The tables INNODB_CMPMEM and INNODB_CMPMEM_RESET contain status information on the compressed pages that reside in the buffer pool. Please consult Chapter 3, InnoDB Data Compression for further information on compressed tables and the use of the buffer pool. The tables INNODB_CMP and INNODB_CMP_RESET should provide more useful statistics on compression.

The InnoDB storage engine uses a so-called “buddy allocator” system to manage memory allocated to pages of various sizes, from 1KB to 16KB. Each row of the two tables described here corresponds to a single page size, except for rows with PAGE_SIZE<1024, which are implementation artifacts. The smallest blocks (PAGE_SIZE=64 or PAGE_SIZE=128, depending on the server platform) are used for keeping track of compressed pages for which no uncompressed page has been allocated in the buffer pool. Other blocks of PAGE_SIZE<1024 should never be allocated (PAGES_USED=0). They exist because the memory allocator allocates smaller blocks by splitting bigger ones into halves.

These two tables have identical contents, but reading from INNODB_CMPMEM_RESET resets the statistics on relocation operations. For example, if every 60 minutes you archived the output of INNODB_CMPMEM_RESET, it would show the hourly statistics. If you never read INNODB_CMPMEM_RESET and monitored the output of INNODB_CMPMEM instead, it would show the cumulated statistics since InnoDB was started.