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Re: [Qemu-devel] [PATCH -V3 7/8] hw/9pfs: Add new virtfs option cache=no

From: Aneesh Kumar K. V
Subject: Re: [Qemu-devel] [PATCH -V3 7/8] hw/9pfs: Add new virtfs option cache=none to skip host page cache
Date: Tue, 15 Mar 2011 14:49:22 +0530
User-agent: Notmuch/0.5-66-g70c5e2c (http://notmuchmail.org) Emacs/23.1.1 (i686-pc-linux-gnu)

On Mon, 14 Mar 2011 10:20:57 +0000, Stefan Hajnoczi <address@hidden> wrote:
> On Sun, Mar 13, 2011 at 7:04 PM, Aneesh Kumar K. V
> <address@hidden> wrote:
> > On Sun, 13 Mar 2011 17:23:50 +0000, Stefan Hajnoczi <address@hidden> wrote:
> >> On Sat, Mar 5, 2011 at 5:52 PM, Aneesh Kumar K.V
> >> <address@hidden> 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.


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