From ee22a9d86921310672aa8775489217f3e2f5e1c6 Mon Sep 17 00:00:00 2001 From: Alberto Garcia Date: Mon, 19 Jun 2017 16:40:08 +0300 Subject: [PATCH] qcow2: Merge the writing of the COW regions with the guest data If the guest tries to write data that results on the allocation of a new cluster, instead of writing the guest data first and then the data from the COW regions, write everything together using one single I/O operation. This can improve the write performance by 25% or more, depending on several factors such as the media type, the cluster size and the I/O request size. Signed-off-by: Alberto Garcia Reviewed-by: Kevin Wolf Signed-off-by: Kevin Wolf --- block/qcow2-cluster.c | 40 ++++++++++++++++++++------- block/qcow2.c | 64 ++++++++++++++++++++++++++++++++++++------- block/qcow2.h | 7 +++++ 3 files changed, 91 insertions(+), 20 deletions(-) diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c index 3ac26d6bf7..01f210187c 100644 --- a/block/qcow2-cluster.c +++ b/block/qcow2-cluster.c @@ -776,6 +776,7 @@ static int perform_cow(BlockDriverState *bs, QCowL2Meta *m) assert(start->nb_bytes <= UINT_MAX - end->nb_bytes); assert(start->nb_bytes + end->nb_bytes <= UINT_MAX - data_bytes); 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) { return 0; @@ -807,7 +808,7 @@ static int perform_cow(BlockDriverState *bs, QCowL2Meta *m) /* The part of the buffer where the end region is located */ end_buffer = start_buffer + buffer_size - end->nb_bytes; - qemu_iovec_init(&qiov, 1); + qemu_iovec_init(&qiov, 2 + (m->data_qiov ? m->data_qiov->niov : 0)); qemu_co_mutex_unlock(&s->lock); /* First we read the existing data from both COW regions. We @@ -842,17 +843,36 @@ static int perform_cow(BlockDriverState *bs, QCowL2Meta *m) } } - /* And now we can write everything */ - qemu_iovec_reset(&qiov); - qemu_iovec_add(&qiov, start_buffer, start->nb_bytes); - ret = do_perform_cow_write(bs, m->alloc_offset, start->offset, &qiov); - if (ret < 0) { - goto fail; + /* And now we can write everything. If we have the guest data we + * can write everything in one single operation */ + if (m->data_qiov) { + qemu_iovec_reset(&qiov); + if (start->nb_bytes) { + qemu_iovec_add(&qiov, start_buffer, start->nb_bytes); + } + qemu_iovec_concat(&qiov, m->data_qiov, 0, data_bytes); + if (end->nb_bytes) { + qemu_iovec_add(&qiov, end_buffer, end->nb_bytes); + } + /* NOTE: we have a write_aio blkdebug event here followed by + * a cow_write one in do_perform_cow_write(), but there's only + * one single I/O operation */ + BLKDBG_EVENT(bs->file, BLKDBG_WRITE_AIO); + ret = do_perform_cow_write(bs, m->alloc_offset, start->offset, &qiov); + } else { + /* If there's no guest data then write both COW regions separately */ + qemu_iovec_reset(&qiov); + qemu_iovec_add(&qiov, start_buffer, start->nb_bytes); + ret = do_perform_cow_write(bs, m->alloc_offset, start->offset, &qiov); + if (ret < 0) { + goto fail; + } + + qemu_iovec_reset(&qiov); + qemu_iovec_add(&qiov, end_buffer, end->nb_bytes); + ret = do_perform_cow_write(bs, m->alloc_offset, end->offset, &qiov); } - qemu_iovec_reset(&qiov); - qemu_iovec_add(&qiov, end_buffer, end->nb_bytes); - ret = do_perform_cow_write(bs, m->alloc_offset, end->offset, &qiov); fail: qemu_co_mutex_lock(&s->lock); diff --git a/block/qcow2.c b/block/qcow2.c index b3ba5daa93..328b1d4fb5 100644 --- a/block/qcow2.c +++ b/block/qcow2.c @@ -1575,6 +1575,44 @@ fail: return ret; } +/* Check if it's possible to merge a write request with the writing of + * the data from the COW regions */ +static bool merge_cow(uint64_t offset, unsigned bytes, + QEMUIOVector *hd_qiov, QCowL2Meta *l2meta) +{ + QCowL2Meta *m; + + for (m = l2meta; m != NULL; m = m->next) { + /* If both COW regions are empty then there's nothing to merge */ + if (m->cow_start.nb_bytes == 0 && m->cow_end.nb_bytes == 0) { + continue; + } + + /* The data (middle) region must be immediately after the + * start region */ + if (l2meta_cow_start(m) + m->cow_start.nb_bytes != offset) { + continue; + } + + /* The end region must be immediately after the data (middle) + * region */ + if (m->offset + m->cow_end.offset != offset + bytes) { + continue; + } + + /* Make sure that adding both COW regions to the QEMUIOVector + * does not exceed IOV_MAX */ + if (hd_qiov->niov > IOV_MAX - 2) { + continue; + } + + m->data_qiov = hd_qiov; + return true; + } + + return false; +} + static coroutine_fn int qcow2_co_pwritev(BlockDriverState *bs, uint64_t offset, uint64_t bytes, QEMUIOVector *qiov, int flags) @@ -1657,16 +1695,22 @@ static coroutine_fn int qcow2_co_pwritev(BlockDriverState *bs, uint64_t offset, goto fail; } - 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) { - 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) { + goto fail; + } } while (l2meta != NULL) { diff --git a/block/qcow2.h b/block/qcow2.h index c26ee0a33d..87b15eb4aa 100644 --- a/block/qcow2.h +++ b/block/qcow2.h @@ -343,6 +343,13 @@ typedef struct QCowL2Meta */ Qcow2COWRegion cow_end; + /** + * 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 + * from @cow_start and @cow_end into one single write operation. + */ + QEMUIOVector *data_qiov; + /** Pointer to next L2Meta of the same write request */ struct QCowL2Meta *next; -- GitLab