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Re: [Qemu-block] [PATCH v11 09/10] qcow2: skip writing zero buffers to e
From: |
Vladimir Sementsov-Ogievskiy |
Subject: |
Re: [Qemu-block] [PATCH v11 09/10] qcow2: skip writing zero buffers to empty COW areas |
Date: |
Fri, 21 Dec 2018 16:16:06 +0000 |
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
- [Qemu-block] [PATCH v11 03/10] quorum: set supported write flags, (continued)
- [Qemu-block] [PATCH v11 03/10] quorum: set supported write flags, Anton Nefedov, 2018/12/18
- [Qemu-block] [PATCH v11 06/10] file-posix: reset fallocate-related flags without CONFIG_FALLOCATE*, Anton Nefedov, 2018/12/18
- [Qemu-block] [PATCH v11 04/10] block: introduce BDRV_REQ_ALLOCATE flag, Anton Nefedov, 2018/12/18
- [Qemu-block] [PATCH v11 05/10] block: treat BDRV_REQ_ALLOCATE as serialising, Anton Nefedov, 2018/12/18
- [Qemu-block] [PATCH v11 08/10] block: support BDRV_REQ_ALLOCATE in passthrough drivers, Anton Nefedov, 2018/12/18
- [Qemu-block] [PATCH v11 07/10] file-posix: support BDRV_REQ_ALLOCATE, Anton Nefedov, 2018/12/18
- [Qemu-block] [PATCH v11 09/10] qcow2: skip writing zero buffers to empty COW areas, Anton Nefedov, 2018/12/18
- Re: [Qemu-block] [PATCH v11 09/10] qcow2: skip writing zero buffers to empty COW areas,
Vladimir Sementsov-Ogievskiy <=
- [Qemu-block] [PATCH v11 10/10] iotest 134: test cluster-misaligned encrypted write, Anton Nefedov, 2018/12/18