From bf3d78ae55ccb276f318168d39d17a36f25221b4 Mon Sep 17 00:00:00 2001 From: Alberto Garcia Date: Wed, 11 Sep 2019 18:16:26 +0300 Subject: qcow2: Stop overwriting compressed clusters one by one handle_alloc() tries to find as many contiguous clusters that need copy-on-write as possible in order to allocate all of them at the same time. However, compressed clusters are only overwritten one by one, so let's say that we have an image with 1024 consecutive compressed clusters: qemu-img create -f qcow2 hd.qcow2 64M for f in `seq 0 64 65472`; do qemu-io -c "write -c ${f}k 64k" hd.qcow2 done In this case trying to overwrite the whole image with one large write request results in 1024 separate allocations: qemu-io -c "write 0 64M" hd.qcow2 This restriction comes from commit 095a9c58ce12afeeb90c2 from 2008. Nowadays QEMU can overwrite multiple compressed clusters just fine, and in fact it already does: as long as the first cluster that handle_alloc() finds is not compressed, all other compressed clusters in the same batch will be overwritten in one go: qemu-img create -f qcow2 hd.qcow2 64M qemu-io -c "write -z 0 64k" hd.qcow2 for f in `seq 64 64 65472`; do qemu-io -c "write -c ${f}k 64k" hd.qcow2 done Compared to the previous one, overwriting this image on my computer goes from 8.35s down to 230ms. Signed-off-by: Alberto Garcia Reviewed-by: John Snow --- block/qcow2-cluster.c | 8 +------- 1 file changed, 1 insertion(+), 7 deletions(-) diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c index f09cc992af..dcacd3c450 100644 --- a/block/qcow2-cluster.c +++ b/block/qcow2-cluster.c @@ -1351,13 +1351,7 @@ static int handle_alloc(BlockDriverState *bs, uint64_t guest_offset, } entry = be64_to_cpu(l2_slice[l2_index]); - - /* For the moment, overwrite compressed clusters one by one */ - if (entry & QCOW_OFLAG_COMPRESSED) { - nb_clusters = 1; - } else { - nb_clusters = count_cow_clusters(bs, nb_clusters, l2_slice, l2_index); - } + nb_clusters = count_cow_clusters(bs, nb_clusters, l2_slice, l2_index); /* This function is only called when there were no non-COW clusters, so if * we can't find any unallocated or COW clusters either, something is -- cgit v1.2.3-55-g7522