| Bug #64170 | why does buf_page_get_gen confirm the page checksum for compressed pages | ||
|---|---|---|---|
| Submitted: | 30 Jan 2012 18:46 | Modified: | 2 Aug 2012 19:51 |
| Reporter: | Mark Callaghan | Email Updates: | |
| Status: | Closed | Impact on me: | |
| Category: | MySQL Server: InnoDB Plugin storage engine | Severity: | S5 (Performance) |
| Version: | OS: | Any | |
| Assigned to: | Marko Mäkelä | CPU Architecture: | Any |
| Tags: | checksum, compressed, innodb | ||
[30 Jan 2012 19:16]
Mark Callaghan
It looks like there might an extra confirmation of the checksum on the write side too in buf_flush_block_write_low when the checksum was set in buf_flush_init_for_writing and buf_LRU_free_block
case BUF_BLOCK_ZIP_DIRTY:
frame = bpage->zip.data;
if (UNIV_LIKELY(srv_use_checksums)) {
ut_a(mach_read_from_4(frame + FIL_PAGE_SPACE_OR_CHKSUM)
== page_zip_calc_checksum(frame, zip_size));
}
[6 Mar 2012 15:16]
Mark Callaghan
After removing other bottlenecks and getting a server into the state where very few pages are on the unzip_LRU, I get 5% more QPS by removing the extra checksum calls.
[9 Mar 2012 20:36]
Mark Callaghan
Performance results are at https://www.facebook.com/note.php?note_id=10150580052310933
[19 Jun 2012 5:40]
MySQL Verification Team
Related: Bug 13397030 - DO NOT CALC UNNECESSARY CHECKSUMS WHEN VALIDATING A PAGE That was fixed in 5.6.4.
[2 Aug 2012 19:51]
John Russell
Added to changelog for 5.6.7, 5.7.0: This fix removes redundant checksum validation on InnoDB pages. The checksum was being verified both when a compressed page was read from disk and when it was uncompressed. Now the verification is only performed when the page is read from disk.
Description: Why does buf_page_get_gen call buf_zip_decompress(..., srv_use_checksums)? The page checksum for a compressed page is confirmed when the page is read from disk via: buf_page_read_low buf_page_io_complete buf_page_is_corrupted page_zip_calc_checksum The checksum confirmation done on the buf_page_get_gen seems to be an extra check and not always a good use of CPU. How to repeat: Look for calls to page_zip_calc_checksum Suggested fix: Disable or make optional via yet another my.cnf parameter checksumming to be done via buf_page_get_gen