Re: [Qemu-devel] [RFC PATCH] qcow2: add a readahead cache for qcow2_decompress_cluster

[Fam Zheng <famz@redhat.com>]

On 2013年12月27日 00:19, Peter Lieven wrote:
> while evaluatiing compressed qcow2 images as a good basis for
> virtual machine templates I found out that there are a lot
> of partly redundant (compressed clusters have common physical
> sectors) and relatively short reads.
>
> This doesn't hurt if the image resides on a local
> filesystem where we can benefit from the local page cache,
> but it adds a lot of penalty when accessing remote images
> on NFS or similar exports.
>
> This patch effectevily implements a readahead of 2 * cluster_size
> which is 2 * 64kB per default resulting in 128kB readahead. This
> is the common setting for Linux for instance.
>
> For example this leads to the following times when converting
> a compressed qcow2 image to a local tmpfs partition.
>
> Old:
> time ./qemu-img convert nfs://10.0.0.1/export/VC-Ubuntu-LTS-12.04.2-64bit.qcow2 /tmp/test.raw
> real	0m24.681s
> user	0m8.597s
> sys	0m4.084s
>
> New:
> time ./qemu-img convert nfs://10.0.0.1/export/VC-Ubuntu-LTS-12.04.2-64bit.qcow2 /tmp/test.raw
> real	0m16.121s
> user	0m7.932s
> sys	0m2.244s
>
> Signed-off-by: Peter Lieven <pl@kamp.de>
> ---
>   block/qcow2-cluster.c |   27 +++++++++++++++++++++++++--
>   block/qcow2.h         |    1 +
>   2 files changed, 26 insertions(+), 2 deletions(-)

I like this idea, but here's a question. Actually, this penalty is 
common to all protocol drivers: curl, gluster, whatever. Readahead is 
not only good for compression processing, but also quite helpful for 
boot: BIOS and GRUB may send sequential 1 sector IO, synchronously, thus 
suffer from high latency of network communication. So I think if we want 
to do this, we will want to share it with other format and protocol 
combinations.

Fam