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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 14:27:44 +0100
User-agent: Mutt/1.14.6 (2020-07-11)

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.

> > 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.

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