Ubuntu Manpage: ovdb - Overview storage method for INN

NAME

       ovdb - Overview storage method for INN

DESCRIPTION

       Ovdb is a storage method that uses the Berkeley DB library to store overview data.  It
       requires version 4.4 or later of the Berkeley DB library (4.7+ is recommended because
       older versions suffer from various issues).

       Ovdb makes use of the full transaction/logging/locking functionality of the Berkeley DB
       environment.  Berkeley DB may be downloaded from
       <http://www.oracle.com/technetwork/database/berkeleydb/overview/index.html> and is needed
       to build the ovdb backend.

UPGRADING

       This is version 2 of ovdb.  If you have a database created with a previous version of ovdb
       (such as the one shipped with INN 2.3.0) your database will need to be upgraded using
       ovdb_init(8).  See the man page ovdb_init(8) for upgrade instructions.

INSTALLATION

       To build ovdb support into INN, specify the option --with-berkeleydb when running the
       configure script.  By default, configure will search for a Berkeley DB tree in several
       likely locations, and choose the highest version (based on the name of the directory,
       e.g., BerkeleyDB.4.4) that it finds.  There will be a message in the configure output
       indicating the chosen pathname.

       You can override this pathname by adding a path to the option, e.g.,
       --with-berkeleydb=/usr/BerkeleyDB.4.4.  This directory is expected to have subdirectories
       include and lib, containing db.h, and the library itself, respectively.

       The ovdb database may take up more disk space for a given spool than the other overview
       methods.  Plan on needing at least 1.1 KB for every article in your spool (not counting
       crossposts).  So, if you have 5 million articles, you'll need at least 5.5 GB of disk
       space for ovdb. With compression enabled, this estimate changes to 0.7 KB per article.
       See the COMPRESSION section below.  Plus, you'll need additional space for transaction
       logs: at least 100 MB.  By default the transaction logs go in the same directory as the
       database.  To improve performance, they can be placed on a different disk -- see the
       DB_CONFIG section.

CONFIGURATION

To enable ovdb, set the ovmethod parameter in inn.conf to "ovdb". The ovdb database is
stored in the directory specified by the pathoverview paramter in inn.conf. This is the
"DB_HOME" directory. To start out, this directory should be empty (other than an optional
DB_CONFIG file; see DB_CONFIG for details) and innd (or makehistory) will create the files
as necessary in that directory. Make sure the directory is owned by the news user.

Other parameters for configuring ovdb are in the ovdb.conf(5) configuration file. See
also the sample ovdb.conf.

cachesize
Size of the memory pool cache, in kilobytes. The cache will have a backing store file
in the DB directory which will be at least as big. In general, the bigger the cache,
the better. Use "ovdb_stat -m" to see cache hit percentages. To make a change of
this parameter take effect, shut down and restart INN (be sure to kill all of the
nnrpds when shutting down). Default is 8000, which is adequate for small to medium
sized servers. Large servers will probably need at least 20000.

compress
If INN was compiled with zlib, and this compress parameter is true, OVDB will compress
overview records that are longer than 600 bytes. See the COMPRESSION section below.

numdbfiles
Overview data is split between this many files. Currently, innd will keep all of the
files open, so don't set this too high or innd may run out of file descriptors. nnrpd
only opens one at a time, regardless. May be set to one, or just a few, but only do
that if your OS supports large (>2G) files. Changing this parameter has no effect on
an already-established database. Default is 32.

txn_nosync
If txn_nosync is set to false, Berkeley DB flushes the log after every transaction.
This minimizes the number of transactions that may be lost in the event of a crash,
but results in significantly degraded performance. Default is true.

useshm
If useshm is set to true, Berkeley DB will use shared memory instead of mmap for its
environment regions (cache, lock, etc). With some platforms, this may improve
performance. Default is false.

shmkey
Sets the shared memory key used by Berkeley DB when 'useshm' is true. Berkeley DB
will create several (usually 5) shared memory segments, using sequentially numbered
keys starting with 'shmkey'. Choose a key that does not conflict with any existing
shared memory segments on your system. Default is 6400.

pagesize
Sets the page size for the DB files (in bytes). Must be a power of 2. Best choices
are 4096 or 8192. The default is 8192. Changing this parameter has no effect on an
already-established database.

minkey
Sets the minimum number of keys per page. See the Berkeley DB documentation for more
info. Default is based on page size and whether compression is enabled:

default_minkey = MAX(2, pagesize / 2600) if compress is false
default_minkey = MAX(2, pagesize / 1500) if compress is true

The lowest allowed minkey is 2. Setting minkey higher than the default is not
recommended, as it will cause the databases to have a lot of overflow pages. Changing
this parameter has no effect on an already-established database.

maxlocks
Sets the Berkeley DB "lk_max" parameter, which is the maximum number of locks that can
exist in the database at the same time. Default is 4000.

nocompact
The nocompact parameter affects expireover's behavior. The expireover function in
ovdb can do its job in one of two ways: by simply deleting expired records from the
database, or by re-writing the overview records into a different location leaving out
the expired records. The first method is faster, but it leaves 'holes' that result in
space that can not immediately be reused. The second method 'compacts' the records by
rewriting them.

If this parameter is set to 0, expireover will compact all newsgroups; if set to 1,
expireover will not compact any newsgroups; and if set to a value greater than one,
expireover will only compact groups that have less than that number of articles.

Experience has shown that compacting has minimal effect (other than making expireover
take longer) so the default is now 1. This parameter will probably be removed in the
future.

readserver
Normally, each nnrpd process directly accesses the Berkeley DB environment. The
process of attaching to the database (and detaching when finished) is fairly
expensive, and can result in high loads in situations when there are lots of reader
connections of relatively short duration.

When the readserver parameter is true, the nnrpds will access overview via a helper
server (ovdb_server -- which is started by ovdb_init). This can also result in
cleaner shutdowns for the database, improving stability and avoiding deadlocks and
corrupted databases. If you are experiencing any instability in ovdb, try setting
this parameter to true. Default is false.

numrsprocs
This parameter is only used when readserver is true. It sets the number of
ovdb_server processes. As each ovdb_server can process only one transaction at a
time, running more servers can improve reader response times. Default is 5.

maxrsconn
This parameter is only used when readserver is true. It sets a maximum number of
readers that a given ovdb_server process will serve at one time. This means the
maximum number of readers for all of the ovdb_server processes is (numrsprocs *
maxrsconn). This does not limit the actual number of readers, since nnrpd will fall
back to opening the database directly if it can't connect to a readserver. Default is
0, which means an umlimited number of connections is allowed.

COMPRESSION

New in this version of OVDB is the ability to compress overview data before it is stored
into the database. In addition to consuming less disk space, compression keeps the
average size of the database keys smaller. This in turn increases the average number of
keys per page, which can significantly improve performance and also helps keep the
database more compact. This feature requires that INN be built with zlib. Only records
larger than 600 bytes get compressed, because that is the point at which compression
starts to become significant.

If compression is not enabled (either from the "compress" option in ovdb.conf or INN was
not built from zlib), the database will be backward compatible with older versions of
OVDB. However, if compression is enabled, the database is marked with a newer version
that will prevent older versions of OVDB from opening the database.

You can upgrade an existing database to use compression simply by setting compress to true
in ovdb.conf. Note that existing records in the database will remain uncompressed; only
new records added after enabling compression will be compressed.

If you disable compression on a database that previously had it enabled, new records will
be stored uncompressed, but the database will still be incompatible with older versions of
OVDB (and will also be incompatible with this version of OVDB if it was not built with
zlib). So to downgrade to a completely uncompressed database you will have to rebuild the
database using makehistory.

DB_CONFIG

A file called DB_CONFIG may be placed in the database directory to customize where the
various database files and transaction logs are written. By default, all of the files are
written in the "DB_HOME" directory. One way to improve performance is to put the
transaction logs on a different disk. To do this, put:

DB_LOG_DIR /path/to/logs

in the DB_CONFIG file. If the pathname you give starts with a /, it is treated as an
absolute path; otherwise, it is relative to the "DB_HOME" directory. Make sure that any
directories you specify exist and have proper ownership/mode before starting INN, because
they won't be created automatically. Also, don't change the DB_CONFIG file while anything
that uses ovdb is running.

Another thing that you can do with this file is to split the overview database across
multiple disks. In the DB_CONFIG file, you can list directories that Berkeley DB will
search when it goes to open a database.

For example, let's say that you have pathoverview set to /mnt/overview and you have four
additional file systems created on /mnt/ov?. You would create a file
"/mnt/overview/DB_CONFIG" containing the following lines:

set_data_dir /mnt/overview
set_data_dir /mnt/ov1
set_data_dir /mnt/ov2
set_data_dir /mnt/ov3
set_data_dir /mnt/ov4

Distribute your ovNNNNN files into the four filesystems. (say, 8 each). When called upon
to open a database file, the db library will look for it in each of the specified
directories (in order). If said file is not found, one will be created in the first of
those directories.

Whenever you change DB_CONFIG or move database files around, make sure all news processes
that use the database are shut down first (including nnrpds).

The DB_CONFIG functionality is part of Berkeley DB itself, rather than something provided
by ovdb. See the Berkeley DB documentation for complete details for the version of
Berkeley DB that you're running.

RUNNING

       When starting the news system, rc.news will invoke ovdb_init.  ovdb_init must be run
       before using the database.  It performs the following tasks:

       •   Creates the database environment, if necessary.

       •   If the database is idle, it performs a normal recovery.  The recovery will remove
           stale locks, recreate the memory pool cache, and repair any damage caused by a system
           crash or improper shutdown.

       •   Starts the DB housekeeping processes (ovdb_monitor) if they're not already running.

       And when stopping INN, rc.news kills the ovdb_monitor processes after the other INN
       processes have been shut down.

DIAGNOSTICS

       Problems relating to ovdb are logged to news.err with "OVDB" in the error message.

       INN programs that use overview will fail to start up if the ovdb_monitor processes aren't
       running.  Be sure to run ovdb_init before running anything that accesses overview.

       Also, INN programs that use overview will fail to start up if the user running them is not
       the "news" user.

       If a program accessing the database crashes, or otherwise exits uncleanly, it might leave
       a stale lock in the database.  This lock could cause other processes to deadlock on that
       stale lock.  To fix this, shut down all news processes (using "kill -9" if necessary) and
       then restart.  ovdb_init should perform a recovery operation which will remove the locks
       and repair damage caused by killing the deadlocked processes.

FILES

       inn.conf
           The ovmethod and pathoverview parameters are relevant to ovdb.

       ovdb.conf
           Optional configuration file for tuning.  See CONFIGURATION above.

       pathoverview
           Directory where the database goes.  Berkeley DB calls it the 'DB_HOME' directory.

       pathoverview/DB_CONFIG
           Optional file to configure the layout of the database files.

       pathrun/ovdb.sem
           A file that gets locked by every process that is accessing the database.  This is used
           by ovdb_init to determine whether the database is active or quiescent.

       pathrun/ovdb_monitor.pid
           Contains the process ID of ovdb_monitor.

TO DO

       Implement a way to limit how many databases can be open at once (to reduce file descriptor
       usage); maybe using something similar to the cache code in ov3.c

HISTORY

       Written by Heath Kehoe <hakehoe@avalon.net> for InterNetNews

       $Id: ovdb.pod 9107 2010-09-14 20:23:07Z eagle $