Db::set_cachesize
Db::set_cachesize |
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#include <db_cxx.h>int Db::set_cachesize(u_int32_t gbytes, u_int32_t bytes, int ncache);
Set the size of the shared memory buffer pool -- that is, the cache -- to gbytes gigabytes plus bytes. 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 should be quite a bit 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. For information on tuning the Berkeley DB cache size, see Selecting a cache size.
It is possible to specify caches to Berkeley DB that are large enough so that 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.
Because databases opened within Berkeley DB environments use the cache specified to the environment, it is an error to attempt to set a cache in a database created within an environment.
The Db::set_cachesize interface may be used only to configure Berkeley DB before the Db::open interface is called.
The Db::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.
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 three arguments specified to this interface, separated by whitespace characters, for example, "set_cachesize 1 500 2". Because the DB_CONFIG file is read when the database environment is opened, it will silently overrule configuration done before that time.
The Db::set_cachesize method may fail and throw an exception or return a non-zero error for the following conditions:
Called after Db::open was called.
The specified cache size was impossibly small.
The Db::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. If a catastrophic error has occurred, the Db::set_cachesize method may fail and either return DB_RUNRECOVERY or throw an exception encapsulating DB_RUNRECOVERY, in which case all subsequent Berkeley DB calls will fail in the same way.
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