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[Qemu-devel] Re: bs->enable_write_cache and the guest ABI
From: |
Juan Quintela |
Subject: |
[Qemu-devel] Re: bs->enable_write_cache and the guest ABI |
Date: |
Mon, 08 Mar 2010 11:29:47 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux) |
Avi Kivity <address@hidden> 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.