On Wed, Mar 4, 2009 at 2:10 AM, Peter Rabbitson <rabbit+list@xxxxxxxxx> wrote:
> No takers? Can I provide more info/measurements/whatever? I would
> really like to get to the bottom of this, pretty frustrating.
>
> Peter Rabbitson wrote:
>> Hello,
>>
>> I have a sil 3124 PCI-X SATA card with 4 drives attached to it.
>> When I read simultaneously from all 4 drives, I get the expected
>> ~220MB/s combined read speed, limited by the presence of a 32bit
>> card on the same bus (entire bus runs on 66mhz, mix of 64 and 32
>> bit cards). When I write to the drives however I only get half of
>> that ~106MB/s combined. Can someone shed some light?
>>
>> All measurements are made on an idle system in single-user mode
>>
>>
>> Combined read:
>> root@Arzamas:~/tr# for d in sde sdb sdc sdd ; do dd if="/dev/${d}2" of=/dev/null count=500 bs=1M & done
>> [1] 30243
>> [2] 30244
>> [3] 30245
>> [4] 30246
>> root@Arzamas:~/tr# 500+0 records in
>> 500+0 records out
>> 524288000 bytes (524 MB) copied, 9.23146 s, 56.8 MB/s
>> 500+0 records in
>> 500+0 records out
>> 524288000 bytes (524 MB) copied, 9.3649 s, 56.0 MB/s
>> 500+0 records in
>> 500+0 records out
>> 524288000 bytes (524 MB) copied, 9.59152 s, 54.7 MB/s
>> 500+0 records in
>> 500+0 records out
>> 524288000 bytes (524 MB) copied, 9.60045 s, 54.6 MB/s
>>
>>
>> Combined write:
>> root@Arzamas:~/tr# for d in sde sdb sdc sdd ; do dd of="/dev/${d}2" if=/dev/zero count=500 bs=1M & done
>> [1] 30239
>> [2] 30240
>> [3] 30241
>> [4] 30242
>> root@Arzamas:~/tr# 500+0 records in
>> 500+0 records out
>> 524288000 bytes (524 MB) copied, 19.4508 s, 27.0 MB/s
>> 500+0 records in
>> 500+0 records out
>> 524288000 bytes (524 MB) copied, 19.6314 s, 26.7 MB/s
>> 500+0 records in
>> 500+0 records out
>> 524288000 bytes (524 MB) copied, 19.7663 s, 26.5 MB/s
>> 500+0 records in
>> 500+0 records out
>> 524288000 bytes (524 MB) copied, 19.7598 s, 26.5 MB/s
>>
>>
>> Individual read (making sure disks are good):
>> root@Arzamas:~/tr# for d in sde sdb sdc sdd ; do dd if="/dev/${d}2" of=/dev/null count=500 bs=1M ; done
>> 500+0 records in
>> 500+0 records out
>> 524288000 bytes (524 MB) copied, 5.00963 s, 105 MB/s
>> 500+0 records in
>> 500+0 records out
>> 524288000 bytes (524 MB) copied, 5.15979 s, 102 MB/s
>> 500+0 records in
>> 500+0 records out
>> 524288000 bytes (524 MB) copied, 5.19258 s, 101 MB/s
>> 500+0 records in
>> 500+0 records out
>> 524288000 bytes (524 MB) copied, 5.24153 s, 100 MB/s
>> root@Arzamas:~/tr#
>>
>>
>> Individual write:
>> root@Arzamas:~/tr# for d in sde sdb sdc sdd ; do dd of="/dev/${d}2" if=/dev/zero count=500 bs=1M ; done
<snip>
Did you verify /dev/zero is not the problem.
It has been known to have throughput bugs that limit its ability to
generate zeros. Strange I know.
I always do a quick dd if=/dev/zero of=/dev/null bs=1m count=10000
type test to make sure I have good functionality in /dev/zero and
/dev/null. (Never heard of an issue with /dev/null, but I have to
send those zeros somewhere.)
Greg
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