On Mon, Aug 16, 2021 at 6:50 PM Eric Blake <ebl...@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 <nsof...@redhat.com> > > --- > > Reviewed-by: Eric Blake <ebl...@redhat.com> > > -- > Eric Blake, Principal Software Engineer > Red Hat, Inc. +1-919-301-3266 > Virtualization: qemu.org | libvirt.org
