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

[Stefan Hajnoczi <stefanha@gmail.com>]

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.

Stefan