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SubjectRe: Drive performance bottleneck
Ian Godin wrote:
>
> I am trying to get very fast disk drive performance and I am seeing
> some interesting bottlenecks. We are trying to get 800 MB/sec or more
> (yes, that is megabytes per second). We are currently using PCI-Express
> with a 16 drive raid card (SATA drives). We have achieved that speed,
> but only through the SG (SCSI generic) driver. This is running the
> stock 2.6.10 kernel. And the device is not mounted as a file system. I
> also set the read ahead size on the device to 16KB (which speeds things
> up a lot):

I was trying to reproduce but got distracted by this:
(use page down, if you just want to see the odd result)

puma:/tmp/dd# sg_map
/dev/sg0 /dev/sda
/dev/sg1 /dev/sdb
/dev/sg2 /dev/scd0
/dev/sg3 /dev/sdc
puma:/tmp/dd# time sg_dd if=/dev/sg1 of=/tmp/dd/sg1 bs=64k count=1000
Reducing read to 64 blocks per loop
1000+0 records in
1000+0 records out

real 0m0.187s
user 0m0.001s
sys 0m0.141s
puma:/tmp/dd# time dd if=/dev/sdb of=/tmp/dd/sdb bs=64k count=1000
1000+0 records in
1000+0 records out
65536000 bytes transferred in 1.203468 seconds (54455956 bytes/sec)

real 0m1.219s
user 0m0.001s
sys 0m0.138s
puma:/tmp/dd# ls -l
total 128000
-rw-r--r-- 1 root root 65536000 Feb 3 17:16 sdb
-rw-r--r-- 1 root root 65536000 Feb 3 17:16 sg1
puma:/tmp/dd# md5sum *
ec31224970ddd3fb74501c8e68327e7b sdb
60d4689227d60e6122f1ffe0ec1b2ad7 sg1
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

See? dd from sdb is not the same as sg1! Is this supposed to happen?

About the 900MB/sec:
This same sg1 (= sdb, which is a single hitachi sata hdd) performes like
this:

puma:/tmp/dd# time sg_dd if=/dev/sg1 of=/dev/null bs=64k count=1000000
time=1
Reducing read to 64 blocks per loop
time to transfer data was 69.784784 secs, 939.12 MB/sec
1000000+0 records in
1000000+0 records out

real 1m9.787s
user 0m0.063s
sys 0m58.115s

I can assure you that this drive can't do more than 60MB/sec sustained.

My only conclusion is that sg (or sg_dd) is broken? ;)

Peace,
Nuno Silva
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