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Re: [Qemu-devel] [GSoC 2010] Pass-through filesystem support.

From: jvrao
Subject: Re: [Qemu-devel] [GSoC 2010] Pass-through filesystem support.
Date: Tue, 13 Apr 2010 12:08:05 -0700
User-agent: Thunderbird (Windows/20100228)

jvrao wrote:
> Alexander Graf wrote:
>> On 12.04.2010, at 13:58, Jamie Lokier wrote:
>>> Mohammed Gamal wrote:
>>>> On Mon, Apr 12, 2010 at 12:29 AM, Jamie Lokier <address@hidden> wrote:
>>>>> Javier Guerra Giraldez wrote:
>>>>>> On Sat, Apr 10, 2010 at 7:42 AM, Mohammed Gamal <address@hidden> wrote:
>>>>>>> On Sat, Apr 10, 2010 at 2:12 PM, Jamie Lokier <address@hidden> wrote:
>>>>>>>> To throw a spanner in, the most widely supported filesystem across
>>>>>>>> operating systems is probably NFS, version 2 :-)
>>>>>>> Remember that Windows usage on a VM is not some rare use case, and
>>>>>>> it'd be a little bit of a pain from a user's perspective to have to
>>>>>>> install a third party NFS client for every VM they use. Having
>>>>>>> something supported on the VM out of the box is a better option IMO.
>>>>>> i don't think virtio-CIFS has any more support out of the box (on any
>>>>>> system) than virtio-9P.
>>>>> It doesn't, but at least network-CIFS tends to work ok and is the
>>>>> method of choice for Windows VMs - when you can setup Samba on the
>>>>> host (which as previously noted you cannot always do non-disruptively
>>>>> with current Sambas).
>>>>> -- Jamie
>>>> I think having support for both 9p and CIFS would be the best option.
>>>> In that case the user will have the option to use either one,
>>>> depending on how their guests support these filesystems. In that case
>>>> I'd prefer to work on CIFS support while the 9p effort can still go
>>>> on. I don't think both efforts are mutually exclusive.
>>>> What do the rest of you guys think?
>>> I only noted NFS because most old OSes do not support CIFS or 9P -
>>> especially all the old unixes.
>>> I don't think old versions of MS-DOS and Windows (95, 98, ME, Nt4?)
>>> even support current CIFS.  They need extra server settings to work
>>> - such as setting passwords on the server to non-encrypted and other quirks.
>>> Meanwhile Windows Vista/2008/7 works better with SMB2, not CIFS, to
>>> properly see symlinks and hard links.
>>> So there is no really nice out of the box file service which works
>>> easily with all guest OSes.
>>> I'm guessing that out of all the filesystems, CIFS is the most widely
>>> supported in recent OSes (released in the last 10 years).  But I'm not
>>> really sure what the state of CIFS is for non-Windows, non-Linux,
>>> non-BSD guests.
>> So what? If you want to have direct host fs access, install guest drivers. 
>> If you can't, set up networking and use CIFS or NFS or whatever.
>>> I'm not sure why 9P is being pursued.  Does anything much support it,
>>> or do all OSes except quite recent Linux need a custom driver for 9P?
>>> Even Linux only got the first commit in the kernel 5 years ago, so
>>> probably it was only about 3 years ago that it will have begun
>>> appearing in stable distros, if at all.  Filesystem passthrough to
>>> Linux guests installed in the last couple of years is a useful
>>> feature, and I know that for many people that is their only use of
>>> KVM, but compared with CIFS' broad support it seems like quite a
>>> narrow goal.
>> The goal is to have something simple and fast. We can fine-tune 9P to align 
>> with the Linux VFS structures, making it really little overhead (and little 
>> headache too). For Windows guests, nothing prevents us to expose yet another 
>> 9P flavor. That again would be aligned well with Windows's VFS and be slim 
>> and fast there.
>> The biggest problem I see with CIFS is that it's a huge beast. There are a 
>> lot of corner cases where it just doesn't fit in. See my previous mail for 
>> more details.
> As Alex mentioned, 9P is chosen for its mere simplicity and easy adaptability.
> NFS and CIFS does not give that flexibility. As we mentioned in the patch 
> series, we are 
> already seeing better numbers with 9P. Looking ahead 9P can embed KVM/QEMU 
> knowledge
> to share physical resources like memory/cache between the host and the guest.
> I think looking into the windows side of 9P client would be great option too. 
> The current patch on the mailing list supports .U (unix) protocol and will be 
> introducing
> .L (Linux) variant as we move forward.

Hi Mohammed,
Please let us know once you decide on where your interest lies.
Will be glad to have you on VirtFS (9P) though. :)

- JV

> - JV
>> Alex

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