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From: Juan Quintela <quintela@redhat.com>
In-Reply-To: <4B93BB4A.8090605@redhat.com> (Avi Kivity's message of "Sun, 07
	Mar 2010 16:42:18 +0200")
References: <4B93BB4A.8090605@redhat.com>
Date: Mon, 08 Mar 2010 11:29:47 +0100
Message-ID: <m3vdd7yulg.fsf@trasno.mitica>
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Subject: [Qemu-devel] Re: bs->enable_write_cache and the guest ABI
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To: Avi Kivity <avi@redhat.com>
Cc: qemu-devel <qemu-devel@nongnu.org>, Christoph Hellwig <hch@lst.de>

Avi Kivity <avi@redhat.com> wrote:
> block.c says:
>
>>     /*
>>      * Yes, BDRV_O_NOCACHE aka O_DIRECT means we have to present a
>>      * write cache to the guest.  We do need the fdatasync to flush
>>      * out transactions for block allocations, and we maybe have a
>>      * volatile write cache in our backing device to deal with.
>>      */
>>     if (flags & (BDRV_O_CACHE_WB|BDRV_O_NOCACHE))
>>         bs->enable_write_cache = 1;
>
> 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.

For RHEL I setted with adding enable_write_cache to the migration
state.  As you state, that value is guest visible.  I can update that
patches to qemu.  When I migrated from an old version, I just set that
value to 0.

What do you think?

Later, Juan.