Experiencing disk latency problems from your PERC controller?

[Cian Cullinan]

If so, then try turning off "Patrol Read" either from the PERC bios,
or via megapr (http://ftp1.us.dell.com/scsi-raid/MegaPR_Linux_A02.tar.gz).

Comparing some representative iostat output from a PowerEdge 2800,
Perc 4e/Di, 4x15,000rpm SCSI disks in RAID10:(apologies for the
formatting):

With PR:
avg-cpu:  %user   %nice    %sys %iowait   %idle
           2.02    0.00    5.33   17.40   75.24

Device:    rrqm/s wrqm/s   r/s   w/s  rsec/s  wsec/s    rkB/s    wkB/s
avgrq-sz avgqu-sz   await  svctm  %util
sda          1.37   1.24 419.19  1.67 17716.62   23.27  8858.31
11.63    42.15     2.83    6.72   1.34  56.47


Without PR:
avg-cpu:  %user   %nice    %sys %iowait   %idle
           3.13    0.00    6.21    9.30   81.36

Device:    rrqm/s wrqm/s   r/s   w/s  rsec/s  wsec/s    rkB/s    wkB/s
avgrq-sz avgqu-sz   await  svctm  %util
sda          1.57   2.44 368.23  2.01 18979.53   35.59  9489.77
17.79    51.36     0.72    1.94   1.23  45.40

A couple of things to note
1)iowait drops from 17% to 10%.
2)the queue length drops from about 3 to 1.
3)and most importantly for me, await - the average time (in
milliseconds) for I/O requests  issued to the device to be served -
falls from about 7 to 2.

That last one becomes very important if you are running applications
where data latency is important. In my case video streaming. I was
seeing data starvation in the video server, but only at certain times
(when PR was running and the video wasn't cached in RAM), and more
with certain videos than others (the higher the video bitrate the more
likely for the server not to get the data when it needed it).

I found a few other posts in the archives referencing bandwidth issues
when PR is running, but nothing specific to latency. Hopefully someone
else will find this post before spending days tearing their hair out
playing with kernel vm tunables, RAID parameters and I/O schedulers
:)