Re: [Qemu-devel] [PATCH v7 03/24] block: Release dirty bitmaps in bdrv_close()

[Fam Zheng]

On Mon, 11/16 12:07, John Snow wrote:
> 
> 
> On 11/15/2015 08:27 PM, Fam Zheng wrote:
> > On Fri, 11/13 17:49, John Snow wrote:
> >>
> >>
> >> On 11/12/2015 01:23 AM, Fam Zheng wrote:
> >>> On Mon, 11/09 23:39, Max Reitz wrote:
> >>>> bdrv_delete() is not very happy about deleting BlockDriverStates with
> >>>> dirty bitmaps still attached to them. In the past, we got around that
> >>>> very easily by relying on bdrv_close_all() bypassing bdrv_delete(), and
> >>>> bdrv_close() simply ignoring that condition. We should fix that by
> >>>> releasing all dirty bitmaps in bdrv_close() and drop the assertion in
> >>>> bdrv_delete().
> >>>
> >>> What bitmaps are attached when bdrv_close() is called?  The ones created 
> >>> from
> >>> the monitor should probably be removed by the monitor, and the internal 
> >>> ones
> >>> like in migration and block jobs should probably be removed by stopping 
> >>> the
> >>> respective job.
> >>>
> >>> Fam
> >>>
> >>
> >> Well in this case at least it appears we are still asserting that the
> >> BDS has no job attached, so it shouldn't have any internal bitmaps
> >> weighing it down, which just leaves the ones created by the QMP interface.
> >>
> >> How important is it that we ask the user to remove all of those bitmaps
> >> themselves?
> >>
> >> It might become more important in the future when persistence is an
> >> option and we go to close a transient bitmap -- but persistent bitmaps I
> >> am sure it will be safe to just close out and flush to disk.
> >>
> > 
> > Yes, I was not saying we should expecting user to do that manually, but it
> > would be good to be clear about different types of instances in the code.
> > 
> > For now, it's unlikely a problem.
> > 
> > Fam
> > 
> 
> OK, just pointing out that I think it's unlikely we have any internal
> bitmaps at this point since we assert that we have no jobs.
> 
> We can actually test this, since internal bitmaps are anonymous and
> user-created ones must always have a name.
> 
> Tangent question: If a user closes a BDS node with a transient bitmap
> attached, should we take any special action?
> 
> i.e.; do we offer the user a last chance to save the bitmap somewhere,
> or do we just do what we were asked and hope the user is competent?
> 
> (I assume: No, we let the user shoot themselves in the foot if they want
> to, but I wanted to ask the question.)
> 
> --js
> 

For persistent dirty bitmaps, we must provide an interface with which user can
make sure that, upon the shutting down of VM, the dirty bitmap file is in
consistent with the image it is tracking.

With embedded dirty bitmap in qcow2 image, this shouldn't be a problem though,
because qcow2_close can handle this internally.  But what about the senario
with "raw image" + "incremental backup"?

Fam