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From Wido den Hollander <w...@widodh.nl>
Subject Re: Adding VirtIO SCSI to KVM hypervisors
Date Sat, 21 Jan 2017 20:56:26 GMT

> 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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