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Re: [Qemu-devel] bs->enable_write_cache and the guest ABI
From: |
Christoph Hellwig |
Subject: |
Re: [Qemu-devel] bs->enable_write_cache and the guest ABI |
Date: |
Mon, 8 Mar 2010 10:39:08 +0100 |
User-agent: |
Mutt/1.3.28i |
On Sun, Mar 07, 2010 at 04:42:18PM +0200, Avi Kivity wrote:
> This means that if I start a guest with cache=writethrough and then
> restart (or live migrate) it with cache=none, then the guest will see a
> change, even though the user only changed the drive's backing, not
> something guest visible. In the case of live migration, the guest will
> not even notice the change and we may be at risk of data loss.
>
> For 0.13 I propose setting enable_write_cache to true unconditionally.
> For 0.12 the question is more difficult, since we'll be changing the
> guest ABI. Given that guests are unlikely not to be able to cope with
> write caches, and that the alternative is data loss, I believe that's
> also the right solution there.
Setting it to true unconditionally will cause performance degradation
for cache=writethrough devices, as we now have to drain the queue in
the guest for no reason at all.
I think the better option would be to move the cache setting to qdev
property on the block device at it's a device visible setting.