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 InnoDB COMPRESSED format).

The PROCESS privilege is required to query this table.

These tables contain the following columns:

Column Name Description
PAGE_SIZE Compressed page size, in bytes. This value is unique in the table; other values are totals which refer to pages of this size.
BUFFER_POOL_INSTANCE Buffer Pool identifier. From MariaDB 10.5.1 returns a value of 0, since multiple InnoDB buffer pool instances has been removed.
PAGES_USED Number of pages of the size PAGE_SIZE which are currently in the buffer pool.
PAGES_FREE Number 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_OPS How 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_TIME Time (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 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 InnoDB compressed tables:

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https://mariadb.com/kb/en/information-schema-innodb_cmpmem-and-innodb_cmpmem_reset-tables/