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OVDB(5)			  InterNetNews Documentation		       OVDB(5)

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/database-technologies/berkeleydb/overview/index.html
       <http://www.oracle.com/technetwork/database/database-
       technologies/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-bdb when
       running the configure script.  By default, configure will search for
       Berkeley DB in default search paths; there will be a message in the
       configure output indicating the pathname that will be used.

       You can override this pathname by adding a path to the option, for
       instance --with-bdb=/usr/BerkeleyDB.4.4.	 This directory is expected to
       have subdirectories include and lib (lib32 and lib64 are also checked),
       containing respectively db.h, and the library itself.  In case non-
       standard paths to the Berkeley DB libraries are used, one or both of
       the options --with-bdb-include and --with-bdb-lib can be given to
       configure with a path.

       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
       parameter 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 9593 2013-12-27 21:16:09Z iulius $

SEE ALSO
       inn.conf(5), innd(8), nnrpd(8), ovdb_init(8), ovdb_monitor(8),
       ovdb_stat(8)

       Berkeley DB documentation:  in the docs directory of the Berkeley DB
       source distribution, or on the Oracle Berkeley DB web page
       (http://www.oracle.com/technetwork/database/database-technologies/berkeleydb/overview/index.html
       <http://www.oracle.com/technetwork/database/database-
       technologies/berkeleydb/overview/index.html>).

INN 2.6.0			  2015-09-12			       OVDB(5)
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