Re: [PATCH v4] virtio-blk: add discard and write zeroes features to specification

[Stefan Hajnoczi <stefanha@redhat.com>]

On Fri, Mar 09, 2018 at 10:44:23AM +0800, Changpeng Liu wrote:
> Existing virtio-blk protocol doesn't have DISCARD/WRITE ZEROES support,
> this will impact the performance when using SSD backend over file systems.
> 
> Here is the proposal to extend existing virtio-blk protocol to support
> DISCARD/WRITE ZEROES commands.
> 
> Basic idea here is using 16 Bytes payload to support 1 descriptor, users
> can put several segments together with 1 DISCARD/WRITE ZEROES command.
> 
> struct virtio_blk_discard_write_zeroes {
>        le64 sector;
>        le32 num_sectors;
>        struct {
>                le32 unmap:1;
>                le32 reserved:31;
>        } flags;
> };
> 
> For the purpose to support such feature, we need to introduce 2 new feature
> flags: VIRTIO_BLK_F_DISCARD/VIRTIO_BLK_F_WRITE_ZEROES, and 2 new command
> types: VIRTIO_BLK_T_DISCARD/VIRTIO_BLK_T_WRITE_ZEROES. Also we introduce
> several new parameters in the configuration space of virtio-blk:
> max_discard_sectors/max_discard_seg/max_write_zeroes_sectors.
> These parameters will tell the OS what's the granularity when
> issuing such commands.
> 
> If both DISCARD and WRITE ZEROES are supported, unmap flag bit maybe used
> for WRITE ZEROES command with DISCARD bit enabled.
> 
> Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>
> ---
>  content.tex | 84 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 81 insertions(+), 3 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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