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]
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Phil Bettinson
Sent: Monday, December 04, 2006 4:50 AM
To: [email protected]
Subject: Re: Excessive IO wait with megaraid card

 > We have a (4) sun v40z's here to run some oracle databases. They have 6
 > > HD's in a hardware raid off of a LSI Logic MegaRAID 413Y (as cleaned
 > > from /proc/scsi/megaraid/1)
 > >
 > > Performance is fine doing it's main job (serving about 6 Lightly used
 > > Oracle databases).
 > >
 > > We are finding, however, that should you attempt to run a large tar 
job
 > > on the system, all four of the quad opteron processors end up stuck 
with
 > > 100% IO wait (when tarring say about a 4gb oracle database).
 > >
 > > We've tried using the megaraid2 driver which did improve things a bit,
 > > the system is a little more responsive during the tar, but not by much.
 > > Does anyone have any ideas?
 > >
 > > Either how we narrow down the problem/get more info, or (ideally ;)) a
 > > fix to our rather annoying problem? It seems to appear on all four
 > > boxses, using RHEL U7 (installed with U0, but up2dated), and using
 > > redhat-made drivers.

 >Do you have writeback-cache enabled on the logical drives? And do find 
 >out which controller you have, 413Y is just the firmware version  :)
 >LSI has software on their site to manage the controller from linux.
 >
 >In my experience, there is something significantly wrong with Megaraid
 >when WB-cache is not enabled on the controller. I have numbers for
 >sequental writing on RHEL4, which uses a different driver though.
 >
 >320-1 2-disk raid1
 >       controller WB-off write 15mb/s
 >       controller WB-on write  34mb/s
 >       read                    49mb/s
 >
 >These numbers are simply 'time sh -c "dd if=/dev/zero of=foo bs=256k
 >count=32k ; sync" and count the transfer rate from that.

The card is a Megaraid 320-2X. I have to say that I echo your findings. 
Having enabled write-back on the card, performance improves dramatically.

For now at least, I think our problem is "Fixed". Thanks for the help on 
this one.

Phil


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			Phil Bettinson

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Swyddog Gweinyddu Systemau	Systems Administrator

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