Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics

[Liu Yuan <namei.unix@gmail.com>]

On 01/09/2013 06:46 PM, Liu Yuan wrote:
>> 1) how slower is QEMU's emulated-writethrough mode for writes, due to
>> > the extra requests?
>> > 
> I'll collect some numbers on it.
> 

Okay I got some nubmers. I run three sheep daemon on the same host to
emulate a 3 nodes cluster, Sheepdog image has 3 copies and I put
Sheepdog client cache and Sheepdog backend storage on a tmpfs.

Guest and Host are all Linux 3.7. I start QEMU by following command:

$ qemu-system-x86_64 --enable-kvm -drive
file=~/images/test1,if=virtio,cache=writeback -smp 2 -cpu host -m 1024
-drive file=sheepdog:test,if=virtio,cache=writeback

I run 5 times 'dd if=/dev/urandom of=/dev/vdb bs=1M count=100
oflag=direct' and get the average nubmer:

    emulated (write + flush)  old impl (single write)
            13.3 M/s               13.7 M/s

boost percentage: (13.7 - 13.3)/13.3 = 3%.

If boost number is not big, but if we run QEMU and Sheep daemon on the
separate boxes, we'll expect of bigger boost because the overhead of
extra 'flush' request is increased.

Besides performance, I think backward compatibility is more important:
  1 if we run a old kernel host (quite possible for a long running
server) which doesn't support WCE, then we will never have a chance to
choose writethrough cache for guest OS against new QEMU (most users tend
to update user space tools to exclude bugs)
  2 The upper layer software which relies on the 'cache=xxx' to choose
cache mode will fail its assumption against new QEMU.

and my proposal (adding another field to BlockDriverState to allow
self-interpretation cache flag) can work well with current block layer:

Sheepdog driver behavior:
   cache flags            WCE toggled and resulting behavior
   writethrough           writethrough
   writeback              writetrhough (writeback + flush)

We can see that the writeback behavior for Guest-WCE toggling is the
same as expected. The difference is that if we set cache=writethrough,
guest can't change it via WCE toggling.

Thanks,
Yuan