iowait


How iowait impacts System performance
System or any server’s performance heavily depends on IOPS. And if MySql is running on the System, it requires disk of high IOPS. Lots of select queries running simultaneously cause high iowait if disk dont have high IOPS. High iowait means cpu is waiting for I/O access. That increase the more cpu utilization and more processes in queue. That mean high cpuload on server.

We can simple find the process which causing high I/O using below command:

iotop
we can use below to check current iowait on server.

iostat

Also mpstat command can we be used for %iowait.

[user1@linux-vm ~]$ mpstat
Linux 2.6.18-194.el5 (linux-vm) 12/21/2013

07:34:11 PM CPU %user %nice %sys %iowait %irq %soft %steal %idle intr/s
07:34:11 PM all 13.96 0.01 2.19 2.98 0.10 0.31 0.00 80.45 519.11
Another command that helps us analysing bottleneck in system is vmstat

[user1@linux-vm ~]$ vmstat
procs ———–memory———- —swap– —–io—- –system– —–cpu——
r b swpd free buff cache si so bi bo in cs us sy id wa st
1 3 8 22280400 6788 1033804 0 0 212 1732 0 0 14 3 80 3 0

 

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