Re: weird throughput on write to SATA disk

[kenneth johansson <ken@kenjo.org>]

On Mon, 2008-10-13 at 14:34 -0700, Grant Grundler wrote:
> On Mon, Oct 13, 2008 at 2:22 PM, kenneth johansson <ken@kenjo.org> 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@kenjo.org> 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"?

I changed to use O_DIRECT and it's much more consistent now. 69-75 with
74 about 95% of the time. the disk is supposed to have 105 115 sustained
data rate.