RE: Excessive IO wait with megaraid card

 >From: "Chris Wornell" <CWornell-KWjJJj/cdYZWk0Htik3J/[email 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-KWjJJj/cdYZWk0Htik3J/[email 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

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