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SubjectRe: [PATCH] Give kjournald a IOPRIO_CLASS_RT io priority
On Fri, 3 Oct 2008 09:58:49 +1000
Dave Chinner <david@fromorbit.com> wrote:

> On Thu, Oct 02, 2008 at 02:37:13PM -0700, Andrew Morton wrote:
> > On Thu, 2 Oct 2008 21:22:23 +0200 Jens Axboe <jens.axboe@oracle.com> wrote:
> > > Can we agree on this patch?
> >
> > This change will cause _all_ kjournald writeout to have elevated
> > priority. The majority of that writeout (in data=ordered mode) is file
> > data, which we didn't intend to change.
> >
> > The risk here is that this will *worsen* latency for plain old read(),
> > because now kjournald writeout will be favoured.
> >
> > There is in fact a good argument for _reducing_ kjournald's IO
> > priority, not increasing it!
> >
> > A better approach might be to mark the relevant buffers/bios as needing
> > higher priority at submit_bh() time (if that's possible). At least
> > that way we don't accidentally elevate the priority of the bulk data.
>
> You can do that for submit_bio() by calling bio_set_prio() before
> submision - I did that for elevating only the XFS journal I/O.
> submit_bh() doesn't have any way of passing a priority through to it
> right now...

Yup. There are plenty of spare bits in buffer_head.b_state.
set_buffer_kludge()?



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