Re: [Qemu-devel] [PATCH -V3 7/8] hw/9pfs: Add new virtfs option cache=none to skip host page cache

["Aneesh Kumar K. V" <aneesh.kumar@linux.vnet.ibm.com>]

On Mon, 14 Mar 2011 10:20:57 +0000, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> On Sun, Mar 13, 2011 at 7:04 PM, Aneesh Kumar K. V
> <aneesh.kumar@linux.vnet.ibm.com> wrote:
> > On Sun, 13 Mar 2011 17:23:50 +0000, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> >> On Sat, Mar 5, 2011 at 5:52 PM, Aneesh Kumar K.V
> >> <aneesh.kumar@linux.vnet.ibm.com> wrote:
> >> > cache=none implies the file are opened in the host with O_SYNC open flag
> >>
> >> O_SYNC does not bypass the host page cache.  It ensures that writes
> >> only complete once data has been written to the disk.
> >>
> >> O_DIRECT is a hint to bypass the host page cache when possible.
> >>
> >> A boolean on|off option would be nicer than an option that takes the
> >> special string "none".  For example, direct=on|off.  It also makes the
> >> code nicer by using bools instead of strdup strings that get leaked.
> >>
> >
> > What i wanted is the O_SYNC behavior. Well the comment should be updated. I
> > want to make sure that we don't have dirty data in host page cache after
> > a write. It is always good to make read hit the page cache
> 
> Why silently enforce O_SYNC on the server side?  The client does not
> know whether or not O_SYNC is in effect, cannot take advantage of that
> knowledge, and cannot control it.
> 
> I think a more useful solution is a 9p client mount option called
> "sync" that caused the client to always add O_SYNC and skip syncfs.
> The whole stack becomes aware of O_SYNC and clients are in control
> over whether or not they need O_SYNC semantics.

The cache=none specifically enables us to ignore the tsyncfs request on
host. tsyncfs on host can be really slow in certain setup.

-aneesh