poor read performance

Ling C. Ho ling at fnal.gov
Sat Jun 30 05:18:22 UTC 2007


Did you see any difference when different block size is used (for 
example, dd with bs=64k or 128k)? Try also change the read-ahead cache. 
blockdev --getra /dev/sdd to see what is the current value, and blockdev 
--setra 8192 /dev/sdd to change it. 8192 is a good number that has been 
working well for me for the similar size setup.


Glen Dosey wrote:
> I am seeing what seems to be a notable limit on read performance of an
> ext3 filesystem. If anyone could offer some insight it would be helpful.
> Background:
> 12 x 500G SATA disks in a Hardware RAID enclosure connected via 2Gb/s FC
> to a 4 x 2.6 Ghz system with 4GB ram running RHEL4.5. Initially the
> enclosure was configured RAID5 10+1 parity, although I've also tried
> RAID 50 and currently RAID 0. I've varied chunk sizes from 64-256K. 
> Problem:
> No matter what I do I cannot get the ext3 read performance above
> ~90MB/s. Under virtually every configuration listed above the write
> performance is greater than the read performance. I've run a large
> number of Bonnie++ and IOzone tests, but for the sake of simplicity in
> this email I'll just refer to simple dd's with /dev/zero.
> Details:
> Under the current RAID0 setup I see the following when dd'ing.
> DD 4G from /dev/zero to /dev/sdd disk (no filesystem) & sync
> 28 seconds
> DD 4G from /dev/sdd to /dev/null 32 seconds
> DD 4G to ext3 on /dev/sdd & sync 32 seconds
> DD 4G from ext3 file to /dev/null 48 seconds.
> I've been watching the port usage on the FC switch and it verifies what
> I am seeing, Writes max out near 2Gb/s but reads hit some artificial
> limit around 90 MB/s and never ever exceed it with the filesystem,
> regardless of they underlying RAID configuration. Without a filesystem
> the reads are atleast 50% faster, and it can be seen on the FC switch
> graphs as well.
> Any help or thoughts would be appreciated.
> Thanks,
> ~Glen
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