# DbEnv::set_mp_mmapsize |

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    #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 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.

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 method configures operations performed using the specified DbEnv handle, not all operations performed on the underlying database environment.

The DbEnv::set_mp_mmapsize interface 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.

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.

# Errors

The DbEnv::set_mp_mmapsize method may fail and throw an exception or return a non-zero error for the following conditions:

EINVAL
  An invalid flag value or parameter was specified.

  Called after DbEnv::open was called.

The DbEnv::set_mp_mmapsize 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 DbEnv::set_mp_mmapsize method may fail and either return DB_RUNRECOVERY or throw a DbRunRecoveryException, in which case all subsequent Berkeley DB calls will fail in the same way.

# Class

DbEnv, DbMpoolFile

# See Also

Memory Pools and Related Methods`

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