Users have reported that qcow2 internal snapshot creation is slow. I filed a bug report at https://bugs.launchpad.net/qemu/+bug/1126369.
This patch series reduces the time for qcow2 internal snapshot creation significantly, from more than 3 minutes to under 1 second. In the process of removing unnecessary cache flushes, I also stumbled upon instances of bdrv_flush(bs->file) where we really want to flush metadata updates. Since qcow2 caches metadata this actually does not write out the metadata updates to disk! The fix is either bdrv_flush(bs) or a more specific cache flush (e.g. refcount block cache). This series passes qemu-iotests. v2: * Use qcow2_cache_set_dependency() in qcow2_alloc_bytes() instead of forcing a flush [Kevin] * Add bdrv_flush() error code paths [Kevin] Stefan Hajnoczi (6): qcow2: flush refcount cache correctly in alloc_refcount_block() qcow2: flush refcount cache correctly in qcow2_write_snapshots() qcow2: set L2 cache dependency in qcow2_alloc_bytes() qcow2: flush in qcow2_update_snapshot_refcount() qcow2: drop flush in update_cluster_refcount() qcow2: drop unnecessary flush in qcow2_update_snapshot_refcount() block/qcow2-refcount.c | 24 ++++++++++++++---------- block/qcow2-snapshot.c | 10 ++++------ 2 files changed, 18 insertions(+), 16 deletions(-) -- 1.8.1.4
