Information Schema INNODB_CMPMEM and INNODB_CMPMEM_RESET Tables

The INNODB_CMPMEM and INNODB_CMPMEM_RESET tables contain status information on compressed pages in the buffer pool (see XtraDB/InnoDB COMPRESSED format).

The PROCESS privilege is required to query this table.

These tables contain the following columns:

Column NameDescription
PAGE_SIZECompressed page size, in bytes. This value is unique in the table; other values are totals which refer to pages of this size.
BUFFER_POOL_INSTANCEID of the buffer pool instance.
PAGES_USEDNumber of pages of the size PAGE_SIZE which are currently in the buffer pool.
PAGES_FREENumber of pages of the size PAGE_SIZE which are currently free, and thus are available for allocation. This value represents the buffer pool's fragmentation. A totally unfragmented buffer pool has at most 1 free page.
RELOCATION_OPSHow many times a page of the size PAGE_SIZE has been relocated. This happens when data exceeds a page (because a row must be copied into a new page) and when two pages are merged (because their data shrunk and can now be contained in one page).
RELOCATION_TIMETime (in seconds) spent in relocation operations for pages of the size PAGE_SIZE. This column is reset when the INNODB_CMPMEM_RESET table is queried.

These tables can be used to measure the effectiveness of XtraDB/InnoDB table compression. When you have to decide a value for KEY_BLOCK_SIZE, you can create more than one version of the table (one for each candidate value) and run a realistic workload on them. Then, these tables can be used to see how the operations performed with different page sizes.

INNODB_CMPMEM and INNODB_CMPMEM_RESET have the same columns and always contain the same values, but when INNODB_CMPMEM_RESET is queried, the RELOCATION_TIME column from both the tables are cleared. INNODB_CMPMEM_RESET can be used, for example, if a script periodically logs the performances of compression in the last period of time. INNODB_CMPMEM can be used to see the cumulated statistics.

Example

SELECT * FROM information_schema.INNODB_CMPMEM\G
********************** 1. row **********************
            page_size: 1024
 buffer_pool_instance: 0
           pages_used: 0
           pages_free: 0
      reloacation_ops: 0
      relocation_time: 0

See Also

Other tables that can be used to monitor XtraDB/InnoDB compressed tables:

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