On 12/03/2018 17:19, Laszlo Ersek wrote: > virtio-scsi already supports discard and write zeroes, and virtio-scsi > can be backed by SSDs. Can you please describe the use case in the > commit message (in a few words) that needs discard / write zeroes but is > ill-served by virtio-scsi? > > I believe the spec (and implementations) should be conservative about > feature duplication unless there's a good reason for it. I don't doubt > you have a good reason; can you please name it in the commit message? > I'm not questioning your motives, I'd just like to see them documented > in the commit log.
Hi Laszlo, the basic reason is that virtio-scsi is slower than virtio-blk. :) Indeed DISCARD/WRITE ZEROES was a reason to use virtio-scsi instead of virtio-blk back when it was created, but at the time DISCARD was considered a thin provisioning feature; that is, the idea was that guests could cooperate with the host to save host disk space and to speed up maintenance operation such as live disk migration. Nowadays, DISCARD has evolved into a basic disk maintenance operation and IMO it fits within the scope of virtio-blk, as the backend of choice for high performance I/O devices. (Though I wouldn't be surprised however if, in a few years, virtio-blk has become obsolete and we'll just use NVMe. Recent versions of the NVMe spec have taken some inspiration from Xen and virtio ring buffers, and perform great as virtual devices; this is a similar "convergence" to what's happening in virtio for the new packed ring format, just in the opposite direction). Paolo --------------------------------------------------------------------- To unsubscribe, e-mail: virtio-dev-unsubscr...@lists.oasis-open.org For additional commands, e-mail: virtio-dev-h...@lists.oasis-open.org
