Re: [Qemu-block] [PATCH v11 09/10] qcow2: skip writing zero buffers to empty COW areas

[Vladimir Sementsov-Ogievskiy]

18.12.2018 10:57, 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>
> ---
>   qapi/block-core.json       |  4 +-
>   block/qcow2.h              |  6 +++
>   block/qcow2-cluster.c      |  2 +-
>   block/qcow2.c              | 89 +++++++++++++++++++++++++++++++++++++-
>   block/trace-events         |  1 +
>   tests/qemu-iotests/060     |  7 ++-
>   tests/qemu-iotests/060.out |  5 ++-
>   7 files changed, 108 insertions(+), 6 deletions(-)
> 
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index 762000f31f..204528b3f6 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -3009,6 +3009,8 @@
>   #
>   # @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',
> @@ -3027,7 +3029,7 @@
>               '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 a98d24500b..32d2c04bfa 100644
> --- a/block/qcow2.h
> +++ b/block/qcow2.h
> @@ -386,6 +386,12 @@ typedef struct QCowL2Meta
>        */
>       Qcow2COWRegion cow_end;
>   
> +    /*
> +     * Indicates that COW regions are already handled and do not require
> +     * any more processing.
> +     */
> +    bool skip_cow;
> +
>       /**

hmm, around it, all comments starts from '/**', so, I think, yours should too.
(this note doesn't touch your other comments)

>        * 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 e2737429f5..23e0702027 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 4897abae5e..161b935962 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -2021,6 +2021,11 @@ 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) {
> @@ -2046,6 +2051,77 @@ 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);
> +}
> +
> +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,

here you add m->cow_start.offset ...[1]

> +                          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;
> +        }
> +
> +        /*
> +         * Conventional place for qcow2_pre_write_overlap_check() but in this
> +         * case it is already done for these clusters
> +         */

to be honest, check is done for original request not, not for cow_regions, and,
on the other hand,
do_perform_cow_write _does_ the check.

So, I see two consistent ways:
1. you just add check here, like in do_perform_cow_write
2. improve check in qcow2_co_pwritev to cover the whole area, and drop checks 
from
    perform_cow.

I prefer 1, as it more helpful in case of bugs in the code (just check exactly 
what
you are going to write in the next write).

> +
> +        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,

[1]... and here - not.
And perform_cow uses start too.
hmm..

On the other hand, here we just want to zero all allocated clusters, it is not 
related
to cow regions. So, ok.

> +                                    m->nb_clusters * s->cluster_size,
> +                                    BDRV_REQ_ALLOCATE);
> +        if (ret < 0) {
> +            if (ret != -ENOTSUP && ret != -EAGAIN) {
> +                return ret;
> +            }
> +            continue;
> +        }
> +
> +        trace_qcow2_skip_cow(qemu_coroutine_self(), m->offset, 
> m->nb_clusters);
> +        m->skip_cow = true;
> +    }
> +    return 0;
> +}
> +

[...]


with pre-write-check added:
Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>



-- 
Best regards,
Vladimir