R515/H700 high iowait

Pavel Mateja pavel at netsafe.cz
Wed Jul 18 03:43:12 CDT 2012

> Hi Guys,
> I was wondering if anyone has had any problems either on the R515 or
> with the H700 card where there are very high io wait times when multiple
> writers write to the same raid group.  Basically we first noticed this
> due to inconsistent buffered IO performance under heavy IO load.  We saw
> IO wait times as high as 6+ seconds.
> After that we started testing with fio.  What we noticed is that if
> writing to say a large raid0 with 7-8 drives we could achieve 800MB/s,
> but only with large (256MB) IOs using direct IO and a single "job" (ie
> fio process).  Simply increasing the number of jobs to 2 reduced
> performance to 80MB/s and dramatically increased IO wait times.  The
> controller has a BBU and the raid array was configured with adaptive
> readahead and writeback cache.
> One thought is that perhaps this is not a problem with the raid
> controller, but some kind of preemption issue with the io scheduler.  At
> once point early on I tried switching from cfq to deadline with little
> apparent change, though I have not yet done exhaustive tests.  I will
> try to record blktrace output soon to see if that provides any insight
> into what is going on.
> In the mean time, has anyone else seen anything like this?

what about filesystem? Ext3 with barrier enabled?
Pavel Mateja

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