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Berkeley DB Reference Guide:
  Memory Pool Subsystem

### Berkeley DB and the memory pool

The Memory Pool subsystem is the general-purpose shared memory buffer pool used by Berkeley DB. This module is useful outside of the Berkeley DB package for processes that require page-oriented, shared and cached file access.

A memory pool is a memory cache shared among any number of threads of control. The DB_INIT_MPOOL flag to the DB_ENV->open method opens and optionally creates a memory pool. When that pool is no longer in use, it should be closed using the DB_ENV->close method.

The DB_ENV->memp_fcreate method returns a DB_MPOOLFILE handle on an underlying file within the memory pool. The file may be opened using the DB_MPOOLFILE->open method. The DB_MPOOLFILE->get method is used to retrieve pages from files in the pool. All retrieved pages must be subsequently returned using the DB_MPOOLFILE->put method. At the time pages are returned, they may be marked dirty, which causes them to be written to the underlying file before being discarded from the pool. If there is insufficient room to bring a new page in the pool, a page is selected to be discarded from the pool using a least-recently-used algorithm. Pages in files may also be explicitly marked clean or dirty using the DB_MPOOLFILE->set method. All dirty pages in the pool from the file may be flushed using the DB_MPOOLFILE->sync method. When the file handle is no longer in use, it should be closed using the DB_MPOOLFILE->close method.

There are additional configuration interfaces that apply when opening a new file in the memory pool:

- The DB_MPOOLFILE->set_clear_len method specifies the number of bytes to clear when creating a new page in the memory pool.
- The DB_MPOOLFILE->set_fileid method specifies a unique ID associated with the file.
- The DB_MPOOLFILE->set_ftype method specifies the type of file for the purposes of page input and output processing.
- The DB_MPOOLFILE->set_lsn_offset method specifies the byte offset of each page's log sequence number (DB_LSN) for the purposes of transaction checkpoints.
- The DB_MPOOLFILE->set_pgcookie method specifies an application provided argument for the purposes of page input and output processing.

There are additional interfaces for the memory pool as a whole:

- It is possible to gradually flush buffers from the pool in order to maintain a consistent percentage of clean buffers in the pool using the DB_ENV->memp_trickle method.
- Because special-purpose processing may be necessary when pages are read or written (for example, compression or endian conversion), the DB_ENV->memp_register function allows applications to specify automatic input and output processing in these cases.
- The db_stat utility uses the DB_ENV->memp_stat method to display statistics about the efficiency of the pool.
- All dirty pages in the pool may be flushed using the DB_ENV->memp_sync method. In addition, DB_ENV->memp_sync takes an argument that is specific to database systems, and which allows the memory pool to be flushed up to a specified log sequence number (DB_LSN).
- The entire pool may be discarded using the DB_ENV->remove method.

Memory Pools and Related Methods | Description
--- | ---
DB_ENV->set_cachesize | Set the environment cache size
DB_ENV->set_mp_mmapsize | Set maximum mapped-in database file size
DB_ENV->memp_register | Register input/output functions for a file in a memory pool
DB_ENV->memp_stat | Return memory pool statistics
DB_ENV->memp_sync | Flush pages from a memory pool
DB_ENV->memp_trickle | Trickle flush pages from a memory pool
DB_ENV->memp_fcreate | Open a file in a memory pool
DB_MPOOLFILE->close | Close a file in a memory pool
DB_MPOOLFILE->get | Get page from a file in a memory pool
DB_MPOOLFILE->open | Open a file in a memory pool
DB_MPOOLFILE->put | Return a page to a memory pool
DB_MPOOLFILE->set | Set memory pool page status
DB_MPOOLFILE->sync | Flush pages from a file in a memory pool
DB_MPOOLFILE->set_clear_len | Set file page bytes to be cleared
DB_MPOOLFILE->set_fileid | Set file unique identifier
DB_MPOOLFILE->set_ftype | Set file type
DB_MPOOLFILE->set_lsn_offset | Set file log-sequence-number offset
DB_MPOOLFILE->set_pgcookie | Set file cookie for pgin/pgout

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