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

[Vladimir Sementsov-Ogievskiy]

14.05.2018 09:41, Fam Zheng wrote:
> On Wed, 04/18 17:00, 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.
> Why do we prevent source storing persistent bitmaps by clearing the flag instead
> of making the bitmap code more BDRV_O_INACTIVE-aware, so it is _not_ stored
> when/after inactivation?

Bitmaps are stored exactly on inactivation. So, we need some
flag, that on following inactivation, we don't want to save them. And 
this flag
is .persistent flag.

Of course, we can't save them if BDRV_O_INACTIVE is already set

> > 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.
> >
> > 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
> Is this "cont" on the source even though dst is already up? How will this work?
> Isn't it expected that dst is using the image?

Dr. David described this case, look at his answer.

> > 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?
> This is not safe. No I/O should be done to the image after inactivation.
>
> > Inactive disk implies, that it may be changed by somebody other, isn't it?
> Yes.
>
> > 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.

So, you agree, that dropping all bitmaps after inactivation is good 
idea.. The second question, is it possible to not drop them? Is there a 
way, to check that disk was not changed after pair of 
inactivate-invalidate? I have an idea:
we can create small-bitmap, which will not migrate through migration 
channel, but remain persistent. It will be very small (big granularity, 
to not increase downtime of migration), so after invalidate we can check 
this bitmap. If it is empty, we are happy, we can "activate" all our 
bitmaps and make them persistent again. If not, we have two ways:

1. drop all bitmaps
2. merge small-bitmap to all our bitmaps

> > --
> > Best regards,
> > Vladimir


--
Best regards,
Vladimir