OVDB(5)



OVDB(5)                   InterNetNews Documentation                   OVDB(5)

NAME
       ovdb - Overview storage method for INN

DESCRIPTION
       The ovdb overview 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).

       The ovdb overview method 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>
       and is needed to build the ovdb backend.

UPGRADING
       There are several versions of the ovdb storage method:

       o Version 1, the initial version shipped with INN 2.3.0 up to
         INN 2.3.5.

       o Version 2, with improved performance, since INN 2.4.0.

       o Version 3, corresponding to version 2 with compression enabled,
         starting with INN 2.5.0.

       If you have a database created with a previous version of ovdb, your
       database will need to be upgraded using ovdb_init.  See the
       ovdb_init(8) man page for upgrade instructions, as well as the
       COMPRESSION section below.

       Note that when the Berkeley DB library is updated to a newer version,
       the ovdb database also needs being upgraded.

INSTALLATION
       If the Berkeley DB library is found at configure time, INN will be
       built with Berkeley DB support unless the --without-bdb flag is
       explicitly passed to configure.  By default, configure will search for
       Berkeley DB in standard locations; 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 the ovdb overview method, 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.  Also, make sure the directory is owned by the news user.

       Other parameters for configuring ovdb are in the ovdb.conf
       configuration file.  The following parameters can be set in that file:

       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.

       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 nnrpd processes when shutting down).  Default is 8000
           (KB), which is adequate for small to medium-sized servers.  Large
           servers will probably need at least 20000 (KB).

       ncache
           Number of regions across which to split the cache.  The region size
           is equal to cachesize divided by ncache.  Default is 1 for ncache,
           that is to say the cache will be allocated contiguously in memory.

       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 (> 2 GB) 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 information.  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 the behaviour of expireover.  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 1.  This parameter
           will probably be removed in the future.

       readserver
           When the readserver parameter is set to false, 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 set to true, the nnrpd processes
           will access overview via a helper server (ovdb_server -- which is
           started by ovdb_init).  All ovdb reads will then be funnelled
           through a single process with a cleaner interface to the underlying
           Berkeley DB database.  This will result in cleaner shutdowns for
           the database, improving stability and avoiding deadlocks, timing
           issues and corrupted databases.  That's why you should try to set
           this parameter to true if you are experiencing any instability in
           the ovdb overview method.

           Default value is true.

       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 an
           ovdb_server.  Default is 0, which means an unlimited number of
           connections is allowed.

COMPRESSION
       The ovdb storage method has 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 with zlib support), 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 INN was not built with zlib
       support).  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
       (pathoverview in inn.conf) 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/ovX.  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
       nnrpd processes).

       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 the ovdb_init
       program.  See the ovdb_init(8) man page for information about the tasks
       it performs.  ovdb_init must be run before using the database.

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

       pathetc/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 legacy ov3.c file.

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

       $Id: ovdb.pod 10241 2018-02-04 15:38:19Z iulius $

SEE ALSO
       inn.conf(5), innd(8), makehistory(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>).

INN 2.6.4                         2018-02-05                           OVDB(5)

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