Re: [Qemu-devel] [PATCH v10 8/9] qcow2: skip writing zero buffers to empty COW areas

[Vladimir Sementsov-Ogievskiy]

03.12.2018 13:14, Anton Nefedov wrote:
> If COW areas of the newly allocated clusters are zeroes on the backing image,
> efficient bdrv_write_zeroes(flags=BDRV_REQ_ALLOCATE) can be used on the whole
> cluster instead of writing explicit zero buffers later in perform_cow().
> 
> iotest 060:
> write to the discarded cluster does not trigger COW anymore.
> Use a backing image instead.
> 
> Signed-off-by: Anton Nefedov <address@hidden>
> Reviewed-by: Alberto Garcia <address@hidden>
> ---
>   qapi/block-core.json       |  4 +-
>   block/qcow2.h              |  6 +++
>   block/qcow2-cluster.c      |  2 +-
>   block/qcow2.c              | 80 +++++++++++++++++++++++++++++++++++++-
>   block/trace-events         |  1 +
>   tests/qemu-iotests/060     | 26 ++++++++-----
>   tests/qemu-iotests/060.out |  5 ++-
>   7 files changed, 109 insertions(+), 15 deletions(-)
> 
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index d4fe710836..50598aa8fe 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -3004,6 +3004,8 @@
>   #
>   # @cor_write: a write due to copy-on-read (since 2.11)
>   #
> +# @cluster_alloc_space: an allocation of file space for a cluster (since 4.0)
> +#
>   # Since: 2.9
>   ##
>   { 'enum': 'BlkdebugEvent', 'prefix': 'BLKDBG',
> @@ -3022,7 +3024,7 @@
>               'pwritev_rmw_tail', 'pwritev_rmw_after_tail', 'pwritev',
>               'pwritev_zero', 'pwritev_done', 'empty_image_prepare',
>               'l1_shrink_write_table', 'l1_shrink_free_l2_clusters',
> -            'cor_write'] }
> +            'cor_write', 'cluster_alloc_space'] }
>   
>   ##
>   # @BlkdebugInjectErrorOptions:
> diff --git a/block/qcow2.h b/block/qcow2.h
> index 8662b68575..8a64077897 100644
> --- a/block/qcow2.h
> +++ b/block/qcow2.h
> @@ -389,6 +389,12 @@ typedef struct QCowL2Meta
>        */
>       Qcow2COWRegion cow_end;
>   
> +    /**
> +     * Indicates that COW regions are already handled and do not require
> +     * any more processing.
> +     */
> +    bool skip_cow;
> +
>       /**
>        * The I/O vector with the data from the actual guest write request.
>        * If non-NULL, this is meant to be merged together with the data
> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> index d37fe08b3d..3685c5f67e 100644
> --- a/block/qcow2-cluster.c
> +++ b/block/qcow2-cluster.c
> @@ -806,7 +806,7 @@ static int perform_cow(BlockDriverState *bs, QCowL2Meta 
> *m)
>       assert(start->offset + start->nb_bytes <= end->offset);
>       assert(!m->data_qiov || m->data_qiov->size == data_bytes);
>   
> -    if (start->nb_bytes == 0 && end->nb_bytes == 0) {
> +    if ((start->nb_bytes == 0 && end->nb_bytes == 0) || m->skip_cow) {
>           return 0;
>       }
>   
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 991d6ac91b..027188a1a3 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -2015,6 +2015,11 @@ static bool merge_cow(uint64_t offset, unsigned bytes,
>               continue;
>           }
>   
> +        /* If COW regions are handled already, skip this too */
> +        if (m->skip_cow) {
> +            continue;
> +        }
> +
>           /* The data (middle) region must be immediately after the
>            * start region */
>           if (l2meta_cow_start(m) + m->cow_start.nb_bytes != offset) {
> @@ -2040,6 +2045,68 @@ static bool merge_cow(uint64_t offset, unsigned bytes,
>       return false;
>   }
>   
> +static bool is_unallocated(BlockDriverState *bs, int64_t offset, int64_t 
> bytes)
> +{
> +    int64_t nr;
> +    return !bytes ||
> +        (!bdrv_is_allocated_above(bs, NULL, offset, bytes, &nr) && nr == 
> bytes);

bdrv_is_allocated_above may return error < 0

> +}
> +
> +static bool is_zero_cow(BlockDriverState *bs, QCowL2Meta *m)
> +{
> +    /* This check is designed for optimization shortcut so it must be
> +     * efficient.
> +     * Instead of is_zero(), use is_unallocated() as it is faster (but not
> +     * as accurate and can result in false negatives). */
> +    return is_unallocated(bs, m->offset + m->cow_start.offset,
> +                          m->cow_start.nb_bytes) &&
> +           is_unallocated(bs, m->offset + m->cow_end.offset,
> +                          m->cow_end.nb_bytes);
> +}
> +
> +static int handle_alloc_space(BlockDriverState *bs, QCowL2Meta *l2meta)
> +{
> +    BDRVQcow2State *s = bs->opaque;
> +    QCowL2Meta *m;
> +
> +    if (!(bs->file->bs->supported_zero_flags & BDRV_REQ_ALLOCATE)) {
> +        return 0;
> +    }
> +
> +    if (bs->encrypted) {
> +        return 0;
> +    }
> +
> +    for (m = l2meta; m != NULL; m = m->next) {
> +        int ret;
> +
> +        if (!m->cow_start.nb_bytes && !m->cow_end.nb_bytes) {
> +            continue;
> +        }
> +
> +        if (!is_zero_cow(bs, m)) {
> +            continue;
> +        }
> +
> +        BLKDBG_EVENT(bs->file, BLKDBG_CLUSTER_ALLOC_SPACE);
> +        /* instead of writing zero COW buffers,
> +           efficiently zero out the whole clusters */
> +        ret = bdrv_co_pwrite_zeroes(bs->file, m->alloc_offset,
> +                                    m->nb_clusters * s->cluster_size,
> +                                    BDRV_REQ_ALLOCATE);
> +        if (ret < 0) {
> +            if (ret != -ENOTSUP && ret != -EAGAIN) {

why do you ignore ENOTSUP?

And also, hmm, should not we have problems with serializing? If already started 
unaligned request,
it is serializing.. And now, we are going to start nested serializing request, 
which will definitely
return EAGAIN..

> +                return ret;
> +            }
> +            continue;
> +        }
> +
> +        trace_qcow2_skip_cow(qemu_coroutine_self(), m->offset, 
> m->nb_clusters);
> +        m->skip_cow = true;
> +    }
> +    return 0;
> +}
> +
>   static coroutine_fn int qcow2_co_pwritev(BlockDriverState *bs, uint64_t 
> offset,
>                                            uint64_t bytes, QEMUIOVector *qiov,
>                                            int flags)
> @@ -2122,24 +2189,33 @@ static coroutine_fn int 
> qcow2_co_pwritev(BlockDriverState *bs, uint64_t offset,
>               goto fail;
>           }
>   
> +        qemu_co_mutex_unlock(&s->lock);
> +
> +        ret = handle_alloc_space(bs, l2meta);
> +        if (ret < 0) {
> +            qemu_co_mutex_lock(&s->lock);
> +            goto fail;
> +        }
> +
>           /* If we need to do COW, check if it's possible to merge the
>            * writing of the guest data together with that of the COW regions.
>            * If it's not possible (or not necessary) then write the
>            * guest data now. */
>           if (!merge_cow(offset, cur_bytes, &hd_qiov, l2meta)) {
> -            qemu_co_mutex_unlock(&s->lock);
>               BLKDBG_EVENT(bs->file, BLKDBG_WRITE_AIO);
>               trace_qcow2_writev_data(qemu_coroutine_self(),
>                                       cluster_offset + offset_in_cluster);
>               ret = bdrv_co_pwritev(bs->file,
>                                     cluster_offset + offset_in_cluster,
>                                     cur_bytes, &hd_qiov, 0);
> -            qemu_co_mutex_lock(&s->lock);
>               if (ret < 0) {
> +                qemu_co_mutex_lock(&s->lock);
>                   goto fail;
>               }
>           }
>   
> +        qemu_co_mutex_lock(&s->lock);
> +
>           ret = qcow2_handle_l2meta(bs, &l2meta, true);
>           if (ret) {
>               goto fail;
> diff --git a/block/trace-events b/block/trace-events
> index 3e8c47bb24..f3f6d66dcc 100644
> --- a/block/trace-events
> +++ b/block/trace-events
> @@ -69,6 +69,7 @@ qcow2_writev_done_part(void *co, int cur_bytes) "co %p 
> cur_bytes %d"
>   qcow2_writev_data(void *co, uint64_t offset) "co %p offset 0x%" PRIx64
>   qcow2_pwrite_zeroes_start_req(void *co, int64_t offset, int count) "co %p 
> offset 0x%" PRIx64 " count %d"
>   qcow2_pwrite_zeroes(void *co, int64_t offset, int count) "co %p offset 0x%" 
> PRIx64 " count %d"
> +qcow2_skip_cow(void *co, uint64_t offset, int nb_clusters) "co %p offset 
> 0x%" PRIx64 " nb_clusters %d"
>   
>   # block/qcow2-cluster.c
>   qcow2_alloc_clusters_offset(void *co, uint64_t offset, int bytes) "co %p 
> offset 0x%" PRIx64 " bytes %d"
> diff --git a/tests/qemu-iotests/060 b/tests/qemu-iotests/060
> index af0588ae9a..089df94657 100755
> --- a/tests/qemu-iotests/060
> +++ b/tests/qemu-iotests/060
> @@ -150,27 +150,33 @@ $QEMU_IO -c "$OPEN_RO" -c "read -P 1 0 512" | 
> _filter_qemu_io
>   echo
>   echo "=== Testing overlap while COW is in flight ==="
>   echo
> +BACKING_IMG=$TEST_IMG.base
> +TEST_IMG=$BACKING_IMG _make_test_img 1G
> +
> +$QEMU_IO -c 'write 64k 64k' "$BACKING_IMG" | _filter_qemu_io
> +
>   # compat=0.10 is required in order to make the following discard actually
> -# unallocate the sector rather than make it a zero sector - we want COW, 
> after
> -# all.
> -IMGOPTS='compat=0.10' _make_test_img 1G
> +# unallocate the sector rather than make it a zero sector as we would like
> +# to reuse it for another guest offset
> +IMGOPTS='compat=0.10' _make_test_img -b "$BACKING_IMG" 1G
>   # Write two clusters, the second one enforces creation of an L2 table after
>   # the first data cluster.
>   $QEMU_IO -c 'write 0k 64k' -c 'write 512M 64k' "$TEST_IMG" | _filter_qemu_io
> -# Discard the first cluster. This cluster will soon enough be reallocated and
> -# used for COW.
> +# Discard the first cluster. This cluster will soon enough be reallocated
>   $QEMU_IO -c 'discard 0k 64k' "$TEST_IMG" | _filter_qemu_io
>   # Now, corrupt the image by marking the second L2 table cluster as free.
>   poke_file "$TEST_IMG" '131084' "\x00\x00" # 0x2000c
> -# Start a write operation requiring COW on the image stopping it right before
> -# doing the read; then, trigger the corruption prevention by writing 
> anything to
> -# any unallocated cluster, leading to an attempt to overwrite the second L2
> +# Start a write operation requiring COW on the image;
> +# this write will reuse the host offset released by a previous discard.
> +# Stop it right before doing the read.
> +# Then, trigger the corruption prevention by writing anything to
> +# another unallocated cluster, leading to an attempt to overwrite the second 
> L2
>   # table. Finally, resume the COW write and see it fail (but not crash).
>   echo "open -o file.driver=blkdebug $TEST_IMG
>   break cow_read 0
> -aio_write 0k 1k
> +aio_write 64k 1k
>   wait_break 0
> -write 64k 64k
> +write 128k 64k
>   resume 0" | $QEMU_IO | _filter_qemu_io
>   
>   echo
> diff --git a/tests/qemu-iotests/060.out b/tests/qemu-iotests/060.out
> index d67c6234a4..ec8f15d2f0 100644
> --- a/tests/qemu-iotests/060.out
> +++ b/tests/qemu-iotests/060.out
> @@ -97,7 +97,10 @@ read 512/512 bytes at offset 0
>   
>   === Testing overlap while COW is in flight ===
>   
> -Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1073741824
> +Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=1073741824
> +wrote 65536/65536 bytes at offset 65536
> +64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
> +Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1073741824 
> backing_file=TEST_DIR/t.IMGFMT.base
>   wrote 65536/65536 bytes at offset 0
>   64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
>   wrote 65536/65536 bytes at offset 536870912
> 


-- 
Best regards,
Vladimir