On 21/12/2018 7:16 PM, Vladimir Sementsov-Ogievskiy wrote: > 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 <anton.nefe...@virtuozzo.com> >> --- >> 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) >
that triggers a warning in checkpatch.pl WARNING: Block comments use a leading /* on a separate line >> * 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). > ok diff --git a/block/qcow2.c b/block/qcow2.c index 161b935962..05a7cbebbd 100644 --- a/block/qcow2.c +++ b/block/qcow2.c @@ -2096,16 +2096,18 @@ static int handle_alloc_space(BlockDriverState *bs, QCowL2Meta *l2meta) continue; } - /* - * Conventional place for qcow2_pre_write_overlap_check() but in this - * case it is already done for these clusters - */ - - BLKDBG_EVENT(bs->file, BLKDBG_CLUSTER_ALLOC_SPACE); /* * instead of writing zero COW buffers, * efficiently zero out the whole clusters */ + + ret = qcow2_pre_write_overlap_check(bs, 0, m->alloc_offset, + m->nb_clusters * s->cluster_size); + if (ret < 0) { + return ret; + } + + BLKDBG_EVENT(bs->file, BLKDBG_CLUSTER_ALLOC_SPACE); ret = bdrv_co_pwrite_zeroes(bs->file, m->alloc_offset, m->nb_clusters * s->cluster_size, BDRV_REQ_ALLOCATE); >> + >> + 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 <vsement...@virtuozzo.com> > > >