Re: [Qemu-devel] [PATCH] migration, xen: Fix block image lock issue on live migration

[Anthony PERARD <anthony.perard@citrix.com>]

On Wed, Oct 04, 2017 at 03:03:49PM +0200, Kevin Wolf wrote:
> Am 02.10.2017 um 21:18 hat Dr. David Alan Gilbert geschrieben:
> > Adding in kwolf;  it looks sane to me; Kevin?
> > If I'm reading this right, this is just after the device state save.
> 
> Is this actual migration? Because the code looks more like it's copied
> and adapted from the snapshot code rather than from the actual migration
> code.

Well the Xen tool stack takes care of the migration, we only need to
save the device states from QEMU, I guess similair to a snapshot.

> If Xen doesn't use the standard mechanisms, I don't know what they need
> to do. Snapshots don't need to inactivate images, but migration does.
> Compared to the normal migration path, this looks very simplistic, so I
> wouldn't be surprised if there was more wrong than just file locking.

I realize now that if one would want to take a snapshot of a running
Xen guest, this xen-save-devices-state qmp command will be called as
well.

So I can see a few options to better handle snapshots, we could:
- Add a new parameter to xen-save-devices-state, "live_migration" which
  could default to 'true' so older version of Xen will still works.
- Create a new qmp command that sole purpose is to call
  bdrv_inactivate_all, I don't know what else this command would have to
  do.
- or just take this patch.

Thanks.

> This looks like it could work as a hack to the problem at hand. Whether
> it is a proper solution, I can't say without investing a lot more time.


-- 
Anthony PERARD