Virtio vs. SCSI disk for Linux KVM guests

[danb35]

The installation guide at http://pve.proxmox.com/wiki/Installation#Virtual_Machines_.28KVM.29 suggests using virtio if the guest OS supports it. I'll be installing a CentOS 6 VM, which does seem to support virtio, but I'm seeing some comments around here suggesting that VIRTIO SCSI might be preferred. I don't see an option that says "VIRTIO SCSI", just "SCSI". Which is preferred, or are they pretty much equivalent? It will be running on local ZFS storage, if it matters.

Edit: Using PVE 4.0-57.

 

[Q-wulf]

Check out this thread:
http://forum.proxmox.com/threads/24322-Hard-Drive-Passthrough?p=122551#post122551

Might have all you need (else ask and i shall go into more depth).

 

[danb35]

That does seem to have the answer. Virtio it is. Thanks!

 

[sigxcpu]

VIRTIO SCSI brings UNMAP (TRIM) support, so your zvols will shrink if you delete blocks inside the guest.

 

[mmenaz]

But in the GUI I have VIRTIO *or* SCSI, how can I have "VIRTIO SCSI"? Thanks for the clarification
Ok, I reply to myself, then you can go to "Options" and choose "SCSI Controller Type" as "VIRTIO"

 

[sigxcpu]

You select "SCSI" as the disk type and then in "Options" tab you select VIRTIO as the SCSI controller type.

 

[danb35]

That could be a helpful point. Is there a difference in performance between VIRTIO and VIRTIO SCSI?

 

[sigxcpu]

I don't remember exactly but it might be, yes. For me, the recovery of deleted blocks is more important. For example, it is very helpful when doing a full zfs send/receive.

 

[mir]

That could be a helpful point. Is there a difference in performance between VIRTIO and VIRTIO SCSI?

Test I have done shows only marginally better performance with virtio-blk (not scsi) compared to virtio-scsi. Also know that virtio-blk development has stopped in favor of virtio-scsi.

My test setup:
- Illumos ZFS pool
- zvol over iscsi
- VM cache config: nocache
- Qemu 3.2
read: +3% in favor of virtio-blk
write: +1% in favor of virtio-blk

But the numbers has change a lot with every release of Qemu so given that this change will continue I expect to see virtio-scsi outperform virtio-blk with Qemu 3.4.

 

[dietmar]

with Qemu 3.4.

you are using wrong version numbers?

 

[mir]

Sorry, I ment 2.2 and 2.4