Re: [Qemu-devel] Para-virtualized ram-based filesystem?

["Ritchie, Stuart" <Stuart.Ritchie@tellabs.com>]

On 4/16/11 1:52 AM, "Stefan Hajnoczi" <stefanha@gmail.com> wrote:

>On Sat, Apr 16, 2011 at 1:27 AM, Brad Hards <bradh@frogmouth.net> wrote:
>> On Saturday 16 April 2011 09:58:32 Ritchie, Stuart wrote:
>>> How does that sound?
>> As a general user: Confusing.
>>
>> Is there a concrete example (specific applications, specific
>>performance issues,
>> specific requirements) that you can share?
>
>I'm also wondering why you want this.

The reason why we want this is the same reason why anyone would want
mmap() and tmpfs/ramfs in the first place: zero-copy, in-place access to
your data.

>
>Does it matter if the files get pushed out to swap on the host?

We don't run in a swapping environment.  But someone who does and accepts
the performance hit, then I don't see why it would matter.

Ramfs does not kick pages out of the page-cache.  But tmpfs does -- the
host should make this transparent, as it does now.

>
>It's tempting to take advantage of running virtualized but then things
>like migration get in the way.  Have you actually tried out network
>file systems and determined they won't work for some reason?
>
>Stefan

For performance reasons it's very important for our system that the data
be as close to the app as possible.  We can't afford to push data through
an I/O channel.

What I'm really suggesting here is a way for guest applications to mmap()
host memory.

Combine that with a virt-aware robust mutex and you've probably got the
most flexible, performant, inter-guest sharing/communication mechanism
possible.  (Semaphores through a socket?  On the same system?  You gotta
be kidding. :-)

Cheers,
--Stuart


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