Promise PDC20267 ATA RAID, poor write performance

Frode Nordahl frode at nordahl.net
Sat Sep 11 07:05:43 PDT 2004


On Sep 11, 2004, at 12:49, Søren Schmidt wrote:
>> Doing simple tests show very poor write performance:
>> (reboot)
>> # dd if=/dev/zero of=fill bs=1m count=2000
>> 2000+0 records in
>> 2000+0 records out
>> 2097152000 bytes transferred in 588.383886 secs (3564258 bytes/sec)
>> (reboot)
>> # dd if=fill of=/dev/null bs=1m
>> 2000+0 records in
>> 2000+0 records out
>> 2097152000 bytes transferred in 68.492015 secs (30618927 bytes/sec)
>
> The only thing I could think of was if you have WC (write cache) 
> turned off for some reason which yields this result here:

When you say WC, you mean hw.ata.wc, right? It was enabled.

I tried to disable it, and things actually got faster.

# dd if=/dev/zero of=fill bs=1m count=2000
2000+0 records in
2000+0 records out
2097152000 bytes transferred in 346.250049 secs (6056756 bytes/sec)

Maybe there is some timing issues here somewhere, or just flaky cables 
:-/

I'll test with other cables and disks and get back.

atacontrol cap shows that the disk has write cache as well as read 
ahead enabled, is it possible to tune this using FreeBSD tools?

Mvh,
Frode

> pizzabox# dd if=/dev/zero of=fill bs=1m count=2000
> 2000+0 records in
> 2000+0 records out
> 2097152000 bytes transferred in 428.874993 secs (4889891 bytes/sec)
>
> That looks more like your results, but they are also within what I 
> would expect when not using WC..
>
>
> -Søren
>



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