Re: [Qemu-devel] [PATCH v2] qcow2: Set zero flag for discarded clusters

[Stefan Hajnoczi <stefanha@gmail.com>]

On Sat, Feb 08, 2014 at 05:50:02PM +0100, Kevin Wolf wrote:
> Instead of making the backing file contents visible again after a discard
> request, set the zero flag if possible (i.e. on version >= 3).
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> Reviewed-by: Max Reitz <mreitz@redhat.com>
> ---
>  block/qcow2-cluster.c | 22 ++++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> index 25d45d1..9461969 100644
> --- a/block/qcow2-cluster.c
> +++ b/block/qcow2-cluster.c
> @@ -1333,13 +1333,31 @@ static int discard_single_l2(BlockDriverState *bs, uint64_t offset,
>          uint64_t old_offset;
>  
>          old_offset = be64_to_cpu(l2_table[l2_index + i]);
> -        if ((old_offset & L2E_OFFSET_MASK) == 0) {
> +
> +        /*
> +         * Make sure that a discarded area reads back as zeroes for v3 images
> +         * (we cannot do it for v2 without actually writing a zero-filled
> +         * buffer). We can skip the operation if the cluster is already marked
> +         * as zero, or if it's unallocated and we don't have a backing file.
> +         *
> +         * TODO We might want to use bdrv_get_block_status(bs) here, but we're
> +         * holding s->lock, so that doesn't work today.
> +         */
> +        if (!!(old_offset & QCOW_OFLAG_ZERO)) {
> +            continue;
> +        }
> +
> +        if ((old_offset & L2E_OFFSET_MASK) == 0 && !bs->backing_hd) {
>              continue;
>          }
>  
>          /* First remove L2 entries */
>          qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);
> -        l2_table[l2_index + i] = cpu_to_be64(0);
> +        if (s->qcow_version >= 3) {
> +            l2_table[l2_index + i] = cpu_to_be64(QCOW_OFLAG_ZERO);
> +        } else {
> +            l2_table[l2_index + i] = cpu_to_be64(0);
> +        }
>  
>          /* Then decrease the refcount */
>          qcow2_free_any_clusters(bs, old_offset, 1, type);

Oops, this breaks qemu-iotests 046.  I have dropped it from the pull request.

046 1s ... - output mismatch (see 046.out.bad)
--- 046.out	2014-02-03 16:06:23.917196159 +0100
+++ 046.out.bad	2014-02-14 18:02:01.577193856 +0100
@@ -187,10 +187,12 @@
 24 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 8192/8192 bytes at offset 516096
 8 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+Pattern verification failed at offset 524288, 24576 bytes
 read 24576/24576 bytes at offset 524288
 24 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 8192/8192 bytes at offset 548864
 8 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+Pattern verification failed at offset 557056, 8192 bytes
 read 8192/8192 bytes at offset 557056
 8 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 57344/57344 bytes at offset 565248
@@ -199,10 +201,12 @@
 24 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 8192/8192 bytes at offset 647168
 8 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+Pattern verification failed at offset 655360, 24576 bytes
 read 24576/24576 bytes at offset 655360
 24 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 8192/8192 bytes at offset 679936
 8 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+Pattern verification failed at offset 688128, 8192 bytes
 read 8192/8192 bytes at offset 688128
 8 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 57344/57344 bytes at offset 696320
Failures: 046
Failed 1 of 1 tests