Excessive IO wait with megaraid card 2006-12-04 - By Phil Bettinson
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> 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
BEng, RHCT Beng, RHCT
Swyddog Gweinyddu Systemau Systems Administrator
Isadran Cyfrifiaduron Computer Section
Llyfrgell Genedlaethol Cymru National Library of Wales
Aberystwyth, Aberystwyth
Ceredigion, Ceredigion
SY23 3BU SY23 3BU
Ff?n / Phone: 01970 632XX Ffacs / Fax: 01970 615XXX
EBost / Email: NOSPAM@(protected) We / Web: http://www.llgc.org.uk
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