Re: [Qemu-devel] [PATCH 2/2] virtio-blk: disable write cache if not negotiated

[Paolo Bonzini <pbonzini@redhat.com>]

Il 03/07/2012 15:49, Kevin Wolf ha scritto:
>> If the guest does not support flushes, we should run in writethrough mode.
>> > The setting is temporary until the next reset, so that for example the
>> > BIOS will run in writethrough mode while Linux will run with a writeback
>> > cache.
>> > 
>> > VIRTIO_BLK_F_FLUSH has been introduced in Linux 2.6.32 (in 2009) and
>> > was backported to RHEL/CentOS 5.6 (in 2010).  The Windows drivers have
>> > two bugs, which I reported on the Red Hat Bugzilla as bugs 837321 and
>> > 837324.  With these patches they will suffer a performance hit but
>> > gain correctness.
>> > 
>> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> I generally like the idea for a default, but doesn't this override even
> an explicit cache=writeback?

Yes.  It doesn't override cache=unsafe though.

> Are we sure that we want this?

The idea is that this change will overcome Anthony's objections to
switching the default to writeback...

Paolo