RFE: Add an option to enable virtio-scsi for new Nova instances by default

Bug #1803575 reported by s10 on 2018-11-15
This bug affects 1 person
Affects Status Importance Assigned to Milestone
OpenStack Compute (nova)

Bug Description


Currently virtio-scsi is used only for libvirt instances created from the images with properties hw_scsi_model=virtio-scsi and hw_disk_bus=scsi set or from the volumes with the same image metadata.

What is requested: config option in [libvirt] section of the nova.conf to enable virtio-scsi for all new instances by default, even if hw_scsi_model and hw_disk_bus properties for the image is not set.

Why: we want virtio-scsi to be enabled for VMs created from users images even if they don't set this property explicitly, because we want to have most of the vms be able to issue BLKDISCARD with fstrim.

A blueprint should be create.

tags: added: config libvirt
sean mooney (sean-k-mooney) wrote :

this is really a feature request not a bug.

part of the feature has been addressed by a different bluepint but the config option to enable this by defualt is still outstanding
as such im lieave this open to tack the config change.

Changed in nova:
importance: Undecided → Wishlist
status: New → Triaged

While the issue you highlight here is real, this wouldn't be a good solution. The primary issue with this approach is the same issue we have with the '[libvirt] rx_queue_size' and '[libvirt] tx_queue_size' - namely that it can break live migration as two hosts with different values will result in a change in the instance XML. If we were to take this approach, we'd have to store the information as part of the instance and we don't have a way to do this other than via the flavor or image metadata. As such, I'm closing this as WONTFIX.

With that said, we do recognize that there is a definite usability issue here. For context, the libosinfo integration in the libvirt driver [1] was supposed to resolve this kind of issue for us but the implementation of that feature is fundamentally broken and it will probably be ripped out in a future release. We're now working on an improved solution to this broader issue, but it will take a different form to this.

[1] https://specs.openstack.org/openstack/nova-specs/specs/liberty/approved/libvirt-hardware-policy-from-libosinfo.html

Changed in nova:
status: Triaged → Won't Fix
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