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


From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH v5 3/4] qcow2: add shrink image support
Date: Wed, 12 Jul 2017 16:52:51 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

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 <address@hidden>
> Reviewed-by: Max Reitz <address@hidden>
> ---
>  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.

> +    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.

Kevin



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