# DbEnv::set_mp_mmapsize

`

###

    #include <db_cxx.h>

    int
    DbEnv::set_mp_mmapsize(size_t mp_mmapsize);

# Description

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, as available virtual memory is normally much larger than the local cache, and page faults are faster than page copying on many systems. However, in the presence of limited virtual memory it can cause resource starvation, and in the presence of large databases, it can result in immense process sizes.

Set 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 DbEnv::set_mp_mmapsize interface may only be used to configure Berkeley DB before the DbEnv::open interface is called.

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.

The 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. Configuration in the DB_CONFIG file silently overrules configuration done using the DbEnv::set_mp_mmapsize method.

# Errors

EINVAL
  An invalid flag value or parameter was specified.

  Called after DbEnv::open was called.

### Classes

DbEnv, DbMpoolFile

# See Also

DbEnv::set_mp_mmapsize, DbMpoolFile::close, DbMpoolFile::get, DbMpoolFile::open, DbMpoolFile::put, DbMpoolFile::set, DbMpoolFile::sync, DbEnv::memp_register, DbEnv::memp_stat, DbEnv::memp_sync and DbEnv::memp_trickle.`

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