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

[Kevin Wolf]

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