key_buffer_size设置注意事项_MySQL
1.单个key_buffer的大小不能超过4G，如果设置超过4G，就有可能遇到下面3个bug:
http://bugs.mysql.com/bug.php?id=29446
http://bugs.mysql.com/bug.php?id=29419
http://bugs.mysql.com/bug.php?id=5731
2.建议key_buffer设置为物理内存的1/4(针对MyISAM引擎)，甚至是物理内存的30%~40%，如果key_buffer_size设置太大，系统就会频繁的换页，降低系统性能。因为MySQL使用操作系统的缓存来缓存数据，所以我们得为系统留够足够的内存；在很多情况下数据要比索引大得多。
3.如果机器性能优越，可以设置多个key_buffer,分别让不同的key_buffer来缓存专门的索引
上面只是对"新手"来说的，我们还可以更深入地优化key_buffer_size，使用"show status"来查看"Key_read_requests, Key_reads, Key_write_requests 以及Key_writes "，以调整到更适合你的应用的大小，Key_reads/Key_read_requests的大小正常情况下得小于0.01
参考资料：
http://dev.mysql.com/doc/refman/5.0/en/server-system-variables.html#sysvar_key_buffer_size
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