matthias platzer wrote:
hello,
i saw this thread a bit late, but I had /am having the exact same issues on a dual-2-core-cpu opteron box with a 9550SX. (Centos 5 x86_64) What I did to work around them was basically switching to XFS for everything except / (3ware say their cards are fast, but only on XFS) AND using very low nr_requests for every blockdev on the 3ware card. (like 32 or 64). That will limit the iowait times for the cpus and make the 3ware-drives respond faster (see yourself with iostat -x -m 1 while benchmarking). If you can, you could also try _not_ putting the system disks on the 3ware card, because additionally the 3ware driver/card gives writes priority. People suggested the unresponsive system behaviour is because the cpu hanging in iowait for writing and then reading the system binaries won't happen till the writes are done, so the binaries should be on another io path.
All this seem to be symptoms of a very complex issue consisting of kernel bugs/bad drivers/... and they seem to be worst on a AMD/3ware Combination. here is another link: http://bugzilla.kernel.org/show_bug.cgi?id=7372
Actually the real-real fix was to use the 'deadline' or 'noop' scheduler with this card as the default 'cfq' scheduler was designed to work with a single drive and not a multiple drive RAID, so it acts as a govenor on the amount of IO that a single process can send to the device and when you do multiple overlapping IOs performance decreases instead of increases.
Personnally I always use 'deadline' as my scheduler of choice.
Of course if your drivers are broken that will always negatively impact performance.
-Ross
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