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Re: [Qemu-devel] bs->enable_write_cache and the guest ABI


From: Avi Kivity
Subject: Re: [Qemu-devel] bs->enable_write_cache and the guest ABI
Date: Mon, 08 Mar 2010 11:45:47 +0200
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.8) Gecko/20100301 Fedora/3.0.3-1.fc12 Thunderbird/3.0.3

On 03/08/2010 11:39 AM, Christoph Hellwig wrote:
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.

True.

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.

Reasonable. Should default to write cache enabled since that's compatible across all host caching policies; if the user knows they will keep the host caching policy consistent, they can disable it and reap the gain.

Or keep the default as it is now, and make it the user's responsibility to keep track. In this case we need to add a paragraph to the qemu management guide.

--
error compiling committee.c: too many arguments to function





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