Re: [Ext2-devel] Re: [PATCH 2/3] ext3 reservation allow turn off for specifed file

Mingming Cao
2004-10-26T22:59:08Z

On Tue, 2004-10-26 at 13:18, Andrew Morton wrote:
> Mingming Cao <cmm@us.ibm.com> wrote:
> >
> > > I wonder how important this optimisation really is? I bet no applications
> > > are using posix_fadvise(POSIX_FADV_RANDOM) anyway.
> > >
> > I don't know if there is application using the POSIX_FADV_RANDOM. No? If
> > this is the truth, I think we don't need this optimization at present.
> > Logically reservation does not benefit seeky random write, but there is
> > no benchmark showing performance issue so far. We have already provided
> > ways for applications turn off reservation through the existing ioctl
> > for specified file and -o noreservation mount option for the whole
> > filesystem.
>
> Well we definitely don't want to be encouraging application developers to be
> adding ext3-specific ioctls. So we need to work out if any applications
> can get significant benefit from manually disabling reservations and if
> so, wire up fadvise() into filesystems and do it that way.
>
Okey. Also, if there is such a need to disable reservation, maybe we
could think about closing the reservation window dynamically based on
past I/O pattern, when user application did not give any hint.

> Do you know if disabling reservations helps any workloads?
>
Not that I aware of. We have done several microbenchmark on sequential
and random workload, haven't seen regression with reservations so far.
But our testing is certainly not comprehensive.

Mingming