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 <p...@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
