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 Tue, 15 Mar 2011 11:11:46 +0000, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> On Tue, Mar 15, 2011 at 9:19 AM, Aneesh Kumar K. V
> <aneesh.kumar@linux.vnet.ibm.com> wrote:
> > 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.
> 
> If I'm a client with the "sync" mount option all my fids are O_SYNC
> and I do not need to send TSYNCFS requests to the server because my
> fids are already stable.

Having sync mount option is useful, Infact for dotu we already default
O_SYNC on the client side because we don't have tsyncfs. But being able
to avoid the tfsyncfs flush from the server point of view also is
nice. Consider a setup where one doesn't have control on the guest
mount option but can control the qemu export options.

-aneesh