[linux-lvm] Re: IO scheduler, queue depth, nr_requests
Kevin P. Fleming
kpfleming at backtobasicsmgmt.com
Tue Feb 17 01:46:02 UTC 2004
Jens Axboe wrote:
>>By fiddling about today I just found that changing
>>/sys/block/sda/queue/nr_requests from 128 to something above
>>the queue depth of the 3ware controller (256 doesn't work,
>>384 and up do) also fixes the problem.
OK, I have run three sets of bonnie++ tests on my system. This was using
the 2.6-bk kernel as of about midnight GMT of 2004-02-17.
System is single P4HT 2.8GHz (SMP kernel), 1GB RAM (4GB highmem
enabled), Intel 865G chipset, 3ware 8506-8 with six Seagate 160GB
Barracuda SATA disks in a single RAID-5. bonnie++ tests run on an XFS
filesystem (made with default mkfs.xfs parameters using a recent
xfstools package) over a 200GB LVM2 volume (dm-linear).
bonnie++ -r 512 -s 40960 -f -b
Anticipatory scheduler, nr_requests=128 (default)
Seq. Write 53772
Rand. Write 15557
Seq. Read 47730
Rand. Seek 146.5
Anticipatory scheduler, nr_requests=384
Seq. Write 53843
Rand. Write 17595
Seq. Read 44663
Rand. Seek 143.1
Deadline scheduler, nr_requests=384
Seq. Write 54973
Rand. Write 18897
Seq. Read 41476
Rand. Seek 227.3 (!)
Miquel's suggestion has a definite positive effect, except on sequential
reads. This system still doesn't produce anywhere near the throughput
I'd expect though, and still doesn't come close to 2.4 numbers either
(but those are somewhat bogus, as they rely on setting extremely large
readahead settings). There may be some sort of hardware problem that is
limiting me to ~60MB/s, although with 2.4 I was able to get sequential
read numbers around 95MB/s, so I think the hardware is OK.
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