On a busy mysql server MySQLTuner 1.2.0 always recommends to add query_cache_size no matter how I increase the value (I tried up to 512MB). On the other hand it warns that :
Increasing the query_cache size over 128M may reduce performance
Here are the last results:
>> MySQLTuner 1.2.0 - Major Hayden <major@mhtx.net>
>> Bug reports, feature requests, and downloads at http://mysqltuner.com/
>> Run with '--help' for additional options and output filtering-------- General Statistics --------------------------------------------------
[--] Skipped version check for MySQLTuner script
[OK] Currently running supported MySQL version 5.5.25-1~dotdeb.0-log
[OK] Operating on 64-bit architecture-------- Storage Engine Statistics -------------------------------------------
[--] Status: +Archive -BDB -Federated +InnoDB -ISAM -NDBCluster
[--] Data in InnoDB tables: 6G (Tables: 195)
[--] Data in PERFORMANCE_SCHEMA tables: 0B (Tables: 17)
[!!] Total fragmented tables: 51-------- Security Recommendations -------------------------------------------
[OK] All database users have passwords assigned-------- Performance Metrics -------------------------------------------------
[--] Up for: 1d 19h 17m 8s (254M q [1K qps], 5M conn, TX: 139B, RX: 32B)
[--] Reads / Writes: 89% / 11%
[--] Total buffers: 24.2G global + 92.2M per thread (1200 max threads)
[!!] Maximum possible memory usage: 132.2G (139% of installed RAM)
[OK] Slow queries: 0% (2K/254M)
[OK] Highest usage of available connections: 32% (391/1200)
[OK] Key buffer size / total MyISAM indexes: 128.0M/92.0K
[OK] Key buffer hit rate: 100.0% (8B cached / 0 reads)
[OK] Query cache efficiency: 79.9% (181M cached / 226M selects)
[!!] Query cache prunes per day: 1033203
[OK] Sorts requiring temporary tables: 0% (341 temp sorts / 4M sorts)
[OK] Temporary tables created on disk: 14% (760K on disk / 5M total)
[OK] Thread cache hit rate: 99% (676 created / 5M connections)
[OK] Table cache hit rate: 22% (1K open / 8K opened)
[OK] Open file limit used: 0% (49/13K)
[OK] Table locks acquired immediately: 99% (64M immediate / 64M locks)
[OK] InnoDB data size / buffer pool: 6.1G/19.5G-------- Recommendations -----------------------------------------------------
General recommendations:
Run OPTIMIZE TABLE to defragment tables for better performance
Reduce your overall MySQL memory footprint for system stability
Increasing the query_cache size over 128M may reduce performance
Variables to adjust:
*** MySQL's maximum memory usage is dangerously high ***
*** Add RAM before increasing MySQL buffer variables ***
query_cache_size (> 192M) [see warning above]
The server has 76GB ram and dual E5-2650. The load is usually below 2.
I appreciate your hints to interpret the recommendation and optimize the database configs.
MySQL Query Cache Sizing is a blog entry that you may find useful.
A high-level summary is that once the query_cache gets above a certain size then MySQL spends more time managing the cache than using the cache. Every write that affects a queries results then invalidates the results in the cache.
If your application is mostly reading the database then a large query cache size would be beneficial. If your application is write-heavy you would benefit from a lower cache or no cache at all.
Note that there are many other blog posts discussing large query_cache sizes but in the end they all say you have to actually test settings in your environment for the best setting.
Check more discussion of this question.
