Berkeley DB: DbEnv::set_mp_mmapsize
Description: Berkeley DB: An embedded database programmatic toolkit.
Keywords: embedded,database,programmatic,toolkit,b+tree,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++

### DbEnv::set_mp_mmapsize |

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    #include <db_cxx.h>

    int
    DbEnv::set_mp_mmapsize(size_t mp_mmapsize);

    int
    DbEnv::get_mp_mmapsize(size_t *mp_mmapsizep);

### Description: DbEnv::set_mp_mmapsize

Files that are opened read-only in the pool (and that satisfy a few other criteria) are, by default, mapped into the process address space instead of being copied into the local cache. This can result in better-than-usual performance because available virtual memory is normally much larger than the local cache, and page faults are faster than page copying on many systems. However, it can cause resource starvation in the presence of limited virtual memory, and it can result in immense process sizes in the presence of large databases.

The DbEnv::get_mp_mmapsize method sets the maximum file size, in bytes, for a file to be mapped into the process address space. If no value is specified, it defaults to 10MB.

The database environment's maximum mapped file 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_mp_mmapsize", one or more whitespace characters, and the size in bytes. 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_mp_mmapsize method configures operations performed using the specified DbEnv handle, not all operations performed on the underlying database environment.

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

The DbEnv::set_mp_mmapsize 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

mp_mmapsize
  The mp_mmapsize parameter is the maximum file size, in bytes, for a file to be mapped into the process address space.

### Errors

The DbEnv::set_mp_mmapsize 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 method was called after DbEnv::open was called; or if an invalid flag value or parameter was specified.

### Description: DbEnv::get_mp_mmapsize

The DbEnv::get_mp_mmapsize method returns the maximum file map size.

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

The DbEnv::get_mp_mmapsize 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

mp_mmapsizep
  The DbEnv::get_mp_mmapsize method returns the maximum file map size in mp_mmapsizep.

### Class

DbEnv, DbMpoolFile

### See Also

Memory Pools and Related Methods`

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