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Causes of high wait i/o

Posted on 2002-07-18
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I've got this voice application running on AIX.  ever so often you just have to restart that application since everything comes to almost a halt.  I did run just the basic sar.  The only thing i noticed was the wait i/o was 97%  and pct idle was 3%.  How do i determine what the causes are? This may be a seperate question, but how do i set up sar to give the best info plus i think it is supposed to give reports.
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Question by:xoxomos
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gandalf94305 earned 100 total points
ID: 7163591
Voice applications may cause significant data traffic to your disks. A large fraction of io waits indicates one of two things:

- your disks are too slow for the volume, i.e., data arrival rates are higher than rates of storing things on disk

- there is some exclusive locking going one in your application and the disks aren't really the problem.

If the system gradually slows down, this would add a bias towards the first possibility. I suggest you have a look at disk transfer rates (iostat) and at the same time at page faults and swapping rates (vmstat). Processes may appear to be in io wait when they are in fact in page wait.

Well-written voice applications will use asynchronous io to avoid getting into io wait situations where not writing is the bottleneck but rather synchronously waiting for the results to be delivered.

High io wait rates may also occur if your application writes data in very small chunks to disk (check the number of syscalls per second, I suspect that to be rather high, in situations where your system slows down, the number of interrupts is probably also high).

And finally, the source may not be the disks but the network or some slow serial line you connect to. In that case, check data transfer rates over these communication links.

So, these are my ideas... let's see if this is of any help.

Cheers,
--gandalf.
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by:xoxomos
ID: 7172393
Thanks.
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