Re: weird throughput on write to SATA disk

[Grant Grundler]

On Mon, Oct 13, 2008 at 2:22 PM, kenneth johansson <ken@xxxxxxxxx> wrote:
> On Mon, 2008-10-13 at 14:07 -0700, Grant Grundler wrote:
>> On Mon, Oct 13, 2008 at 12:19 PM, kenneth johansson <ken@xxxxxxxxx> wrote:
>> > When writing data to /dev/sdb (that is the whole disk and no filesystem)
>> > it starts out ok around 100MB/sec but soon end up doing considerably
>> > worse. For prolonged times lasting several minutes it hovers in the
>> > 20-30MB/sec.
>>
>> Is the application using "O_DIRECT"?
>
> no just open then write no fancy stuff
>
>> If not, could be issue with how VM writeback is (not) working.
>
> the access pattern is the simplest possible. Is there any knobs I could
> try to adjust.
>
>> Can you reproduce this using fio or even "dd"?
>> (See git://git.kernel.dk/fio)
>
> dd is having the same effect.

Can you try "dd oflag=direct if=/dev/null of=/dev/sdb bs=64k"?

Or try with "sgp_dd"? It's part of sg3-utils package.

thanks,
grant
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/