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
The DbEnv::set_mp_mmapsize interface may be called at any time during the life of
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.
The DbEnv::set_mp_mmapsize method may fail and throw an exception or return a non-zero error for the following conditions:
- 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
in which case all subsequent Berkeley DB calls will fail in the same way.
Memory Pools and Related Methods
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