On 08/24/2010 04:29 PM, Anthony Liguori wrote:
I'm not sure this patch is needed in the first place.
If you have a sequence of operations like:
0) receive guest write request Z
1) submit write A
2) write A completes
3) submit write B
4) write B completes
5) report guest write Z complete
You're adding a:
4.5) sync write B
Which is ultimately unnecessary if what you care about is avoiding
reordering of step (2) and (4). When a write() request completes,
you're guaranteed that a subsequent read() request will return the
written data. That's always true. If I could do a write(A) followed
by a write(B) and then read()=A, no software would actually function
correctly.
It's important to make sure that you don't get image corruption if
(2) happens but not (4). But I think that's okay in qcow2 today.
It's about metadata writes. If an operation changes metadata, we must
sync it to disk before writing any data or other metadata which
depends on it, regardless of any promises to the guest.