Re: [Qemu-devel] [Qemu-block] [PATCH v2] qcow2: allocate cluster_cache/cluster_data on demand

[Stefan Hajnoczi <stefanha@gmail.com>]

On Mon, Aug 21, 2017 at 02:55:30PM +0100, Stefan Hajnoczi wrote:
> Most qcow2 files are uncompressed so it is wasteful to allocate (32 + 1)
> * cluster_size + 512 bytes upfront.  Allocate s->cluster_cache and
> s->cluster_data when the first read operation is performance on a
> compressed cluster.
> 
> The buffers are freed in .bdrv_close().  .bdrv_open() no longer has any
> code paths that can allocate these buffers, so remove the free functions
> in the error code path.
> 
> This patch can result in significant memory savings when many qcow2
> disks are attached or backing file chains are long:
> 
> Before 12.81% (1,023,193,088B)
> After   5.36% (393,893,888B)
> 
> Reported-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> Tested-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> Cc: Kevin Wolf <kwolf@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
> v2:
>  * Changed EIO to ENOMEM [Eric]
>  * Added Alexey's Tested-by
> ---
>  block/qcow2-cluster.c | 17 +++++++++++++++++
>  block/qcow2.c         | 12 ------------
>  2 files changed, 17 insertions(+), 12 deletions(-)

Thanks, applied to my block-next tree:
https://github.com/stefanha/qemu/commits/block-next

Stefan