High IOWAIT on 3ware Controllers

First, you'll need to be running a 2.6 kernel or a RHEL backported kernel with support for iowait statistics otherwise you'll iowait will stay at 0.0%.

I've tried a million and one things to resolve the matter, but the fact remains that iowait is ridiculously high with 3ware cards on Linux.

During normal operations iowait stay between 0% and 20%. However do some continous IO such as tarring a directory up .e.g tar cf joeblog.tar joeblog/ and watch IO hover near 100% on all CPUs for the duration of the operation (top -d1). On a 2.4 system this was literally starving mySQL and other services of resources, eventually mySQL would throw too many connections since it was unable to process the queries quick enough, they build up whilst starved. We upgraded to 2.6 and things have improved since the scheduler is better, but the high iowait remain and the extra latency is still present.

Has anyone had success with 3ware RAID cards on Linux and has actually monitored the iowait?

 

 

 

 

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