NAME
cnid_dbd - implement access to CNID databases through a dedicated
daemon process
SYNOPSIS
cnid_dbd dbdir ctrlfd clntfd logconfig_string
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 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.
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 reqular 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
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, 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
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.
SEE ALSO
cnid_metad(8), afpd(8), dbd(1)