Re: [Qemu-devel] [PATCH v5 3/4] qcow2: add shrink image support

[Kevin Wolf <kwolf@redhat.com>]

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Am 12.07.2017 um 18:58 hat Max Reitz geschrieben:
> On 2017-07-12 16:52, Kevin Wolf wrote:
> > Am 12.07.2017 um 13:46 hat Pavel Butsykin geschrieben:
> >> This patch add shrinking of the image file for qcow2. As a result, this allows
> >> us to reduce the virtual image size and free up space on the disk without
> >> copying the image. Image can be fragmented and shrink is done by punching holes
> >> in the image file.
> >>
> >> Signed-off-by: Pavel Butsykin <pbutsykin@virtuozzo.com>
> >> Reviewed-by: Max Reitz <mreitz@redhat.com>
> >> ---
> >>  block/qcow2-cluster.c  |  40 ++++++++++++++++++
> >>  block/qcow2-refcount.c | 110 +++++++++++++++++++++++++++++++++++++++++++++++++
> >>  block/qcow2.c          |  43 +++++++++++++++----
> >>  block/qcow2.h          |  14 +++++++
> >>  qapi/block-core.json   |   3 +-
> >>  5 files changed, 200 insertions(+), 10 deletions(-)
> >>
> >> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> >> index f06c08f64c..518429c64b 100644
> >> --- a/block/qcow2-cluster.c
> >> +++ b/block/qcow2-cluster.c
> >> @@ -32,6 +32,46 @@
> >>  #include "qemu/bswap.h"
> >>  #include "trace.h"
> >>  
> >> +int qcow2_shrink_l1_table(BlockDriverState *bs, uint64_t exact_size)
> >> +{
> >> +    BDRVQcow2State *s = bs->opaque;
> >> +    int new_l1_size, i, ret;
> >> +
> >> +    if (exact_size >= s->l1_size) {
> >> +        return 0;
> >> +    }
> >> +
> >> +    new_l1_size = exact_size;
> >> +
> >> +#ifdef DEBUG_ALLOC2
> >> +    fprintf(stderr, "shrink l1_table from %d to %d\n", s->l1_size, new_l1_size);
> >> +#endif
> >> +
> >> +    BLKDBG_EVENT(bs->file, BLKDBG_L1_SHRINK_WRITE_TABLE);
> >> +    ret = bdrv_pwrite_zeroes(bs->file, s->l1_table_offset +
> >> +                                       sizeof(uint64_t) * new_l1_size,
> >> +                             (s->l1_size - new_l1_size) * sizeof(uint64_t), 0);
> >> +    if (ret < 0) {
> >> +        return ret;
> >> +    }
> >> +
> >> +    ret = bdrv_flush(bs->file->bs);
> >> +    if (ret < 0) {
> >> +        return ret;
> >> +    }
> > 
> > If we have an error here (or after a partial bdrv_pwrite_zeroes()), we
> > have entries zeroed out on disk, but in memory we still have the
> > original L1 table.
> > 
> > Should the in-memory L1 table be zeroed first? Then we can't
> > accidentally reuse stale entries, but would have to allocate new ones,
> > which would get on-disk state and in-memory state back in sync again.
> 
> Yes, I thought of the same.  But this implies that the allocation is
> able to modify the L1 table, and I find that unlikely if that
> bdrv_flush() failed already...
> 
> So I concluded not to have an opinion on which order is better.

Well, let me ask the other way round: Is there any disadvantage in first
zeroing the in-memory table and then writing to the image?

If I have a choice between "always safe" and "not completely safe, but I
think it's unlikely to happen", especially in image formats, then I will
certainly take the "always safe".

> >> +    BLKDBG_EVENT(bs->file, BLKDBG_L1_SHRINK_FREE_L2_CLUSTERS);
> >> +    for (i = s->l1_size - 1; i > new_l1_size - 1; i--) {
> >> +        if ((s->l1_table[i] & L1E_OFFSET_MASK) == 0) {
> >> +            continue;
> >> +        }
> >> +        qcow2_free_clusters(bs, s->l1_table[i] & L1E_OFFSET_MASK,
> >> +                            s->cluster_size, QCOW2_DISCARD_ALWAYS);
> >> +        s->l1_table[i] = 0;
> >> +    }
> >> +    return 0;
> >> +}
> >> +
> >>  int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
> >>                          bool exact_size)
> >>  {
> > 
> > I haven't checked qcow2_shrink_reftable() for similar kinds of problems,
> > I hope Max has.
> 
> Well, it's exactly the same there.

Ok, so I'll object to this code without really having looked at it.

Kevin

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