cnid_dbd
8
01 Jan 2012
@NETATALK_VERSION@

# cnid_dbd

implement access to CNID databases through a dedicated daemon process

          cnid_dbd

          cnid_dbd

              -v
              -V

## DESCRIPTION

`cnid_dbd` provides an interface for storage and retrieval of catalog node IDs (CNIDs) and related information to the afpd daemon. CNIDs are a component of Macintosh based file systems with semantics that map not easily onto Unix file systems. This makes separate storage in a database necessary. `cnid_dbd` is part of the CNID backend framework of afpd and implements the dbd backend.

`cnid_dbd` is never started via the command line or system startup scripts but only by the cnid_metad daemon. There is one instance of `cnid_dbd` per netatalk volume.

`cnid_dbd` uses the Berkeley DB database library and uses transactionally protected updates. The dbd backend with transactions will avoid corruption of the CNID database even if the system crashes unexpectedly.

`cnid_dbd` inherits the effective userid and groupid from cnid_metad on startup, which is normally caused by afpd serving a netatalk volume to a client. It changes to the Berkeley DB database home directory dbdir that is associated with the volume. If the userid inherited from cnid_metad is 0 (root), `cnid_dbd` will change userid and groupid to the owner and group of the database home directory. Otherwise, it will continue to use the inherited values. `cnid_dbd` will then attempt to open the database and start serving requests using filedescriptor clntfd. Subsequent instances of afpd that want to access the same volume are redirected to the running `cnid_dbd` process by cnid_metad via the filedescriptor ctrlfd.

`cnid_dbd` can be configured to run forever or to exit after a period of inactivity. If `cnid_dbd` receives a TERM or an INT signal it will exit cleanly after flushing dirty database buffers to disk and closing Berkeley DB database environments. It is safe to terminate `cnid_dbd` this way, it will be restarted when necessary. Other signals are not handled and will cause an immediate exit, possibly leaving the CNID database in an inconsistent state (no transactions) or losing recent updates during recovery (transactions).

The Berkeley DB database subsystem will create files named log.xxxxxxxxxx in the database home directory dbdir, where xxxxxxxxxx is a monotonically increasing integer. These files contain the transactional database changes. They will be removed regularly, unless the logfile_autoremove option is specified in the db_param configuration file (see below) with a value of 0 (default 1).

## OPTIONS

`-v, -V`

  Show version and exit.

## CONFIGURATION

`cnid_dbd` reads configuration information from the file db_param in the database directory dbdir on startup. If the file does not exist or a parameter is not listed, suitable default values are used. The format for a single parameter is the parameter name, followed by one or more spaces, followed by the parameter value, followed by a newline. The following parameters are currently recognized:

logfile_autoremove

  If set to 0, unused Berkeley DB transactional logfiles (log.xxxxxxxxxx in the database home directory) are not removed on startup of `cnid_dbd` and on a regular basis. Default: 1.

cachesize

  Determines the size of the Berkeley DB cache in kilobytes. Default: 8192. Each `cnid_dbd` process grabs that much memory on top of its normal memory footprint. It can be used to tune database performance. The db_stat utility with the `-m` option that comes with Berkley DB can help you determine ether you need to change this value. The default is pretty conservative so that a large percentage of requests should be satisfied from the cache directly. If memory is not a bottleneck on your system you might want to leave it at that value. The Berkeley DB Tutorial and Reference Guide has a section Selecting a cache size that gives more detailed information.

flush_frequency
flush_interval

  flush_frequency (Default: 1000) and flush_interval (Default: 1800) control how often changes to the database are checkpointed. Both of these operations are performed if either i) more than flush_frequency requests have been received or ii) more than flush_interval seconds have elapsed since the last save/checkpoint. Be careful to check your harddisk configuration for on disk cache settings. Many IDE disks just cache writes as the default behaviour, so even flushing database files to disk will not have the desired effect.

fd_table_size

  is the maximum number of connections (filedescriptors) that can be open for afpd client processes in cnid_dbd. Default: 512. If this number is exceeded, one of the existing connections is closed and reused. The affected afpd process will transparently reconnect later, which causes slight overhead. On the other hand, setting this parameter too high could affect performance in `cnid_dbd` since all descriptors have to be checked in a `select()` system call, or worse, you might exceed the per process limit of open file descriptors on your system. It is safe to set the value to 1 on volumes where only one afpd client process is expected to run, e.g. home directories.

idle_timeout

  is the number of seconds of inactivity before an idle `cnid_dbd` exits. Default: 600. Set this to 0 to disable the timeout.

## UPDATING

Note that the first version to appear after Netatalk 2.1 ie Netatalk 2.1.1, will support BerkeleyDB updates on the fly without manual intervention. In other words Netatalk 2.1 does contain code to prepare the BerkeleyDB database for upgrades and to upgrade it in case it has been prepared before. That means it can't upgrade a 2.0.x version because that one didn't prepare the database.

In order to update between older Netatalk releases using different BerkeleyDB library versions, follow this steps:

- Stop the to be upgraded old version of Netatalk

- Using the old BerkeleyDB utilities run `db_recover -h <path to .AppleDB>`

- Using the new BerkeleyDB utilities run `db_upgrade -v -h <path to .AppleDB> -f cnid2.db`

- Again using the new BerkeleyDB utilities run `db_checkpoint -1 -h <path to .AppleDB>`

- Start the the new version of Netatalk

## SEE ALSO

cnid_metad 8 , afpd 8 , dbd 1