我在Ubuntu 18.04上有MySQL 5.7.23。这是一个VPS，有2个vCores和8吉布RAM。服务器正在托管两个WordPress网站。每个网站都有一个7.5 MB的数据库。所以总共有15MB的数据库。
我试着用mysqltuner优化数据库。它每次都推荐我做同样的事情：
Variables to adjust:
tmp_table_size (> 64M)
max_heap_table_size (> 64M)
我从16M开始，然后我被建议增加到32M，现在它建议我增加到64M或更多。但为什么呢？我所有数据库的总大小约为15米。
我有足够的内存，可用内存大约为6,5 GiB，所以我可以为MySQL使用更多的内存。
mysqltuner的完整日志是
root@web:~# perl mysqltuner.pl
>> MySQLTuner 1.7.10 - Major Hayden <[email protected]>
>> Bug reports, feature requests, and downloads at http://mysqltuner.com/
>> Run with '--help' for additional options and output filtering
[--] Skipped version check for MySQLTuner script
[OK] Logged in using credentials from Debian maintenance account.
[OK] Currently running supported MySQL version 5.7.23-0ubuntu0.18.04.1
[OK] Operating on 64-bit architecture
-------- Log file Recommendations ---------------------------------------------- --------------------
[--] Log file: /var/log/mysql/error.log(0B)
[OK] Log file /var/log/mysql/error.log exists
[OK] Log file /var/log/mysql/error.log is readable.
[!!] Log file /var/log/mysql/error.log is empty
[OK] Log file /var/log/mysql/error.log is smaller than 32 Mb
[OK] /var/log/mysql/error.log doesn't contain any warning.
[OK] /var/log/mysql/error.log doesn't contain any error.
[--] 0 start(s) detected in /var/log/mysql/error.log
[--] 0 shutdown(s) detected in /var/log/mysql/error.log
-------- Storage Engine Statistics --------------------------------------------- --------------------
[--] Status: +ARCHIVE +BLACKHOLE +CSV -FEDERATED +InnoDB +MEMORY +MRG_MYISAM +My ISAM +PERFORMANCE_SCHEMA
[--] Data in InnoDB tables: 14.2M (Tables: 45)
[OK] Total fragmented tables: 0
-------- Security Recommendations ---------------------------------------------- --------------------
[OK] There are no anonymous accounts for any database users
[OK] All database users have passwords assigned
[--] Bug #80860 MySQL 5.7: Avoid testing password when validate_password is acti vated
-------- CVE Security Recommendations ------------------------------------------ --------------------
[--] Skipped due to --cvefile option undefined
-------- Performance Metrics --------------------------------------------------- --------------------
[--] Up for: 1d 2h 43m 42s (84K q [0.879 qps], 1K conn, TX: 441M, RX: 12M)
[--] Reads / Writes: 98% / 2%
[--] Binary logging is disabled
[--] Physical Memory : 7.8G
[--] Max MySQL memory : 384.4M
[--] Other process memory: 252.8M
[--] Total buffers: 224.0M global + 1.1M per thread (151 max threads)
[--] P_S Max memory usage: 72B
[--] Galera GCache Max memory usage: 0B
[OK] Maximum reached memory usage: 228.3M (2.86% of installed RAM)
[OK] Maximum possible memory usage: 384.4M (4.82% of installed RAM)
[OK] Overall possible memory usage with other process is compatible with memory available
[OK] Slow queries: 0% (0/84K)
[OK] Highest usage of available connections: 2% (4/151)
[OK] Aborted connections: 0.00% (0/1561)
[OK] Query cache is disabled by default due to mutex contention on multiprocesso r machines.
[OK] Sorts requiring temporary tables: 0% (0 temp sorts / 12K sorts)
[OK] No joins without indexes
[!!] Temporary tables created on disk: 44% (3K on disk / 7K total)
[OK] Thread cache hit rate: 99% (4 created / 1K connections)
[OK] Table cache hit rate: 77% (1K open / 1K opened)
[OK] Open file limit used: 1% (56/5K)
[OK] Table locks acquired immediately: 100% (202 immediate / 202 locks)
-------- Performance schema ---------------------------------------------------- --------------------
[--] Memory used by P_S: 72B
[--] Sys schema is installed.
-------- ThreadPool Metrics ---------------------------------------------------- --------------------
[--] ThreadPool stat is disabled.
-------- MyISAM Metrics -------------------------------------------------------- --------------------
[!!] Key buffer used: 18.3% (3M used / 16M cache)
[OK] Key buffer size / total MyISAM indexes: 16.0M/43.0K
[OK] Read Key buffer hit rate: 96.3% (190 cached / 7 reads)
-------- InnoDB Metrics -------------------------------------------------------- --------------------
[--] InnoDB is enabled.
[--] InnoDB Thread Concurrency: 0
[OK] InnoDB File per table is activated
[OK] InnoDB buffer pool / data size: 128.0M/14.2M
[OK] Ratio InnoDB log file size / InnoDB Buffer pool size: 16.0M * 2/128.0M shou ld be equal 25%
[OK] InnoDB buffer pool instances: 1
[--] Number of InnoDB Buffer Pool Chunk : 1 for 1 Buffer Pool Instance(s)
[OK] Innodb_buffer_pool_size aligned with Innodb_buffer_pool_chunk_size & Innodb _buffer_pool_instances
[OK] InnoDB Read buffer efficiency: 99.96% (2124360 hits/ 2125302 total)
[!!] InnoDB Write Log efficiency: 83.91% (9286 hits/ 11067 total)
[OK] InnoDB log waits: 0.00% (0 waits / 1781 writes)
-------- AriaDB Metrics -------------------------------------------------------- --------------------
[--] AriaDB is disabled.
-------- TokuDB Metrics -------------------------------------------------------- --------------------
[--] TokuDB is disabled.
-------- XtraDB Metrics -------------------------------------------------------- --------------------
[--] XtraDB is disabled.
-------- RocksDB Metrics ------------------------------------------------------- --------------------
[--] RocksDB is disabled.
-------- Spider Metrics -------------------------------------------------------- --------------------
[--] Spider is disabled.
-------- Connect Metrics ------------------------------------------------------- --------------------
[--] Connect is disabled.
-------- Galera Metrics -------------------------------------------------------- --------------------
[--] Galera is disabled.
-------- Replication Metrics --------------------------------------------------- --------------------
[--] Galera Synchronous replication: NO
[--] No replication slave(s) for this server.
[--] Binlog format: ROW
[--] XA support enabled: ON
[--] Semi synchronous replication Master: Not Activated
[--] Semi synchronous replication Slave: Not Activated
[--] This is a standalone server
-------- Recommendations ------------------------------------------------------- --------------------
General recommendations:
When making adjustments, make tmp_table_size/max_heap_table_size equal
Reduce your SELECT DISTINCT queries which have no LIMIT clause
Variables to adjust:
tmp_table_size (> 64M)
max_heap_table_size (> 64M)
最佳答案
你应该接受mysqltuner的建议。它的建议不是在了解数据库或查询的情况下提出的，有时只是基于不完整信息的猜测。
增加tmp_table_大小的建议可能是基于读取指示在磁盘上创建了某些临时表的状态值。
mysql> show global status like 'created%tables';
+-------------------------+-------+
| Variable_name | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 35 |
| Created_tmp_tables | 274 |
+-------------------------+-------+
“啊哈！”mysqltuner说：“13%的临时表都在磁盘上。如果我们允许更大的临时表留在RAM中，它们可能会留在RAM中。两倍的tmp_table_size！“
但有些临时表总是在磁盘上创建的。
https://dev.mysql.com/doc/refman/5.7/en/internal-temporary-tables.html说：
有些查询条件阻止使用内存中的临时表，
在这种情况下，服务器将使用磁盘上的表：
表中存在BLOB或文本列。这包括
用户定义的变量具有字符串值，因为它们被处理
作为BLOB列或文本列，具体取决于其值是否为二进制
或非二进制字符串。
任何长度大于512的字符串列的存在
（二进制字符串的字节，非二进制字符串的字符）
如果使用UNION或UNION ALL，请选择list。
SHOW COLUMNS和descripe语句使用BLOB作为某些
列，因此用于结果的临时表是磁盘上的
桌子。
mysqltuner如何区分由于太大而进入磁盘的临时表与由于上述条件之一而进入磁盘的临时表之间的区别？
它分辨不出区别。它必须读取您的查询日志并分析每个查询，但它不会这样做。它只知道磁盘上有多少临时表，而不知道为什么。
Mysqltuner只假设一个临时表存储在磁盘上，因为它大于tmp_table_size，并且它假设增加tmp_table_size会有一些好处。