Re: [Qemu-devel] Restoring bitmaps after failed/cancelled migration

[Vladimir Sementsov-Ogievskiy]

16.05.2018 18:32, Kevin Wolf wrote:
> Am 16.05.2018 um 17:10 hat Vladimir Sementsov-Ogievskiy geschrieben:
> > 16.05.2018 15:47, Kevin Wolf wrote:
> > > Am 14.05.2018 um 12:09 hat Vladimir Sementsov-Ogievskiy geschrieben:
> > > > 14.05.2018 09:41, Fam Zheng wrote:
> > > > > On Wed, 04/18 17:00, Vladimir Sementsov-Ogievskiy wrote:
> > > > > > Is it possible, that target will change the disk, and then we return control
> > > > > > to the source? In this case bitmaps will be invalid. So, should not we drop
> > > > > > all the bitmaps on inactivate?
> > > > > Yes, dropping all live bitmaps upon inactivate sounds reasonable. If the dst
> > > > > fails to start, and we want to resume VM at src, we could (optionally?) reload
> > > > > the persistent bitmaps, I guess.
> > > > Reload from where? We didn't store them.
> > > Maybe this just means that it turns out that not storing them was a bad
> > > idea?
> > >
> > > What was the motivation for not storing the bitmap? The additional
> > > downtime? Is it really that bad, though? Bitmaps should be fairly small
> > > for the usual image sizes and writing them out should be quick.
> > What are usual ones? A bitmap of standard granularity of 64k for 16Tb disk
> > is ~30mb. If we have several such bitmaps it may be significant downtime.
> We could have an in-memory bitmap that tracks which parts of the
> persistent bitmap are dirty so that you don't have to write out the
> whole 30 MB during the migration downtime, but can already flush most of
> the persistent bitmap before the VM is stopped.
>
> Kevin

Yes it looks possible. But how to control that downtime? Introduce 
migration state, with specific _pending function? However, it may be not 
necessary.

Anyway, I think we don't need to store it.

If we decided to resume source, bitmap is already in memory, why to 
reload it? If someone already killed source (which was in paused mode), 
it is inconsistent anyway and loss of dirty bitmap is not the worst 
possible problem.

So, finally, it looks safe enough, just to make bitmaps on source 
persistent again (or better, introduce another way to skip storing (may 
be with additional flag, so everybody will be happy), not dropping 
persistent flag). And, after source resume, we have one of the following 
situations:

1. disk was not changed during migration, so, all is ok and we have bitmaps
2. disk was changed. bitmaps are inconsistent. But not only bitmaps, the 
whole vm state is inconsistent with it's disk. This case is a bug in 
management layer and it should never happen. And possibly, we need some 
separate way, to catch such cases.

--
Best regards,
Vladimir