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

[Eric Blake]

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.

> 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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