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.


-- 

  Jussi


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