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

From: John Snow
Subject: Re: [Qemu-devel] Restoring bitmaps after failed/cancelled migration
Date: Fri, 11 May 2018 17:23:39 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0

On 04/18/2018 10:00 AM, Vladimir Sementsov-Ogievskiy wrote:
> Hi all.
> We now have the following problem:
> If dirty-bitmaps migration capability is enabled, persistance flag is
> dropped for all migrated bitmaps, to prevent their storing to the
> storage on inactivate. It works ok, persistence itself is migrated
> through the migration channel. But on source, bitmaps becomes not
> persistent, so if we, for some reasons, want to continue using source
> vm, we'll lose bitmaps on stop/start.

Sorry for not following along more carefully, which kind of migration
are we talking about in this case?

> It's simple to fix it: just make bitmaps persistent again on invalidate
> [1].. But I have some questions.
> 1. What are possible cases? I think about the following:
> a. migration cancel or fail, then invalidate
> b. migration success, then qmp cont => invalidate
> c. migration success, then stop/start (there was no invalidate, so [1]
> will not work)
> 2. Is it safe at all, saving bitmaps after inactivate, even without
> persistence?
> Inactive disk implies, that it may be changed by somebody other, isn't
> it? 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?

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