[Virtio-fs] Can not set high msize with virtio-9p (Was: Re: virtiofs vs 9p performance)

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, Sep 25, 2020 at 10:06:41AM +0200, Christian Schoenebeck wrote:
> On Freitag, 25. September 2020 00:10:23 CEST Vivek Goyal wrote:
> > In my testing, with cache=none, virtiofs performed better than 9p in
> > all the fio jobs I was running. For the case of cache=auto  for virtiofs
> > (with xattr enabled), 9p performed better in certain write workloads. I
> > have identified root cause of that problem and working on
> > HANDLE_KILLPRIV_V2 patches to improve WRITE performance of virtiofs
> > with cache=auto and xattr enabled.
> 
> Please note, when it comes to performance aspects, you should set a reasonable 
> high value for 'msize' on 9p client side:
> https://wiki.qemu.org/Documentation/9psetup#msize

Hi Christian,

I am not able to set msize to a higher value. If I try to specify msize
16MB, and then read back msize from /proc/mounts, it sees to cap it
at 512000. Is that intended?

$ mount -t 9p -o trans=virtio,version=9p2000.L,cache=none,msize=16777216 hostShared /mnt/virtio-9p

$ cat /proc/mounts | grep 9p
hostShared /mnt/virtio-9p 9p rw,sync,dirsync,relatime,access=client,msize=512000,trans=virtio 0 0

I am using 5.11 kernel.

Thanks
Vivek