Re: [Qemu-devel] [PATCH 1/2] hw/9pfs: Add new virtfs option cache=writethrough to skip host page cache

[Aneesh Kumar K.V]

On Sat, 8 Oct 2011 12:24:37 +0100, Stefan Hajnoczi <address@hidden> wrote:
> On Fri, Oct 7, 2011 at 7:46 AM, Aneesh Kumar K.V
> <address@hidden> wrote:
> > cache=writethrough implies the file are opened in the host with O_SYNC open 
> > flag
> >
> > Signed-off-by: Aneesh Kumar K.V <address@hidden>
> > ---
> >  fsdev/file-op-9p.h         |    1 +
> >  fsdev/qemu-fsdev.c         |   10 ++++++++--
> >  fsdev/qemu-fsdev.h         |    2 ++
> >  hw/9pfs/virtio-9p-device.c |    5 +++++
> >  hw/9pfs/virtio-9p.c        |   24 ++++++++++++++++++------
> >  qemu-config.c              |    6 ++++++
> >  qemu-options.hx            |   17 ++++++++++++-----
> >  vl.c                       |    6 ++++++
> >  8 files changed, 58 insertions(+), 13 deletions(-)
> 
> When would this be used?  For serving up vanilla 9P?
> 
> I think 9P.u and 9P.l have support for fsync(2) while vanilla 9P does not.
> 

TFSYNC is added by 9p.L. So we would need this for 9p.u. Another use
case is to ensure that we don't leave pages on host as dirty. That would
ensure that large writeback from a guest don't result in large number of
dirty pages on the host, thereby resulting in writeback in the host. It
would be needed for predictable I/O behavior in a setup where we have
multiple guest.

-aneesh