Re: weird throughput on write to SATA disk

[Grant Grundler]

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"?
If not, could be issue with how VM writeback is (not) working.

Can you reproduce this using fio or even "dd"?
(See git://git.kernel.dk/fio)

grant

>
> This happens on two different SATA disk one samsung and one seagate and
> on both 2.6.26 and 2.6.27 kernel. The disk are connected to the external
> esata connector. Controller is 00:1f.2 SATA controller: Intel
> Corporation 82801HB (ICH8) 4 port SATA AHCI Controller (rev 02)
>
> What could be the reason for this ?? reading is no problem.
>
>
>
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