Re: [Qemu-block] [Qemu-devel] [PATCH v2 10/11] blockjob: refactor backup_start as backup_job_create

[John Snow]

On 10/10/2016 04:57 AM, Kevin Wolf wrote:
> Am 07.10.2016 um 20:39 hat John Snow geschrieben:
> > On 09/30/2016 06:00 PM, John Snow wrote:
> > > Refactor backup_start as backup_job_create, which only creates the job,
> > > but does not automatically start it. The old interface, 'backup_start',
> > > is not kept in favor of limiting the number of nearly-identical iterfaces

(Ah yes, 'iterfaces.')

> > > that would have to be edited to keep up with QAPI changes in the future.
> > >
> > > Callers that wish to synchronously start the backup_block_job can
> > > instead just call block_job_start immediately after calling
> > > backup_job_create.
> > >
> > > Transactions are updated to use the new interface, calling block_job_start
> > > only during the .commit phase, which helps prevent race conditions where
> > > jobs may finish before we even finish building the transaction. This may
> > > happen, for instance, during empty block backup jobs.
> >
> > Sadly for me, I realized this patch has a potential problem. When we
> > were adding the bitmap operations, it became clear that the
> > atomicity point was during .prepare, not .commit.
> >
> > e.g. the bitmap is cleared or created during prepare, and backup_run
> > installs its Write Notifier at that point in time, too.
>
> Strictly speaking that's wrong then.

I agree, though I do remember this coming up during the bitmap review 
process that the current point-in-time spot is during prepare at the moment.

I do think that while it's at least a consistent model (The model where 
we do in fact commit during .prepare(), and simply undo or revert during 
.abort(), and only clean or remove undo-cache in .commit()) it certainly 
violates the principle of least surprise and is a little rude...

> The write notifier doesn't really hurt because it is never triggered
> between prepare and commit (we're holding the lock) and it can just be
> removed again.
>
> Clearing the bitmap is a bug because the caller could expect that the
> bitmap is in its original state if the transaction fails. I doubt this
> is a problem in practice, but we should fix it anyway.

We make a backup to undo the process if it fails. I only mention it to 
emphasize that the atomic point appears to be during prepare. In 
practice we hold the locks for the whole process, but... I think Paolo 
may be actively trying to change that.

> By the way, why did we allow to add a 'bitmap' option for DriveBackup
> without adding it to BlockdevBackup at the same time?

I don't remember. I'm not sure anyone ever audited it to convince 
themselves it was a useful or safe thing to do. I believe at the time I 
was pushing for bitmaps in DriveBackup, Fam was still authoring the 
BlockdevBackup interface.

> > By changing BlockJobs to only run on commit, we've severed the
> > atomicity point such that some actions will take effect during
> > prepare, and others at commit.
> >
> > I still think it's the correct thing to do to delay the BlockJobs
> > until the commit phase, so I will start auditing the code to see how
> > hard it is to shift the atomicity point to commit instead. If it's
> > possible to do that, I think from the POV of the managing
> > application, having the atomicity point be
> >
> > Feel free to chime in with suggestions and counterpoints until then.
>
> I agree that jobs have to be started only at commit. There may be other
> things that are currently happening in prepare that really should be
> moved as well, but unless moving one thing but not the other doesn't
> break anything that was working, we can fix one thing at a time.
>
> Kevin

Alright, let's give this a whirl.

We have 8 transaction actions:

drive_backup
blockdev_backup
block_dirty_bitmap_add
block_dirty_bitmap_clear
abort
blockdev_snapshot
blockdev_snapshot_sync
blockdev_snapshot_internal_sync

Drive and Blockdev backup are already modified to behave point-in-time 
at time of .commit() by changing them to only begin running once the 
commit phase occurs.

Bitmap add and clear are trivial to rework; clear just moves the call to 
clear in commit, with possibly some action taken to prevent the bitmap 
from become used by some other process in the meantime. Add is easy to 
rework too, we can create it during prepare but reset it back to zero 
during commit if necessary.

Abort needs no changes.

blockdev_snapshot[_sync] actually appears to already be doing the right 
thing, by only installing the new top layer during commit, which makes 
this action inconsistent by current semantics, but requires no changes 
to move to the desired new semantics.

That leaves only the internal snapshot to worry about, which does 
admittedly look like quite the yak to shave. It's a bit out of scope for 
me, but Kevin, do you think this is possible?

Looks like implementations are qcow2, rbd, and sheepdog. I imagine this 
would need to be split into prepare and commit semantics to accommodate 
this change... though we don't have any meaningful control over the rbd 
implementation.

Any thoughts? I could conceivably just change everything over to working 
primarily during .commit(), and just argue that the locks held for the 
transaction are sufficient to leave the internal snapshot alone "for 
now," ...

--js