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From Simon Weller <swel...@ena.com>
Subject RE: Adding VirtIO SCSI to KVM hypervisors
Date Sat, 21 Jan 2017 21:22:01 GMT
For the record, we've been looking into this as well.
Has anyone tried it with Windows VMs before? The standard virtio driver doesn't support spanned
disks and that's something we'd really like to enable for our customers.



Simon Weller/615-312-6068

-----Original Message-----
From: Wido den Hollander [wido@widodh.nl]
Received: Saturday, 21 Jan 2017, 2:56PM
To: Syed Ahmed [sahmed@cloudops.com]; dev@cloudstack.apache.org [dev@cloudstack.apache.org]
Subject: Re: Adding VirtIO SCSI to KVM hypervisors


> Op 21 januari 2017 om 16:15 schreef Syed Ahmed <sahmed@cloudops.com>:
>
>
> Wido,
>
> Were you thinking of adding this as a global setting? I can see why it will
> be useful. I'm happy to review any ideas you might have around this.
>

Well, not really. We don't have any structure for this in place right now to define what type
of driver/disk we present to a guest.

See my answer below.

> Thanks,
> -Syed
> On Sat, Jan 21, 2017 at 04:46 Laszlo Hornyak <laszlo.hornyak@gmail.com>
> wrote:
>
> > Hi Wido,
> >
> > If I understand correctly from the documentation and your examples, virtio
> > provides virtio interface to the guest while virtio-scsi provides scsi
> > interface, therefore an IaaS service should not replace it without user
> > request / approval. It would be probably better to let the user set what
> > kind of IO interface the VM needs.
> >

You'd say, but we already do those. Some Operating Systems get a IDE disk, others a SCSI disk
and when Linux guest support it according to our database we use VirtIO.

CloudStack has no way of telling how to present a volume to a guest. I think it would be a
bit to much to just make that configurable. That would mean extra database entries, API calls.
A bit overkill imho in this case.

VirtIO SCSI is supported by all Linux distributions for a very long time.

Wido

> > Best regards,
> > Laszlo
> >
> > On Fri, Jan 20, 2017 at 10:21 PM, Wido den Hollander <wido@widodh.nl>
> > wrote:
> >
> > > Hi,
> > >
> > > VirtIO SCSI [0] has been supported a while now by Linux and all kernels,
> > > but inside CloudStack we are not using it. There is a issue for this [1].
> > >
> > > It would bring more (theoretical) performance to VMs, but one of the
> > > motivators (for me) is that we can support TRIM/DISCARD [2].
> > >
> > > This would allow for RBD images on Ceph to shrink, but it can also give
> > > back free space on QCOW2 images if quests run fstrim. Something all
> > modern
> > > distributions all do weekly in a CRON.
> > >
> > > Now, it is simple to swap VirtIO for VirtIO SCSI. This would however mean
> > > that disks inside VMs are then called /dev/sdX instead of /dev/vdX.
> > >
> > > For GRUB and such this is no problems. This usually work on UUIDs and/or
> > > labels, but for static mounts on /dev/vdb1 for example things break.
> > >
> > > We currently don't have any configuration method on how we want to
> > present
> > > a disk to a guest, so when attaching a volume we can't say that we want
> > to
> > > use a different driver. If we think that a Operating System supports
> > VirtIO
> > > we use that driver in KVM.
> > >
> > > Any suggestion on how to add VirtIO SCSI support?
> > >
> > > Wido
> > >
> > >
> > > [0]: http://wiki.qemu.org/Features/VirtioSCSI
> > > [1]: https://issues.apache.org/jira/browse/CLOUDSTACK-8239
> > > [2]: https://issues.apache.org/jira/browse/CLOUDSTACK-8104
> > >
> >
> >
> >
> > --
> >
> > EOF
> >

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