mysiam表参数详解

http://imysql.cn/2008_09_27_deep_into_mysql_sort_buffer

http://my.oschina.net/realfighter/blog/364420

要想提高MySQL的性能，首先就是必须对mysql的配置参数进行了解，在了解了mysql的配置参数后，根据自己的项目需要以及运行环境，再做出相应的调整，那么以下这些参数是经过对mysql的官方配置参数说明、网上资料查询，以及自己的各种实验得出的个人结论。

1. skip-external-locking：跳过外部锁定。要明白这个参数，必须先了解external-locking（外部锁定，作用是为MYISAM数据表在多进程【多个服务公用同一个数据库目录】访问下锁定），大多数情况下，我们的mysql服务都是单进程服务的，从mysql官网上看，skip-external-locking参数默认情况下是ON的，

   mysql> show variables like ‘%skip%‘;
   +------------------------+-------+
   | Variable_name          | Value |
   +------------------------+-------+
   | skip_external_locking  | ON    |

   在配置文件[mysqld]下开启这个参数OK。

2. key_buffer_size = 256M：为MYISAM数据表开启供线程共享的索引缓存。我们的项目中数据表基本上用的是INNODB引擎，所以这个参数暂时不进行调整，有需要的可以参考http://database.51cto.com/art/201010/229939.htm
3. max_allowed_packet = 16M：服务端最大允许接收的数据包大小。在没有调整该配置项的时候，服务端默认是4M。当然这个参数和mysql（默认16M）和mysqldump（默认为24M，我已经调整为16M）中的数据包大小有关系，一般情况下1M就可以，官方建议如果使用了blog或者更大的字符串时进行该参数的调整，一般情况下，数据库会被初始化为net_buffer_length（最小1024byte，最大是1M，默认是16KB）的大小。
4. table_open_cache = 512：所有线程打开表的数目（默认设置大小为1000）。如果opened_tables很大并且不经常使用flush tables，官方建议我们增加该参数的大小。这个值并不是越大越好，需要根据实际情况下open_tables和opened_tables的综合进行调整，详细可见http://www.cnblogs.com/suredandan/p/4010931.html
5. sort_buffer_size = 512K：需要排序 会话 的缓存大小，是针对每一个connection的，这个值也不会越大越好，默认大小是256kb，过大的配置会消耗更多的内存。我个人还没有测试，可以查看http://bbs.chinaunix.net/thread-1805254-1-1.html
6. read_buffer_size = 512K：为需要全表扫描的MYISAM数据表线程指定缓存，也是针对每个connection的，这个参数暂时我也不需要太关注。

   Each thread that does a sequential scan for a MyISAM table allocates a buffer of this size (in bytes)
   for each table it scans. If you do many sequential scans, you might want to increase this value, which
   defaults to 131072. The value of this variable should be a multiple of 4KB. If it is set to a value that is not
   a multiple of 4KB, its value will be rounded down to the nearest multiple of 4KB.
   This option is also used in the following context for all search engines:
   • For caching the indexes in a temporary file (not a temporary table), when sorting rows for ORDER BY.
   • For bulk insert into partitions.
   • For caching results of nested queries.
   and in one other storage engine-specific way: to determine the memory block size for MEMORY tables.
   The maximum permissible setting for read_buffer_size is 2GB.
   For more information about memory use during different operations, see Section 8.11.4.1, “How MySQL
   Uses Memory”.

7. read_rnd_buffer_size = 1M：首先，该变量可以被任何存储引擎使用，当从一个已经排序的键值表中读取行时，会先从该缓冲区中获取而不再从磁盘上获取。默认为256K。

   This variable is used for reads from MyISAM tables, and, for any storage engine, for Multi-Range Read
   optimization.
   When reading rows from a MyISAM table in sorted order following a key-sorting operation, the rows are
   read through this buffer to avoid disk seeks. See Section 8.2.1.15, “ORDER BY Optimization”. Setting
   the variable to a large value can improve ORDER BY performance by a lot. However, this is a buffer
   Server System Variables
   627
   allocated for each client, so you should not set the global variable to a large value. Instead, change the
   session variable only from within those clients that need to run large queries.
   The maximum permissible setting for read_rnd_buffer_size is 2GB.

   另外可参见http://bbs.chinaunix.net/forum.php?mod=viewthread&tid=3642777&highlight=

8. thread_cache_size = 18：有多少线程供服务缓存使用。

   How many threads the server should cache for reuse. When a client disconnects, the client‘s threads
   are put in the cache if there are fewer than thread_cache_size threads there. Requests for threads
   are satisfied by reusing threads taken from the cache if possible, and only when the cache is empty is
   a new thread created. This variable can be increased to improve performance if you have a lot of new
   connections. Normally, this does not provide a notable performance improvement if you have a good
   thread implementation. However, if your server sees hundreds of connections per second you should
   normally set thread_cache_size high enough so that most new connections use cached threads. By
   examining the difference between the Connections and Threads_created status variables, you can
   see how efficient the thread cache is. For details, see Section 5.1.6, “Server Status Variables”.
   The default value is based on the following formula, capped to a limit of 100:
   8 + (max_connections / 100)
   This variable has no effect for the embedded server (libmysqld) and as of MySQL 5.7.2 is no longer
   visible within the embedded server.

9. query_cache_size= 8M：分配给查询缓存的内存大小。要配合query_cache_type使用，默认是不开启的。只从该参数的表面介绍来看，似乎值设置的越大，带来的效果会更好，但是请注意，查询缓存的工作原理，一个select语句过来后，数据库将查询结果缓存到cache中，等同样的select查询过来后，如果这段时间内该查询结果没有发生变化时，数据库将cache中将缓存结果返回，那么假如查询的相关数据表增删改特别多的话，数据表变更的这段时间内，要将cache失效，然后再更新数据，对于增删改来说，花费的时间就很多了，所以要有所权衡，这个参数我会在将来进行相关测试数据整理。

   By default, the query cache is
   disabled. This is achieved using a default value of 1M, with a default for query_cache_type of
   0. (To reduce overhead significantly if you set the size to 0, you should also start the server with
   query_cache_type=0.
   The permissible values are multiples of 1024; other values are rounded down to the nearest multiple.
   Note that query_cache_size bytes of memory are allocated even if query_cache_type is set to 0.
   See Section 8.9.3.3, “Query Cache Configuration”, for more information.
   The query cache needs a minimum size of about 40KB to allocate its structures. (The exact size
   depends on system architecture.) If you set the value of query_cache_size too small, a warning will
   occur, as described in Section 8.9.3.3, “Query Cache Configuration”.

10. query_cache_type = 1：1表示缓存所有查询，2表示缓存select sql_cache的查询，看如下内容
    

    0 or OFF Do not cache results in or retrieve results from the query cache. Note that
    this does not deallocate the query cache buffer. To do that, you should set
    query_cache_size to 0.
    1 or ON Cache all cacheable query results except for those that begin with SELECT
    SQL_NO_CACHE.
    2 or DEMAND Cache results only for cacheable queries that begin with SELECT SQL_CACHE.

11. set global max_connections = 500：注意这个是通过命令行设置最大连接数，不是配置在配置文件的，因为我在配置文件里面尝试修改，重启mysql服务后并没有起效，通过该参数设置以后，重启服务后，依然没有起效，如果有朋友知道这个原因的话，请告知。如果说你的项目使用的是spring的连接池的时候，我认为spring个connection就对应的这个连接。根据你项目的需求而定。

key_buffer_size：

Index blocks for MyISAM tables are buffered and are shared by all threads. key_buffer_size is the size of the buffer used for index blocks. The key buffer is also known as the key cache.