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

[Eric Blake]

On Sun, Sep 19, 2021 at 10:56:58PM +0300, Nir Soffer wrote:
> 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?

It's on my tree:
https://repo.or.cz/qemu/ericb.git/shortlog/refs/heads/nbd

I just haven't bundled up enough patches for a pull request yet.  I'll
get one out this week, probably with a few more patches included.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org