From bf4a66eed423694cfa0b3b5692211264022e2b98 Mon Sep 17 00:00:00 2001 From: Alberto Garcia Date: Fri, 10 Jul 2020 18:13:09 +0200 Subject: [PATCH] qcow2: Add subcluster support to handle_alloc_space() The bdrv_co_pwrite_zeroes() call here fills complete clusters with zeroes, but it can happen that some subclusters are not part of the write request or the copy-on-write. This patch makes sure that only the affected subclusters are overwritten. A potential improvement would be to also fill with zeroes the other subclusters if we can guarantee that we are not overwriting existing data. However this would waste more disk space, so we should first evaluate if it's really worth doing. Signed-off-by: Alberto Garcia Reviewed-by: Max Reitz Reviewed-by: Vladimir Sementsov-Ogievskiy Message-Id: Signed-off-by: Max Reitz --- block/qcow2.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff --git a/block/qcow2.c b/block/qcow2.c index 60192f1be3..9990535c46 100644 --- a/block/qcow2.c +++ b/block/qcow2.c @@ -2421,6 +2421,9 @@ static int handle_alloc_space(BlockDriverState *bs, QCowL2Meta *l2meta) for (m = l2meta; m != NULL; m = m->next) { int ret; + uint64_t start_offset = m->alloc_offset + m->cow_start.offset; + unsigned nb_bytes = m->cow_end.offset + m->cow_end.nb_bytes - + m->cow_start.offset; if (!m->cow_start.nb_bytes && !m->cow_end.nb_bytes) { continue; @@ -2435,16 +2438,14 @@ static int handle_alloc_space(BlockDriverState *bs, QCowL2Meta *l2meta) * efficiently zero out the whole clusters */ - ret = qcow2_pre_write_overlap_check(bs, 0, m->alloc_offset, - m->nb_clusters * s->cluster_size, + ret = qcow2_pre_write_overlap_check(bs, 0, start_offset, nb_bytes, true); if (ret < 0) { return ret; } BLKDBG_EVENT(bs->file, BLKDBG_CLUSTER_ALLOC_SPACE); - ret = bdrv_co_pwrite_zeroes(s->data_file, m->alloc_offset, - m->nb_clusters * s->cluster_size, + ret = bdrv_co_pwrite_zeroes(s->data_file, start_offset, nb_bytes, BDRV_REQ_NO_FALLBACK); if (ret < 0) { if (ret != -ENOTSUP && ret != -EAGAIN) { -- 2.30.2