Re: [Qemu-devel] [PATCH 2/2] qcow2: truncate the tail of the image file after shrinking the image

[Pavel Butsykin]

On 21.09.2017 18:28, Max Reitz wrote:
> On 2017-09-20 15:58, Pavel Butsykin wrote:
> > Now after shrinking the image, at the end of the image file, there might be a
> > tail that probably will never be used. So we can find the last used cluster and
> > cut the tail.
> >
> > Signed-off-by: Pavel Butsykin <address@hidden>
> > ---
> >   block/qcow2-refcount.c | 21 +++++++++++++++++++++
> >   block/qcow2.c          | 19 +++++++++++++++++++
> >   block/qcow2.h          |  1 +
> >   3 files changed, 41 insertions(+)
> >
> > diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
> > index 88d5a3f1ad..5e221a166c 100644
> > --- a/block/qcow2-refcount.c
> > +++ b/block/qcow2-refcount.c
> > @@ -3181,3 +3181,24 @@ out:
> >       g_free(reftable_tmp);
> >       return ret;
> >   }
> > +
> > +int64_t qcow2_get_last_cluster(BlockDriverState *bs, int64_t size)
> > +{
> > +    BDRVQcow2State *s = bs->opaque;
> > +    int64_t i, last_cluster, nb_clusters = size_to_clusters(s, size);
> > +    uint64_t refcount;
> > +
> > +    for (i = 0, last_cluster = 0; i < nb_clusters; i++) {
> > +        int ret = qcow2_get_refcount(bs, i, &refcount);
> > +        if (ret < 0) {
> > +            fprintf(stderr, "Can't get refcount for cluster %" PRId64 ": %s\n",
> > +                    i, strerror(-ret));
> > +            continue;
> > +        }
> > +
> > +        if (refcount > 0) {
> > +            last_cluster = i;
> > +        }
> > +    }
> > +    return last_cluster;
> > +}
>
> Wouldn't it make more sense to start from the end of the image?

If this will reduce the iterations, then yes. But it will depend on the
situation. If you truncate the image more than half, it can increase the
number of iterations. But intuitively it seems that to start from the
end would be better :)
> Max