Re: [Qemu-devel] [Qemu-block] [PATCH v3 01/10] qapi: Add transaction support to block-dirty-bitmap operations

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Fri, May 08, 2015 at 08:29:09AM -0600, Eric Blake wrote:
> On 05/08/2015 07:14 AM, Stefan Hajnoczi wrote:
> 
> > No it doesn't.  Actions have to appear atomic to the qmp_transaction
> > caller.  Both approaches achieve that so they are both correct in
> > isolation.
> > 
> > The ambiguity is whether "commit the changes" for .commit() means
> > "changes take effect" or "discard stashed state, making undo
> > impossible".
> > 
> > I think the "discard stashed state, making undo impossible"
> > interpretation is good because .commit() is not allowed to fail.  That
> > function should only do things that never fail.
> > 
> >> That's going to get hard to maintain as we add more transactions.
> > 
> > Yes, we need to be consistent and stick to one of the interpretations in
> > order to guarantee ordering.
> > 
> > Unfortunately, there is already an inconsistency:
> > 
> > 1. internal_snapshot - snapshot taken in .prepare()
> > 2. external_snapshot - BDS node appended in .commit()
> > 3. drive_backup - block job started in .prepare()
> > 4. blockdev_backup - block job started in .prepare()
> > 
> > external_snapshot followed by internal_snapshot acts like the reverse
> > ordering!
> 
> Is that fatal, though?

Yes, ordering is critical when add-bitmap or clear-bitmap are combined
with drive-backup.  Typically the drive-backup must happen after
add-bitmap or clear-bitmap.

There is probably no one who uses external and internal snapshots
together in a single 'transaction' command, so my example is contrived
but it's the same problem.

> Let's see if I'm understanding the problem
> correctly: if you start with a.qcow2, then
>  external_snapshot followed by internal_snapshot
> should create b.qcow2 then the internal snapshot inside b.qcow2, while
>  internal_snapshot followed by external_snapshot
> should create the internal snapshot inside a.qcow2, then create b.qcow2
> 
> But since we create the BDS node later than the internal snapshot is
> taken, both sequences currently cause the internal snapshot to live in
> a.qcow2.

Right.  Ordering is not honored :(.

Stefan

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