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Re: [PATCH 2/2] migration: dirty-bitmap: Allow control of bitmap persist

From: Peter Krempa
Subject: Re: [PATCH 2/2] migration: dirty-bitmap: Allow control of bitmap persistence on destination
Date: Wed, 3 Feb 2021 14:39:32 +0100
User-agent: Mutt/1.14.6 (2020-07-11)

On Wed, Feb 03, 2021 at 14:27:49 +0100, Peter Krempa wrote:
> On Wed, Feb 03, 2021 at 16:23:21 +0300, Vladimir Sementsov-Ogievskiy wrote:
> > 03.02.2021 16:00, Peter Krempa wrote:
> > > Bitmap's source persistence is transported over the migration stream and
> > > the destination mirrors it. In some cases the destination might want to
> > > persist bitmaps which are not persistent on the source (e.g. the result
> > > of merge of bitmaps from a number of layers on the source when migrating
> > > into a squashed image)
> > 
> > Why not make merge target on source be persistent itself? Then it will be 
> > persistent on migration destination.
> Because they are temporary on the source. I don't want to make it
> persistent in case of a failure so that it doesn't get written to the
> disk e.g. in case of VM shutdown.

To be a bit more specific, I don't want the bitmaps to stay in the
image, that means that I'd have to also delete them on the source after
a successfull migration before qemu is terminated, which might not even
be possible since source deactivates storage after migration.

So making them persistent on source is impossible.

> > 
> > > but currently it would need to create another set
> > > of persistent bitmaps and merge them.
> > > 
> > > This adds 'dest-persistent' optional property to
> > > 'BitmapMigrationBitmapAlias' which when present overrides the bitmap
> > > presence state from the source.
> > 
> > It's seems simpler to make a separate qmp command 
> > block-dirty-bitmap-make-persistent.. Didn't you consider this way?
> I'm not sure how the internals work entirely. In my case it's way
> simpler to do this setup when generating the mapping which I need to do
> anyways rather than calling separate commands.

Similarly here, after a successful migration I'd have to go and make all
the bitmaps persistent, which is an extra step, and also a vector for
possible failures after migration which also doesn't seem appealing.

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