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

[Kevin Wolf]

On Tue, Feb 19, 2013 at 04:45:00PM +0100, Stefan Hajnoczi wrote:
> 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.
> 
> Stefan Hajnoczi (6):
>   qcow2: flush refcount cache correctly in alloc_refcount_block()
>   qcow2: flush refcount cache correctly in qcow2_write_snapshots()
>   qcow2: flush caches 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 | 12 +++---------
>  block/qcow2-snapshot.c |  7 +------
>  2 files changed, 4 insertions(+), 15 deletions(-)

You touch a few bdrv_flush() callers that don't check the return value.
Could you use the opportunity to fix that as well?

Kevin