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


From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH 0/6] qcow2: cache flush fixes and performance improvements
Date: Tue, 19 Feb 2013 14:28:22 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130110 Thunderbird/17.0.2

On 02/19/2013 08:45 AM, 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(-)

Always fun when a net decrease in code results in better performance.

Series: Reviewed-by: Eric Blake <address@hidden>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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