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


From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH 0/6] qcow2: cache flush fixes and performance improvements
Date: Wed, 20 Feb 2013 12:38:29 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

On Wed, Feb 20, 2013 at 11:27:47AM +0100, Kevin Wolf wrote:
> 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?

Sure, will send a v2.

Stefan



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