MySQL 8.4 Release Notes
Example 17.1 Using the Compression Information Schema Tables
The following is sample output from a database that contains
compressed tables (see Section 17.9, “InnoDB Table and Page Compression”,
INNODB_CMP,
INNODB_CMP_PER_INDEX, and
INNODB_CMPMEM).
The following table shows the contents of
INFORMATION_SCHEMA.INNODB_CMP
under a light workload.
The only compressed page size that the buffer pool contains is
8K. Compressing or uncompressing pages has consumed less than
a second since the time the statistics were reset, because the
columns COMPRESS_TIME and
UNCOMPRESS_TIME are zero.
| page size | compress ops | compress ops ok | compress time | uncompress ops | uncompress time |
|---|---|---|---|---|---|
| 1024 | 0 | 0 | 0 | 0 | 0 |
| 2048 | 0 | 0 | 0 | 0 | 0 |
| 4096 | 0 | 0 | 0 | 0 | 0 |
| 8192 | 1048 | 921 | 0 | 61 | 0 |
| 16384 | 0 | 0 | 0 | 0 | 0 |
According to INNODB_CMPMEM, there
are 6169 compressed 8KB pages in the
buffer pool. The only
other allocated block size is 64 bytes. The smallest
PAGE_SIZE in
INNODB_CMPMEM is used for block
descriptors of those compressed pages for which no
uncompressed page exists in the buffer pool. We see that there
are 5910 such pages. Indirectly, we see that 259 (6169-5910)
compressed pages also exist in the buffer pool in uncompressed
form.
The following table shows the contents of
INFORMATION_SCHEMA.INNODB_CMPMEM
under a light workload.
Some memory is unusable due to fragmentation of the memory
allocator for compressed pages:
SUM(PAGE_SIZE*PAGES_FREE)=6784. This is
because small memory allocation requests are fulfilled by
splitting bigger blocks, starting from the 16K blocks that are
allocated from the main buffer pool, using the buddy
allocation system. The fragmentation is this low because some
allocated blocks have been relocated (copied) to form bigger
adjacent free blocks. This copying of
SUM(PAGE_SIZE*RELOCATION_OPS) bytes has
consumed less than a second
(SUM(RELOCATION_TIME)=0).