FreeBSD. They just happen to be the ones that we use.

     --------------------------------------------------------------

    5.3.1 4mm (DAT: Digital Audio Tape)

   Archive Python 28454

   Archive Python 04687

   HP C1533A

   HP C1534A

   HP 35450A

   HP 35470A

   HP 35480A

   SDT-5000

   Wangtek 6200

     --------------------------------------------------------------

    5.3.2 8mm (Exabyte)

   EXB-8200

   EXB-8500

   EXB-8505

     --------------------------------------------------------------

    5.3.3 QIC (Quarter-Inch Cartridge)

   Archive Anaconda 2750

   Archive Viper 60

   Archive Viper 150

   Archive Viper 2525

   Tandberg TDC 3600

   Tandberg TDC 3620

   Tandberg TDC 3800

   Tandberg TDC 4222

   Wangtek 5525ES

     --------------------------------------------------------------

    5.3.4 DLT (Digital Linear Tape)

   Digital TZ87

     --------------------------------------------------------------

    5.3.5 Mini-Cartridge

   Conner CTMS 3200

   Exabyte 2501

     --------------------------------------------------------------

    5.3.6 Autoloaders/Changers

   Hewlett-Packard HP C1553A Autoloading DDS2

     --------------------------------------------------------------

  5.4 * IDE drives

     --------------------------------------------------------------

  5.5 Floppy drives

   Conner 420R

     --------------------------------------------------------------

  5.6 * Parallel port drives

     --------------------------------------------------------------

  5.7 Detailed Information

    5.7.1 Archive Anaconda 2750

   The boot message identifier for this drive is ARCHIVE ANCDA 2750
   28077 -003 type 1 removable SCSI 2

   This is a QIC tape drive.

   Native capacity is 1.35GB when using QIC-1350 tapes. This drive
   will read and write QIC-150 (DC6150), QIC-250 (DC6250), and
   QIC-525 (DC6525) tapes as well.

   Data transfer rate is 350kB/s using dump(8). Rates of 530kB/s have
   been reported when using Amanda

   Production of this drive has been discontinued.

   The SCSI bus connector on this tape drive is reversed from that on
   most other SCSI devices. Make sure that you have enough SCSI cable
   to twist the cable one-half turn before and after the Archive
   Anaconda tape drive, or turn your other SCSI devices upside-down.

   Two kernel code changes are required to use this drive. This drive
   will not work as delivered.

   If you have a SCSI-2 controller, short jumper 6. Otherwise, the
   drive behaves are a SCSI-1 device. When operating as a SCSI-1
   device, this drive, "locks" the SCSI bus during some tape
   operations, including: fsf, rewind, and rewoffl.

   If you are using the NCR SCSI controllers, patch the file
   /usr/src/sys/pci/ncr.c (as shown below). Build and install a new
   kernel.

 *** 4831,4835 ****
                 };
        
 !               if (np->latetime>4) {
                         /*
                         **      Although we tried to wake it up,
 --- 4831,4836 ----
                 };

 !               if (np->latetime>1200) {
                         /*
                         **      Although we tried to wake it up,

   Reported by: Jonathan M. Bresler <jmb@FreeBSD.org>

     --------------------------------------------------------------

    5.7.2 Archive Python 28454

   The boot message identifier for this drive is ARCHIVE Python
   28454-XXX4ASB type 1 removable SCSI 2 density code 0x8c, 512-byte
   blocks

   This is a DDS-1 tape drive.

   Native capacity is 2.5GB on 90m tapes.

   Data transfer rate is XXX.

   This drive was repackaged by Sun Microsystems as model 595-3067.

   Reported by: Bob Bishop <rb@gid.co.uk>

   Throughput is in the 1.5 MByte/sec range, however this will drop
   if the disks and tape drive are on the same SCSI controller.

   Reported by: Robert E. Seastrom <rs@seastrom.com>

     --------------------------------------------------------------

    5.7.3 Archive Python 04687

   The boot message identifier for this drive is ARCHIVE Python
   04687-XXX 6580 Removable Sequential Access SCSI-2 device

   This is a DAT-DDS-2 drive.

   Native capacity is 4GB when using 120m tapes.

   This drive supports hardware data compression. Switch 4 controls
   MRS (Media Recognition System). MRS tapes have stripes on the
   transparent leader. Switch 4 off enables MRS, on disables MRS.

   Parity is controlled by switch 5. Switch 5 on to enable parity
   control. Compression is enabled with Switch 6 off. It is possible
   to override compression with the SCSI MODE SELECT command (see
   mt(1)).

   Data transfer rate is 800kB/s.

     --------------------------------------------------------------

    5.7.4 Archive Viper 60

   The boot message identifier for this drive is ARCHIVE VIPER 60
   21116 -007 type 1 removable SCSI 1

   This is a QIC tape drive.

   Native capacity is 60MB.

   Data transfer rate is XXX.

   Production of this drive has been discontinued.

   Reported by: Philippe Regnauld <regnauld@hsc.fr>

     --------------------------------------------------------------

    5.7.5 Archive Viper 150

   The boot message identifier for this drive is ARCHIVE VIPER 150
   21531 -004 Archive Viper 150 is a known rogue type 1 removable
   SCSI 1. A multitude of firmware revisions exist for this drive.
   Your drive may report different numbers (e.g 21247 -005.

   This is a QIC tape drive.

   Native capacity is 150/250MB. Both 150MB (DC6150) and 250MB
   (DC6250) tapes have the recording format. The 250MB tapes are
   approximately 67% longer than the 150MB tapes. This drive can read
   120MB tapes as well. It can not write 120MB tapes.

   Data transfer rate is 100kB/s

   This drive reads and writes DC6150 (150MB) and DC6250 (250MB)
   tapes.

   This drives quirks are known and pre-compiled into the SCSI tape
   device driver (st(4)).

   Under FreeBSD 2.2-CURRENT, use mt blocksize 512 to set the
   blocksize. (The particular drive had firmware revision 21247 -005.
   Other firmware revisions may behave differently) Previous versions
   of FreeBSD did not have this problem.

   Production of this drive has been discontinued.

   Reported by: Pedro A M Vazquez <vazquez@IQM.Unicamp.BR>

   Michael Smith <msmith@FreeBSD.org>

     --------------------------------------------------------------

    5.7.6 Archive Viper 2525

   The boot message identifier for this drive is ARCHIVE VIPER 2525
   25462 -011 type 1 removable SCSI 1

   This is a QIC tape drive.

   Native capacity is 525MB.

   Data transfer rate is 180kB/s at 90 inches/sec.

   The drive reads QIC-525, QIC-150, QIC-120 and QIC-24 tapes. Writes
   QIC-525, QIC-150, and QIC-120.

   Firmware revisions prior to 25462 -011 are bug ridden and will not
   function properly.

   Production of this drive has been discontinued.

     --------------------------------------------------------------

    5.7.7 Conner 420R

   The boot message identifier for this drive is Conner tape.

   This is a floppy controller, mini cartridge tape drive.

   Native capacity is XXXX

   Data transfer rate is XXX

   The drive uses QIC-80 tape cartridges.

   Reported by: Mark Hannon <mark@seeware.DIALix.oz.au>

     --------------------------------------------------------------

    5.7.8 Conner CTMS 3200

   The boot message identifier for this drive is CONNER CTMS 3200
   7.00 type 1 removable SCSI 2.

   This is a mini cartridge tape drive.

   Native capacity is XXXX

   Data transfer rate is XXX

   The drive uses QIC-3080 tape cartridges.

   Reported by: Thomas S. Traylor <tst@titan.cs.mci.com>

     --------------------------------------------------------------

    5.7.9 DEC TZ87

   The boot message identifier for this drive is DEC TZ87 (C) DEC
   9206 type 1 removable SCSI 2 density code 0x19

   This is a DLT tape drive.

   Native capacity is 10GB.

   This drive supports hardware data compression.

   Data transfer rate is 1.2MB/s.

   This drive is identical to the Quantum DLT2000. The drive firmware
   can be set to emulate several well-known drives, including an
   Exabyte 8mm drive.

   Reported by: Wilko Bulte <wilko@FreeBSD.org>

     --------------------------------------------------------------

    5.7.10 Exabyte EXB-2501

   The boot message identifier for this drive is EXABYTE EXB-2501

   This is a mini-cartridge tape drive.

   Native capacity is 1GB when using MC3000XL mini cartridges.

   Data transfer rate is XXX

   This drive can read and write DC2300 (550MB), DC2750 (750MB),
   MC3000 (750MB), and MC3000XL (1GB) mini cartridges.

   WARNING: This drive does not meet the SCSI-2 specifications. The
   drive locks up completely in response to a SCSI MODE_SELECT
   command unless there is a formatted tape in the drive. Before
   using this drive, set the tape blocksize with

 # mt -f /dev/st0ctl.0 blocksize 1024

   Before using a mini cartridge for the first time, the mini
   cartridge must be formatted. FreeBSD 2.1.0-RELEASE and earlier:

 # /sbin/scsi -f /dev/rst0.ctl -s 600 -c "4 0 0 0 0 0"

   (Alternatively, fetch a copy of the scsiformat shell script from
   FreeBSD 2.1.5/2.2.) FreeBSD 2.1.5 and later:

 # /sbin/scsiformat -q -w /dev/rst0.ctl

   Right now, this drive cannot really be recommended for FreeBSD.

   Reported by: Bob Beaulieu <ez@eztravel.com>

     --------------------------------------------------------------

    5.7.11 Exabyte EXB-8200

   The boot message identifier for this drive is EXABYTE EXB-8200
   252X type 1 removable SCSI 1

   This is an 8mm tape drive.

   Native capacity is 2.3GB.

   Data transfer rate is 270kB/s.

   This drive is fairly slow in responding to the SCSI bus during
   boot. A custom kernel may be required (set SCSI_DELAY to 10
   seconds).

   There are a large number of firmware configurations for this
   drive, some have been customized to a particular vendor's
   hardware. The firmware can be changed via EPROM replacement.

   Production of this drive has been discontinued.

   Reported by: Michael Smith <msmith@FreeBSD.org>

     --------------------------------------------------------------

    5.7.12 Exabyte EXB-8500

   The boot message identifier for this drive is EXABYTE
   EXB-8500-85Qanx0 0415 type 1 removable SCSI 2

   This is an 8mm tape drive.

   Native capacity is 5GB.

   Data transfer rate is 300kB/s.

   Reported by: Greg Lehey <grog@lemis.de>

     --------------------------------------------------------------

    5.7.13 Exabyte EXB-8505

   The boot message identifier for this drive is EXABYTE
   EXB-85058SQANXR1 05B0 type 1 removable SCSI 2

   This is an 8mm tape drive which supports compression, and is
   upward compatible with the EXB-5200 and EXB-8500.

   Native capacity is 5GB.

   The drive supports hardware data compression.

   Data transfer rate is 300kB/s.

   Reported by: Glen Foster <gfoster@gfoster.com>

     --------------------------------------------------------------

    5.7.14 Hewlett-Packard HP C1533A

   The boot message identifier for this drive is HP C1533A 9503 type
   1 removable SCSI 2.

   This is a DDS-2 tape drive. DDS-2 means hardware data compression
   and narrower tracks for increased data capacity.

   Native capacity is 4GB when using 120m tapes. This drive supports
   hardware data compression.

   Data transfer rate is 510kB/s.

   This drive is used in Hewlett-Packard's SureStore 6000eU and 6000i
   tape drives and C1533A DDS-2 DAT drive.

   The drive has a block of 8 dip switches. The proper settings for
   FreeBSD are: 1 ON; 2 ON; 3 OFF; 4 ON; 5 ON; 6 ON; 7 ON; 8 ON.

   switch 1 switch 2                      Result                      
   On       On       Compression enabled at power-on, with host       
                     control                                          
   On       Off      Compression enabled at power-on, no host control 
   Off      On       Compression disabled at power-on, with host      
                     control                                          
   Off      Off      Compression disabled at power-on, no host        
                     control                                          

   Switch 3 controls MRS (Media Recognition System). MRS tapes have
   stripes on the transparent leader. These identify the tape as DDS
   (Digital Data Storage) grade media. Tapes that do not have the
   stripes will be treated as write-protected. Switch 3 OFF enables
   MRS. Switch 3 ON disables MRS.

   See HP SureStore Tape Products and Hewlett-Packard Disk and Tape
   Technical Information for more information on configuring this
   drive.

   Warning: Quality control on these drives varies greatly. One
   FreeBSD core-team member has returned 2 of these drives. Neither
   lasted more than 5 months.

   Reported by: Stefan Esser <se@FreeBSD.org>

     --------------------------------------------------------------

    5.7.15 Hewlett-Packard HP 1534A

   The boot message identifier for this drive is HP HP35470A T503
   type 1 removable SCSI 2 Sequential-Access density code 0x13,
   variable blocks.

   This is a DDS-1 tape drive. DDS-1 is the original DAT tape format.

   Native capacity is 2GB when using 90m tapes.

   Data transfer rate is 183kB/s.

   The same mechanism is used in Hewlett-Packard's SureStore 2000i
   tape drive, C35470A DDS format DAT drive, C1534A DDS format DAT
   drive and HP C1536A DDS format DAT drive.

   The HP C1534A DDS format DAT drive has two indicator lights, one
   green and one amber. The green one indicates tape action: slow
   flash during load, steady when loaded, fast flash during
   read/write operations. The amber one indicates warnings: slow
   flash when cleaning is required or tape is nearing the end of its
   useful life, steady indicates an hard fault. (factory service
   required?)

   Reported by Gary Crutcher <gcrutchr@nightflight.com>

     --------------------------------------------------------------

    5.7.16 Hewlett-Packard HP C1553A Autoloading DDS2

   The boot message identifier for this drive is "".

   This is a DDS-2 tape drive with a tape changer. DDS-2 means
   hardware data compression and narrower tracks for increased data
   capacity.

   Native capacity is 24GB when using 120m tapes. This drive supports
   hardware data compression.

   Data transfer rate is 510kB/s (native).

   This drive is used in Hewlett-Packard's SureStore 12000e tape
   drive.

   The drive has two selectors on the rear panel. The selector closer
   to the fan is SCSI id. The other selector should be set to 7.

   There are four internal switches. These should be set: 1 ON; 2 ON;
   3 ON; 4 OFF.

   At present the kernel drivers do not automatically change tapes at
   the end of a volume. This shell script can be used to change
   tapes:

 #!/bin/sh
 PATH="/sbin:/usr/sbin:/bin:/usr/bin"; export PATH

 usage()
 {
         echo "Usage: dds_changer [123456ne] raw-device-name
         echo "1..6 = Select cartridge"
         echo "next cartridge"
         echo "eject magazine"
         exit 2
 }

 if [ $# -ne 2 ] ; then
         usage
 fi

 cdb3=0
 cdb4=0
 cdb5=0

 case $1 in
         [123456])
                 cdb3=$1
                 cdb4=1
                 ;;
         n)
                 ;;
         e)
                 cdb5=0x80
                 ;;
         ?)
                 usage
                 ;;
 esac

 scsi -f $2 -s 100 -c "1b 0 0 $cdb3 $cdb4 $cdb5"

     --------------------------------------------------------------

    5.7.17 Hewlett-Packard HP 35450A

   The boot message identifier for this drive is HP HP35450A -A C620
   type 1 removable SCSI 2 Sequential-Access density code 0x13

   This is a DDS-1 tape drive. DDS-1 is the original DAT tape format.

   Native capacity is 1.2GB.

   Data transfer rate is 160kB/s.

   Reported by: Mark Thompson <mark.a.thompson@pobox.com>

     --------------------------------------------------------------

    5.7.18 Hewlett-Packard HP 35470A

   The boot message identifier for this drive is HP HP35470A 9 09
   type 1 removable SCSI 2

   This is a DDS-1 tape drive. DDS-1 is the original DAT tape format.

   Native capacity is 2GB when using 90m tapes.

   Data transfer rate is 183kB/s.

   The same mechanism is used in Hewlett-Packard's SureStore 2000i
   tape drive, C35470A DDS format DAT drive, C1534A DDS format DAT
   drive, and HP C1536A DDS format DAT drive.

   Warning: Quality control on these drives varies greatly. One
   FreeBSD core-team member has returned 5 of these drives. None
   lasted more than 9 months.

   Reported by: David Dawes <dawes@rf900.physics.usyd.edu.au> (9 09)

     --------------------------------------------------------------

    5.7.19 Hewlett-Packard HP 35480A

   The boot message identifier for this drive is HP HP35480A 1009
   type 1 removable SCSI 2 Sequential-Access density code 0x13.

   This is a DDS-DC tape drive. DDS-DC is DDS-1 with hardware data
   compression. DDS-1 is the original DAT tape format.

   Native capacity is 2GB when using 90m tapes. It cannot handle 120m
   tapes. This drive supports hardware data compression. Please refer
   to the section on HP C1533A for the proper switch settings.

   Data transfer rate is 183kB/s.

   This drive is used in Hewlett-Packard's SureStore 5000eU and 5000i
   tape drives and C35480A DDS format DAT drive..

   This drive will occasionally hang during a tape eject operation
   (mt offline). Pressing the front panel button will eject the tape
   and bring the tape drive back to life.

   WARNING: HP 35480-03110 only. On at least two occasions this tape
   drive when used with FreeBSD 2.1.0, an IBM Server 320 and an 2940W
   SCSI controller resulted in all SCSI disk partitions being lost.
   The problem has not be analyzed or resolved at this time.

     --------------------------------------------------------------

    5.7.20 Sony SDT-5000

   There are at least two significantly different models: one is a
   DDS-1 and the other DDS-2. The DDS-1 version is SDT-5000 3.02. The
   DDS-2 version is SONY SDT-5000 327M. The DDS-2 version has a 1MB
   cache. This cache is able to keep the tape streaming in almost any
   circumstances.

   The boot message identifier for this drive is SONY SDT-5000 3.02
   type 1 removable SCSI 2 Sequential-Access density code 0x13

   Native capacity is 4GB when using 120m tapes. This drive supports
   hardware data compression.

   Data transfer rate is depends upon the model or the drive. The
   rate is 630kB/s for the SONY SDT-5000 327M while compressing the
   data. For the SONY SDT-5000 3.02, the data transfer rate is
   225kB/s.

   In order to get this drive to stream, set the blocksize to 512
   bytes (mt blocksize 512) reported by Kenneth Merry
   <ken@ulc199.residence.gatech.edu>.

   SONY SDT-5000 327M information reported by Charles Henrich
   <henrich@msu.edu>.

   Reported by: Jean-Marc Zucconi <jmz@FreeBSD.org>

     --------------------------------------------------------------

    5.7.21 Tandberg TDC 3600

   The boot message identifier for this drive is TANDBERG TDC 3600
   =08: type 1 removable SCSI 2

   This is a QIC tape drive.

   Native capacity is 150/250MB.

   This drive has quirks which are known and work around code is
   present in the SCSI tape device driver (st(4)). Upgrading the
   firmware to XXX version will fix the quirks and provide SCSI 2
   capabilities.

   Data transfer rate is 80kB/s.

   IBM and Emerald units will not work. Replacing the firmware EPROM
   of these units will solve the problem.

   Reported by: Michael Smith <msmith@FreeBSD.org>

     --------------------------------------------------------------

    5.7.22 Tandberg TDC 3620

   This is very similar to the Tandberg TDC 3600 drive.

   Reported by: Jo:rg Wunsch <joerg@FreeBSD.org>

     --------------------------------------------------------------

    5.7.23 Tandberg TDC 3800

   The boot message identifier for this drive is TANDBERG TDC 3800
   =04Y Removable Sequential Access SCSI-2 device

   This is a QIC tape drive.

   Native capacity is 525MB.

   Reported by: Julian Stacey <jhs@FreeBSD.org>

     --------------------------------------------------------------

    5.7.24 Tandberg TDC 4222

   The boot message identifier for this drive is TANDBERG TDC 4222
   =07 type 1 removable SCSI 2

   This is a QIC tape drive.

   Native capacity is 2.5GB. The drive will read all cartridges from
   the 60 MB (DC600A) upwards, and write 150 MB (DC6150) upwards.
   Hardware compression is optionally supported for the 2.5 GB
   cartridges.

   This drives quirks are known and pre-compiled into the SCSI tape
   device driver (st(4)) beginning with FreeBSD 2.2-CURRENT. For
   previous versions of FreeBSD, use mt to read one block from the
   tape, rewind the tape, and then execute the backup program (mt fsr
   1; mt rewind; dump ...)

   Data transfer rate is 600kB/s (vendor claim with compression), 350
   KB/s can even be reached in start/stop mode. The rate decreases
   for smaller cartridges.

   Reported by: Jo:rg Wunsch <joerg@FreeBSD.org>

     --------------------------------------------------------------

    5.7.25 Wangtek 5525ES

   The boot message identifier for this drive is WANGTEK 5525ES SCSI
   REV7 3R1 type 1 removable SCSI 1 density code 0x11, 1024-byte
   blocks

   This is a QIC tape drive.

   Native capacity is 525MB.

   Data transfer rate is 180kB/s.

   The drive reads 60, 120, 150, and 525MB tapes. The drive will not
   write 60MB (DC600 cartridge) tapes. In order to overwrite 120 and
   150 tapes reliably, first erase (mt erase) the tape. 120 and 150
   tapes used a wider track (fewer tracks per tape) than 525MB tapes.
   The "extra" width of the previous tracks is not overwritten, as a
   result the new data lies in a band surrounded on both sides by the
   previous data unless the tape have been erased.

   This drives quirks are known and pre-compiled into the SCSI tape
   device driver (st(4)).

   Other firmware revisions that are known to work are: M75D

   Reported by: Marc van Kempen <marc@bowtie.nl> REV73R1 Andrew
   Gordon <Andrew.Gordon@net-tel.co.uk> M75D

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    5.7.26 Wangtek 6200

   The boot message identifier for this drive is WANGTEK 6200-HS 4B18
   type 1 removable SCSI 2 Sequential-Access density code 0x13

   This is a DDS-1 tape drive.

   Native capacity is 2GB using 90m tapes.

   Data transfer rate is 150kB/s.

   Reported by: Tony Kimball <alk@Think.COM>

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  5.8 * Problem drives

     --------------------------------------------------------------

6 CDROM drives

   Contributed by David O'Brien <obrien@FreeBSD.org>. 23 November
   1997.

   Generally speaking those in The FreeBSD Project prefer SCSI CDROM
   drives over IDE CDROM drives. However not all SCSI CDROM drives
   are equal. Some feel the quality of some SCSI CDROM drives have
   been deteriorating to that of IDE CDROM drives. Toshiba used to be
   the favored stand-by, but many on the SCSI mailing list have found
   displeasure with the 12x speed XM-5701TA as its volume (when
   playing audio CDROMs) is not controllable by the various audio
   player software.

   Another area where SCSI CDROM manufacturers are cutting corners is
   adherence to the SCSI specification. Many SCSI CDROMs will respond
   to multiple LUNs for its target address. Known violators include
   the 6x Teac CD-56S 1.0D.

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