#include <db_cxx.h>

int
DbEnv::set_cachesize(u_int32_t gbytes, u_int32_t bytes, int ncache);

int
DbEnv::get_cachesize(u_int32_t *gbytesp, u_int32_t *bytesp, int *ncachep);

Description: DbEnv::set_cachesize

Set the size of the shared memory buffer pool -- that is, the cache. The cache should be the size of the normal working data set of the application, with some small amount of additional memory for unusual situations. (Note: the working set is not the same as the number of pages accessed simultaneously, and is usually much larger.)

The default cache size is 256KB, and may not be specified as less than 20KB. Any cache size less than 500MB is automatically increased by 25% to account for buffer pool overhead; cache sizes larger than 500MB are used as specified. The current maximum size of a single cache is 4GB. (All sizes are in powers-of-two, that is, 256KB is 2^32 not 256,000.) For information on tuning the Berkeley DB cache size, see Selecting a cache size.

It is possible to specify caches to Berkeley DB larger than 4GB and/or large enough they cannot be allocated contiguously on some architectures. For example, some releases of Solaris limit the amount of memory that may be allocated contiguously by a process. If ncache is 0 or 1, the cache will be allocated contiguously in memory. If it is greater than 1, the cache will be broken up into ncache equally sized, separate pieces of memory.

The database environment's cache size may also be set using the environment's DB_CONFIG file. The syntax of the entry in that file is a single line with the string "set_cachesize", one or more whitespace characters, and the cache size specified in three parts: the gigabytes of cache, the additional bytes of cache, and the number of caches, also separated by whitespace characters. For example, "set_cachesize 2 524288000 3" would create a 2.5GB logical cache, split between three physical caches. Because the DB_CONFIG file is read when the database environment is opened, it will silently overrule configuration done before that time.

The DbEnv::set_cachesize method configures a database environment, not only operations performed using the specified DbEnv handle.

The DbEnv::set_cachesize method may not be called after the DbEnv::open method is called. If the database environment already exists when DbEnv::open is called, the information specified to DbEnv::set_cachesize will be ignored.

The DbEnv::set_cachesize method either returns a non-zero error value or throws an exception that encapsulates a non-zero error value on failure, and returns 0 on success.

Parameters

bytes

The size of the cache is set to gbytes gigabytes plus bytes.

gbytes

The size of the cache is set to gbytes gigabytes plus bytes.

ncache

The ncache parameter is the number of caches to create.

Errors

The DbEnv::set_cachesize method may fail and throw DbException, encapsulating one of the following non-zero errors, or return one of the following non-zero errors:

EINVAL

If the specified cache size was impossibly small; the method was called after DbEnv::open was called; or if an invalid flag value or parameter was specified.

Description: DbEnv::get_cachesize

The DbEnv::get_cachesize method returns the current size and composition of the cache.

The DbEnv::get_cachesize method may be called at any time during the life of the application.

The DbEnv::get_cachesize method either returns a non-zero error value or throws an exception that encapsulates a non-zero error value on failure, and returns 0 on success.

Parameters

bytesp

The bytesp parameter references memory into which the additional bytes of memory in the cache is copied.

gbytesp

The gbytesp parameter references memory into which the gigabytes of memory in the cache is copied.

ncachep

The ncachep parameter references memory into which the number of caches is copied.

Class

See Also