Re: [PATCH] qemu-nbd: Change default cache mode to writeback

[Nir Soffer]

On Mon, Aug 16, 2021 at 6:50 PM Eric Blake <eblake@redhat.com> wrote:
>
> On Fri, Aug 13, 2021 at 11:55:19PM +0300, Nir Soffer wrote:
> > Both qemu and qemu-img use writeback cache mode by default, which is
> > already documented in qemu(1). qemu-nbd uses writethrough cache mode by
> > default, and the default cache mode is not documented.
> >
> > According to the qemu-nbd(8):
> >
> >    --cache=CACHE
> >           The  cache  mode  to be used with the file.  See the
> >           documentation of the emulator's -drive cache=... option for
> >           allowed values.
> >
> > qemu(1) says:
> >
> >     The default mode is cache=writeback.
> >
> > So users have no reason to assume that qemu-nbd is using writethough
> > cache mode. The only hint is the painfully slow writing when using the
> > defaults.
>
> Oh, good catch.  Unfortunately too late for 6.1 proper, but I'll add
> qemu-stable in cc and queue this through my NBD tree for 6.2.

I don't see this in master, lost in your NBD tree?

> > Users can avoid the issue by using --cache=writeback[1] but the defaults
> > should give good performance for the common use case.
> >
> > [1] https://bugzilla.redhat.com/1990656
> >
> > Signed-off-by: Nir Soffer <nsoffer@redhat.com>
> > ---
>
> Reviewed-by: Eric Blake <eblake@redhat.com>
>
> --
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.           +1-919-301-3266
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