Very bad FreeBSD SCSI RAID5 write speed performance

[Andreas Braukmann]

--On Donnerstag, 13. November 2003 14:29 Uhr +0100 Martin Blapp 
<mb at imp.ch> wrote:
>> On Thu, Nov 13, 2003 at 06:38:53AM -0500, Gary Stanley wrote:
>> > root at 64:[/tmp/blah]>dd if=/dev/zero of=/tmp/blah/blah
>> > 89472+0 records in
>> > 89471+0 records out
>> > 45809152 bytes transferred in 8.546312 secs (5360108 bytes/sec)

ServerWorks Chipset, PIII 1 GHz, Adaptec 5400S, Raid-5 over four
spindles (Fujitsu U160, 10kUPM disks, write cache off) on one
channel, -current from end of september:

roach:[/home/ab] # dd if=/dev/zero of=./test bs=128k count=20000
20000+0 records in
20000+0 records out
2621440000 bytes transferred in 66.062844 secs (39681004 bytes/sec)

Another system: Tyan K8S, Adaptec 2200S, RAID-5 over five spindles
(Fujitsu U320, 10kUPM disks, write-cache off) across both channels,
-current from end of september (i386):

opti#  dd if=/dev/zero of=./test bs=128k count=20000
20000+0 records in
20000+0 records out
2621440000 bytes transferred in 57.648042 secs (45473184 bytes/sec)


> Exactly what we get here: ~ 6MB per second with RAID5 and the IPS
> driver. The Adaptec 3200s is similar.

I'll update the Opteron box to a recent kernel and will check
again.


> A single disk has writes to the 53MB to the raw slice. I don't
> know why 5 equal disks on 2 busses are that slow.

What about the write cache settings on the disks and on the
raid-controllers? Both of my controllers are equipped with
battery backup modules for the controller cache and are set
to "write behind caching".
Turning off the controller's write cache leads to raid-5
write performance less than 10 MByte/s.

-Andreas