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

[Pavel Butsykin]

On 21.09.2017 18:30, 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;
>
> Oh, and another thing: If you decide to ignore errors here, I'd at least
> consider the cluster allocated.
>
> Of course it would also be possible not to ignore errors, and instead
> return them to the caller which would then just not truncate the file.

Yes, it seems so safer.

> Max
>
> > +        }
> > +
> > +        if (refcount > 0) {
> > +            last_cluster = i;
> > +        }
> > +    }
> > +    return last_cluster;
> > +}