Re: [PATCH RFC v2 1/5] block: add bitmap-populate job

[Kevin Wolf]

Am 04.06.2020 um 11:16 hat Peter Krempa geschrieben:
> On Thu, Jun 04, 2020 at 11:12:31 +0200, Kevin Wolf wrote:
> > Am 18.05.2020 um 22:49 hat Eric Blake geschrieben:
> > > > +
> > > > +    /* NB: new bitmap is anonymous and enabled */
> > > > +    cluster_size = bdrv_dirty_bitmap_granularity(target_bitmap);
> > > > +    new_bitmap = bdrv_create_dirty_bitmap(bs, cluster_size, NULL, 
> > > > errp);
> > > > +    if (!new_bitmap) {
> > > > +        return NULL;
> > > > +    }
> > > 
> > > This means if the guest writes to the disk while the job is ongoing, the
> > > bitmap will be updated to mark that portion of the bitmap as set, even if 
> > > it
> > > was not allocated at the time the job started.  But then again, the guest
> > > writes are causing allocation, so this seems like the right thing to do.
> > 
> > Is the target bitmap active at the same time, i.e. will it get the
> > correct information only from new_bitmap or are the bits already set in
> > it anyway?
> 
> Yes, libvirt plans to use it with an active non-persistent bitmap which
> will in subsequent steps be merged into others. The bitmap is added in
> the same transaction. The bitmap must be active, because we need to wait
> for the block jobs to finish before it becomes usable and thus can't
> sequence in other operations until later.

A lot of bitmap merging then, because the block job in this series
already creates a temporary internal bitmap that is merged into the
target bitmap on completion. But if the target bitmap is only libvirt's
temporary bitmap to be merged to yet another bitmap, I wonder if this
process shouldn't be simplified.

Kevin