What is the recurring performance problem you are seeing?
Pretty much exactly the symptoms described in
http://bugzilla.kernel.org/show_bug.cgi?id=7372 relating to read
starvation under heavy write IO causing sluggish system response.
I recently graphed the blocks in/blocks out from vmstat 1 for the
same test using each of the four IO schedulers (see the PDF attached
to the article below):
http://community.novacaster.com/showarticle.pl?id=7492
The test was:
dd if=/dev/sda of=/dev/null bs=1M count=4096 &; sleep 5; dd
if=/dev/zero of=./4G bs=1M count=4096 &
Despite appearances, interactive responsiveness subjectively felt
better using deadline than cfq - but this is obviously an atypical
workload and so now I'm focusing on finishing building the machine
completely so I can try profiling the more typical patterns of
activity that it'll experience when in use.
I find myself wondering whether the fact that the array looks like a
single SCSI disk to the OS means that cfq is able to perform better
in terms of interleaving reads and writes to the card but that some
side effect of its work is causing the responsiveness issue at the
same time. Pure speculation on my part - this is way outside my
experience.
I'm also looking into trying an Areca card instead (avoiding LSI
because they're cited as having the same issue in the bugzilla
mentioned above).
S.
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