cnid_dbd — implement access to CNID databases through a dedicated daemon process
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 at most one instance of cnid_dbd per netatalk volume.
cnid_dbd uses the Berkleley 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 uses the same on-disk database format as the cdb backend. It is therefore possible to switch between the two backends as necessary.
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 Berkleley 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 uses logconfig_string
which is passed from cnid_metad to configure its
logging output.
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 Berkleley 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 Berkleley 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 ithe transactional database changes. They will be removed regularily, unless the logfile_autoremove option is specified in the db_param configuration file (see below).
Do not use cnid_dbd for databases on NFS mounted file systems. It makes the whole point of securing database changes properly moot. Use the dbdir: Option in the appropriate AppleVolumes configuration file to put the database onto a local disk.
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:
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 reqular basis. Default: 1.
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 Berkely DB can help you determine wether 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 (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.
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.
is the number of seconds of inactivity before an idle cnid_dbd exits. Default: 600. Set this to 0 to disable the timeout.
In order to update between 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
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.
