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

From: Stefan Hajnoczi
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 11:11:46 +0000

On Tue, Mar 15, 2011 at 9:19 AM, Aneesh Kumar K. V
<address@hidden> wrote:
> 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.

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.


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