Excessive IO wait with megaraid card 2006-12-06 - By Phil Bettinson
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>From: "Chris Wornell" <CWornell@(protected)>
>Subject: RE: Excessive IO wait with megaraid card
>
>Reading through this thread I had a question about how to determine if
>IO is the bottleneck or not. How was it determined that it was the IO
>setup and not the RAM or CPU? Just curious for troubleshooting >purposes.
>
>Thanks,
>
>Chris Wornell
>Network Administrator, Information Technology
>Peerless Systems Corporation
>http://www.peerless.com
>office: 310.727.5723
>fax: 310.727.5715
>mailto:cwornell@(protected)
In my case, Chris, it was pretty damn obvious =). Whenever someone ran
"tar" if you typed "top" the io wait was listed at 100%. Not exactly an
elegeant solution, but a solution nontheless.
There are, of course, several tools out there that can help look for odd
bottlenecks that might not be as obvious as mine was. As a result of
this problem we have Sysstat installed on the machines in question. The
website is pretty helpful, at least enough to get you going
(http://perso.orange.fr/sebastien.godard/index.html). I note, as an
interesting point for the list, that Sysstat 7 now does I/O stats on
NFS. But I digress.
It's fairly simple to install (up2date sysstat), and gives you a timed
readout of some values on your system at various points throughout the day.
00:00:01 CPU %user %nice %system %iowait %idle
00:10:01 all 0.10 0.00 0.12 0.14 99.64
00:20:01 all 0.08 0.00 0.12 0.05 99.75
00:30:01 all 0.08 0.00 0.12 0.02 99.79
00:40:01 all 0.07 0.00 0.11 0.03 99.80
00:50:01 all 0.09 0.00 0.11 0.03 99.77
01:00:01 all 0.08 0.00 0.09 0.02 99.80
01:10:01 all 0.10 0.00 0.13 0.13 99.64
01:20:01 all 0.06 0.00 0.09 0.03 99.82
01:30:01 all 0.02 0.00 0.07 0.02 99.89
(a sample output using sar)
One note, however, is if you are installing on a 64bit machine. The
binaries are in /usr/lib64/sa/sa1 (as opposed to /usr/lib/sa/sa1 as
detailed on the website). I know it appears obvious, but it's detailed
here as it didn't occour to me, and I got an inbox full of cron errors.
There are probably a myriad of tools out there, and a myriad ways of
manipulating Sysstat &c to get more accurate results. It did what we
needed, however.
Hope that helps,
Phil
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Phil Bettinson
BEng, RHCT Beng, RHCT
Swyddog Gweinyddu Systemau Systems Administrator
Isadran Cyfrifiaduron Computer Section
Llyfrgell Genedlaethol Cymru National Library of Wales
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