DbEnv::set_mp_mmapsize()
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## DbEnv::set_mp_mmapsize()

    #include <db_cxx.h>

    int
    DbEnv::set_mp_mmapsize(size_t mp_mmapsize);

Files that are opened read-only in the cache (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::set_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 configured 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 a database environment, not only operations performed using the specified DbEnv handle.

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 a DbException exception, encapsulating one of the following non-zero errors, or return one of the following non-zero errors:

#### EINVAL

An invalid flag value or parameter was specified.

### Class

DbEnv, DbMpoolFile

### See Also

Memory Pools and Related Methods

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