20110806

IO

 

 写道

Disk latency is around 13ms, but it depends on the quality and rotational speed of the hard drive. RAM latency is around 83 nanoseconds.

 

 

性能指标

• IOPS (input/output operations per-second)
• iowait

 

 写道

If your I/O wait percentage is greater than (1/# of CPU cores) then your CPUs are waiting a significant amount of time for the disk subsystem to catch up.

In the output above, I/O wait is 12.1%. This server has 8 cores (via cat /proc/cpuinfo). This is very close to (1/8 cores = 0.125).

 iops的计算公式

 

I/O Operations Per-Sec =	number of disks * Average I/O Operations on 1 disk per-sec
% of read workload + (Raid Factor * % of write workload)

 

iops和磁盘的 rotational speed关系很大

Rotational Speed (rpm)	IOPS
5400	50-80
7200	75-100
10k	125-150
15k	175-210

 

 

 

相关命令：

sar

•   sar -d -p will report activity for each block device and pretty print the device name.

lsof

 

参考

http://blog.scoutapp.com/articles/2011/02/10/understanding-disk-i-o-when-should-you-be-worried

http://www.cmdln.org/2010/04/22/analyzing-io-performance-in-linux/

http://www.performancewiki.com/diskio-tuning.html 

http://www.moeding.net/archives/2-The-problem-with-wait-IO.html

 

Others:

http://blog.scoutapp.com/articles/2009/07/31/understanding-load-averages

http://mirrors.psu.ac.th/pub/bash-howto/shell-tutorial.pdf