Re: [Qemu-devel] Virtio-9p

[Pradeep Kiruvale]

 

 Hi Greg,

Thanks for your reply. 

 

I'm Cc'ing qemu-devel like in your previous posts, so QEMU experts may jump in.

> What I understand from the requirement for our project is if we use
> virtio-blk it caches the pages in the guest. We would like to avoid that

AFAIK this is true when you pass cache=none to -drive on the QEMU command
line. But there are other options such as writeback or writethrough, which
rely on the host page cache.

Yes, we did explore these options already.

 

> and also we want to share those pages across multiple guests and when they
> need some data they should get it from the host instead of using the cached
> data at each guest.
>

So you want all the guests to attach to the same block device or backing file in
the host, correct ? AFAIK we cannot do that with virtio-blk indeed... and virtio-9p
is only about sharing files, not block devices.

Yes, we want to share the files.

 

Maybe you could share a big file between the host and all guests with 9p, and
each guest can use the loop device to access the file as a block device... but
even then, you'd have to deal with concurrent accesses...

> Basically we are trying to cut down the memory foot print of the guests.
>

If you're using KVM and your guests run the same distro or application,
you may try to use KSM (Kernel Same-page Merging) in the host.

We are using these things also, we want to reduce the foot print as much as possible.

Regards,

Pradeep

 

> Regards,
> Pradeep
>
> On 31 March 2016 at 18:12, Greg Kurz <address@hidden> wrote:
>
> > On Wed, 30 Mar 2016 16:27:48 +0200
> > Pradeep Kiruvale <address@hidden> wrote:
> >
> > > Hi Greg,
> > >
> >
> > Hi Pradeep,
> >
> > > Thanks for the reply.
> > >
> > > Let me put it this way, virtio-blk-pci is used for block IO on the
> > devices
> > > shared between the guest and the host.
> >
> > I don't really understand the "devices shared between the guest and the
> > host" wording... virtio-blk-pci exposes a virtio-blk device through PCI
> > to the guest. The virtio-blk device can be backed by a file or a block
> > device from the host.
> >
> > > Here I want to share the file and have QoS between the guests. So I am
> > > using the Virtio-9p-pci.
> > >
> >
> > What file ?
> >
> > > Basically I want to have QoS for virtio-9p-pci.
> > >
> >
> > Can you provide a more detailed scenario on the result you want to reach ?
> >
> > > Regards,
> > > Pradeep
> > >
> >
> > Cheers.
> >
> > --
> > Greg
> >
> > > On 30 March 2016 at 16:13, Greg Kurz <address@hidden> wrote:
> > >
> > > > On Wed, 30 Mar 2016 14:10:38 +0200
> > > > Pradeep Kiruvale <address@hidden> wrote:
> > > >
> > > > > Hi All,
> > > > >
> > > > > Is virtio-9p-pci device only supports the fsdev deices? I am trying
> > to
> > > > use
> > > > > -drive option for applying QoS for block device using Virtio-9p-pci
> > > > device,
> > > > > but failing to create/add a device other than fsdev. Can you please
> > help
> > > > me
> > > > > on this?
> > > > >
> > > > > Regards,
> > > > > Pradeep
> > > >
> > > > Hi Pradeep,
> > > >
> > > > Not sure to catch what you want to do but I confirm that virti-9p-pci
> > only
> > > > supports
> > > > fsdev... if you want a block device, why don't you use virtio-blk-pci ?
> > > >
> > > > Cheers.
> > > >
> > > > --
> > > > Greg
> > > >
> > > >
> >
> >