Virtualization - Mar 2017 - [RFC] virtio_blk: add DISCARD support to virtio_blk driver

Currently virtio_blk driver does not provide discard feature
flag, so the filesystems which built on top of the block device
will not send discard command. This is okay for HDD backend, but
it will impact the performance for SSD backend.

Add a feature flag such as VIRTIO_BLK_F_DISCARD to virtio_blk driver
is very straightforward, but it will extend the exist virtio_blk
protocol, I'm preparing to add this feature, before that, I'd like
to get comments or suggestions from community.

---
1.9.3

On Fri, Mar 17, 2017 at 1:34 AM, Changpeng Liu <changpeng.liu at
intel.com> wrote:
> Currently virtio_blk driver does not provide discard feature
> flag, so the filesystems which built on top of the block device
> will not send discard command. This is okay for HDD backend, but
> it will impact the performance for SSD backend.
>
> Add a feature flag such as VIRTIO_BLK_F_DISCARD to virtio_blk driver
> is very straightforward, but it will extend the exist virtio_blk
> protocol, I'm preparing to add this feature, before that, I'd like
> to get comments or suggestions from community.
Please remember to also send a VIRTIO spec change:
https://www.oasis-open.org/committees/tc_home.php?wg_abbrev=virtio#feedback

This feature was not added to virtio-blk in the past because
virtio-scsi was essentially a replacement that could support all disk
I/O features (thanks to the SCSI command set).  There are many
virtio-blk users and some may not want to migrate to virtio-scsi.  So
it seems reasonable to add a DISCARD command to virtio-blk.

Stefan