[Qemu-devel] [PATCH v2 0/6] qcow2: cache flush fixes and performance improvements

[Stefan Hajnoczi]

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(-)

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1.8.1.4