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

Registered by s10 on 2018-11-20

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.

The proposal is to add 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.

An option to enable virtio-scsi for all VMs, even if they are created from the image or booted from volume created from the image without hw_scsi_model and hw_disk_bug set explicitly gives several benefits:

1) Most of the created VMs will be able to issue BLKDISCARD with fstrim.
2) VMs will be able to have more than 26 volumes attached, up to 255.

Blueprint information

Status:
Not started
Approver:
None
Priority:
Undefined
Drafter:
None
Direction:
Needs approval
Assignee:
None
Definition:
New
Series goal:
None
Implementation:
Unknown
Milestone target:
None

Related branches

Sprints

Whiteboard

"2) VMs will be able to have more than 26 volumes attached, up to 255." overlaps with this Stein approved blueprint that melwitt is working on:

https://blueprints.launchpad.net/nova/+spec/conf-max-attach-volumes

So you might want to talk with her about that. -- mriedem 20181214

Indeed, the "number of volumes attached" part of this blueprint is covered more generally (not only virtio-scsi) by the blueprint I'm working on. Feel free to ping me if you have questions or concerns. -- melwitt 20181214

i have recently triaged the related bug for this https://bugs.launchpad.net/nova/+bug/1803575 and set it to whishlist priortity. is a config option to default to virti-scsi still
a feature you wish to be added to nova? sean-k-mooney 20190614

(?)

Work Items

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