[Libguestfs] [PATCH nbdkit] file: Implement cache=none and fadvise=normal|random|sequential.

[Nir Soffer]

On Fri, Aug 7, 2020 at 5:36 PM Richard W.M. Jones <rjones at redhat.com> wrote:
>
> On Fri, Aug 07, 2020 at 05:29:24PM +0300, Nir Soffer wrote:
> > On Fri, Aug 7, 2020 at 5:07 PM Richard W.M. Jones <rjones at redhat.com> wrote:
> > > These ones?
> > > https://www.redhat.com/archives/libguestfs/2020-August/msg00078.html
> >
> > No, we had a bug when copying image from glance caused sanlock timeouts
> > because of the unpredictable page cache flushes.
> >
> > We tried to use fadvice but it did not help. The only way to avoid such issues
> > is with O_SYNC or O_DIRECT. O_SYNC is much slower but this is the path
> > we took for now in this flow.
>
> I'm interested in more background about this, because while it is true
> that O_DIRECT and POSIX_FADV_DONTNEED are not exactly equivalent, I
> think I've shown here that DONTNEED can be used to avoid polluting the
> page cache.

This fixes the minor issue of polluting the page cache, but it does not help
to avoid stale data in the cache, or unconfrolled flushes.

The bug I mentioned is:
https://bugzilla.redhat.com/1832967

This explains the issue:
https://bugzilla.redhat.com/1247135#c29

And here you can how unrelated I/O is affected by uncontrolled flushes:
https://bugzilla.redhat.com/1247135#c30
https://bugzilla.redhat.com/1247135#c36

Here we compare oflag=nocache,dsync with oflag=direct for NFS and iSCSI disks:
https://gerrit.ovirt.org/c/108912/

cahe=none now is basically oflag=nocache,dsync

>
> Rich.
>
> --
> Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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