最近公司的mysql服务器经常出现阻塞状态。动不动就重启，给用户访问带来了相当的不便。
首先说明一点，每天的访问量并不大每天才2W的访问量，按道理说，访问量再增加一倍这样的服务器也应该足以承受。可是我们的服务器为什么总是这样频频重启呢？经过mysqlreport显示mysql运行的状态如下：
<pre class="brush:php;toolbar:false"><pre class="brush:php;toolbar:false"><pre class="brush:php;toolbar:false"><pre class="brush:php;toolbar:false"><pre class="brush:php;toolbar:false"><pre class="brush:php;toolbar:false"><pre class="brush:php;toolbar:false"><pre class="brush:php;toolbar:false"><pre class="brush:php;toolbar:false"><pre class="brush:php;toolbar:false"><pre class="brush:php;toolbar:false"><pre class="brush:php;toolbar:false"><pre class="brush:php;toolbar:false"><font color="#006bad">MySQL 5.1.28-rc-log uptime 0 0:4:48 Thu Apr 30 14:04:58 2009<br><br>__ Key _________________________________________________________________<br>Buffer used 7.74M of 384.00M %Used: 2.02<br> Current 52.89M %Usage: 13.77<br>Write hit 0.00%<br>Read hit 91.97%<br><br>__ Questions ___________________________________________________________<br>Total 14.46k 50.2/s<br> DMS 9.24k 32.1/s %Total: 63.90<br> QC Hits 3.66k 12.7/s 25.33<br> Com_ 1.04k 3.6/s 7.19<br> COM_QUIT 517 1.8/s 3.58<br> -Unknown 1 0.0/s 0.01<br>Slow 1 s 4 0.0/s 0.03 %DMS: 0.04 Log: OFF<br>DMS 9.24k 32.1/s 63.90<br> SELECT 9.21k 32.0/s 63.75 99.75<br> UPDATE 23 0.1/s 0.16 0.25<br> REPLACE 0 0/s 0.00 0.00<br> DELETE 0 0/s 0.00 0.00<br> INSERT 0 0/s 0.00 0.00<br>Com_ 1.04k 50/s 44.4<br> set_option 523 1.8/s 3.62<br> change_db 510 50/s 44.4<br> show_proces 5 0.0/s 0.03<br><br>__ SELECT and Sort _____________________________________________________<br>Scan 49 0.2/s %SELECT: 0.53<br>Range 16 0.1/s 0.17<br>Full join 2 0.0/s 0.02<br>Range check 0 0/s 0.00<br>Full rng join 0 0/s 0.00<br>Sort scan 55 0.2/s<br>Sort range 475 1.6/s<br>Sort mrg pass 0 0/s<br><br>__ Query Cache _________________________________________________________<br>Memory usage 12.02M of 150.00M %Used: 8.01<br>Block Fragmnt 0.01%<br>Hits 3.66k 12.7/s<br>Inserts 9.14k 31.8/s<br>Insrt:Prune 9.14k:1 31.8/s<br>Hit:Insert 0.40:1<br><br>__ Table Locks _________________________________________________________<br>Waited 0 0/s %Total: 0.00<br>Immediate 10.49k 36.4/s<br><br>__ Tables ______________________________________________________________<br>Open 58 of 512 %Cache: 11.33<br>Opened 64 0.2/s<br><br>__ Connections _________________________________________________________<br>Max used 4 of 2000 %Max: 0.20<br>Total 519 1.8/s<br><br>__ Created Temp ________________________________________________________<br>Disk table 1 0.0/s<br>Table 184 0.6/s Size: 256.0M<br>File 5 0.0/s<br><br>__ Threads _____________________________________________________________<br>Running 2 of 2<br>Cached 2 of 150 %Hit: 99.23<br>Created 4 0.0/s<br>Slow 0 0/s<br><br>__ Aborted _____________________________________________________________<br>Clients 0 0/s<br>Connects 0 0/s<br><br>__ Bytes _______________________________________________________________<br>Sent 20.36M 70.7k/s<br>Received 1.37M 4.7k/s<br><br>__ InnoDB Buffer Pool __________________________________________________<br>Usage 304.00k of 8.00M %Used: 3.71<br>Read hit 84.42%<br>Pages<br> Free 493 %Total: 96.29<br> Data 19 3.71 %Drty: 0.00<br> Misc 0 0.00<br> Latched 0.00<br>Reads 77 0.3/s<br> From file 12 0.0/s 15.58<br> Ahead Rnd 1 0.0/s<br> Ahead Sql 0 0/s<br>Writes 0 0/s<br>Flushes 0 0/s<br>Wait Free 0 0/s<br><br>__ InnoDB Lock _________________________________________________________<br>Waits 0 0/s<br>Current 0<br>Time acquiring<br> Total 0 ms<br> Average 0 ms<br> Max 0 ms<br><br>__ InnoDB Data, Pages, Rows ____________________________________________<br>Data<br> Reads 25 0.1/s<br> Writes 3 0.0/s<br> fsync 3 0.0/s<br> Pending<br> Reads 0<br> Writes 0<br> fsync 0<br><br>Pages<br> Created 0 0/s<br> Read 19 0.1/s<br> Written 0 0/s<br><br>Rows<br> Deleted 0 0/s<br> Inserted 0 0/s<br> Read 0 0/s<br> Updated 0 0/s<br>大家可以看到在Com_中占的比例是相当大的，而一般情况下，这个值应该是3.0%左右。如果这个值过高的话，就说明，你的mysql处理请求中<br>做了一些无谓的工作，占用了大量的系统资源。其中我们看到尤为change_db的占用率比较高。这个值高了，说明我们执行的user database;命令相当多。<br>检查程序，原来我们在执行查询语句的时候应用了mysql_query_db("database","sql");这个语句在每次执行的时候都会自动调用use database;<br>所以我们应该将mysql_db_query 更改成mysql_query();再观察mysql的运行状态，ok。一切正常了。</font>
