PE2950, LSI SAS -> SATA very slow
adam.nielsen at uq.edu.au
Tue May 4 19:57:39 CDT 2010
>>> 10MB/sec does seem extremely poor. we've had read slowness when Linux block
>>> device read-ahead was too small. Once properly configured, performance on
>>> RAID-5 went from 150MB/sec to 500MB/sec, so the difference is dramatic. we
>>> usually tune read-ahead (in Linux) buffers starting from 8MB to 32MB and
>>> benchmark to see what works best. but your problem may be elsewhere... we've
>>> never seen 10MB/sec...
>> Have you got any pointers to where this can be adjusted? It seems
>> (according to Google) there are countless methods and patches, most of
>> which are for rather old kernel versions.
> man blockdev
> blockdev --setra blocks /dev/sda
> It is part of util-linux-ng .
Thanks for the hint! This didn't make much difference for us, but
that's probably because the array is RAID-1, so the array readahead
already matched the individual disk readahead. Strangely enough
increasing the readahead actually reduced the raw speed according to
hdparm -t, which I wouldn't have thought would make a difference.
What did make a huge difference is setting the NCQ depth to 1 (instead
of the default 64) as Tim suggested. This brought raw read speed up
from ~50MB/sec to almost 90MB/sec!
Haven't yet had the machine under a large load so will be interesting to
see how it copes, but now I really want the updated firmware from Dell!
More information about the Linux-PowerEdge