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

[Kevin Wolf <kwolf@redhat.com>]

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Am 11.05.2015 um 15:10 hat Stefan Hajnoczi geschrieben:
> 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.

Is there a reason to include add-bitmap/clear-bitmap in the transaction
rather than preparing everything before the transaction and then only
starting the backup (possibly of multiple disks) in a transaction?

The original assumption was that there are no interdependencies between
different transaction commands and the order is undefined.

Kevin

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