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<title>Berkeley DB: DbEnv::set_cachesize</title>
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<h1>DbEnv::set_cachesize</h1>
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#include <db_cxx.h>
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DbEnv::set_cachesize(u_int32_t gbytes, u_int32_t bytes, int ncache);
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<p>Set the size of the shared memory buffer pool -- that is, the cache --
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to <b>gbytes</b> gigabytes plus <b>bytes</b>. The cache should be
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the size of the normal working data set of the application, with some
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small amount of additional memory for unusual situations. (Note: the
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working set is not the same as the number of pages accessed
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simultaneously, and should be quite a bit larger!)
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<p>The default cache size is 256KB, and may not be specified as less than
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20KB. Any cache size less than 500MB is automatically increased by 25%
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to account for buffer pool overhead; cache sizes larger than 500MB are
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used as specified. The current maximum size of a single cache is 4GB.
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For information on tuning the Berkeley DB cache size, see
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<a href="../ref/am_conf/cachesize.html">Selecting a cache size</a>.
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<p>It is possible to specify caches to Berkeley DB that are large enough so that
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they cannot be allocated contiguously on some architectures. For
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example, some releases of Solaris limit the amount of memory that may
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be allocated contiguously by a process. If <b>ncache</b> is 0 or 1,
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the cache will be allocated contiguously in memory. If it is greater
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than 1, the cache will be broken up into <b>ncache</b> equally sized,
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separate pieces of memory.
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<p>The DbEnv::set_cachesize method configures a database environment, not only operations
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performed using the specified <a href="../api_cxx/env_class.html">DbEnv</a> handle.
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<p>The DbEnv::set_cachesize interface may not be called after the <a href="../api_cxx/env_open.html">DbEnv::open</a>
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If the database environment already exists when
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<a href="../api_cxx/env_open.html">DbEnv::open</a> is called, the information specified to DbEnv::set_cachesize
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<p>The DbEnv::set_cachesize method either returns a non-zero error value or throws an exception that
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encapsulates a non-zero error value on failure, and returns 0 on success.
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<p>The database environment's cache size may also be set using the environment's
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<b>DB_CONFIG</b> file. The syntax of the entry in that file is a
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single line with the string "set_cachesize", one or more whitespace characters,
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and the three arguments specified to this interface, separated by whitespace
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characters, for example, "set_cachesize 1 500 2". Because the <b>DB_CONFIG</b> file is read when the database
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environment is opened, it will silently overrule configuration done
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<p>The DbEnv::set_cachesize method may fail and throw an exception or return a non-zero error for the following conditions:
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<p><dt>EINVAL<dd>An invalid flag value or parameter was specified.
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<p>The specified cache size was impossibly small.
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<a href="../api_cxx/env_open.html">DbEnv::open</a>
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<p>The DbEnv::set_cachesize method may fail and throw an exception or return a non-zero error for errors specified for other Berkeley DB and C library or system methods.
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If a catastrophic error has occurred, the DbEnv::set_cachesize method may fail and
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either return <a href="../ref/program/errorret.html#DB_RUNRECOVERY">DB_RUNRECOVERY</a> or throw a
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<a href="../api_cxx/runrec_class.html">DbRunRecoveryException</a>,
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in which case all subsequent Berkeley DB calls will fail in the same way.
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<a href="../api_cxx/env_class.html">DbEnv</a>
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<a href="../api_cxx/env_list.html">Database Environments and Related Methods</a>
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