On Fri, Jan 18, 2019 at 3:56 PM Roman Penyaev <rpenyaev@xxxxxxx> wrote:
>
> Hi all,
>
> This is an attempt to split DISCARD and WRITE_ZEROES paths on krbd side
> when REQ_NOUNMAP flag is set for a block layer request.
Hi Roman,
I'm working on splitting DISCARD and WRITE_ZEROES handling right now.
The idea is to punt on small and/or unaligned discard requests which
don't actually free up any space but translate into a RADOS zero op.
I'm not changing how WRITE_ZEROES is implemented though, so this is
orthogonal to your work -- just wanted to give a heads up.
>
> Currently both REQ_OP_DISCARD and REQ_OP_WRITE_ZEROES block layer requests
> fall down to CEPH_OSD_OP_ZERO request, which punches holes on osd side.
>
> With a new CEPH_OSD_OP_FLAG_ZERO_NOUNMAP flag for CEPH_OSD_OP_ZERO request
> osd can zero out blocks, instead of punching holes.
REQ_NOUNMAP is just a hint, the block device is free to ignore it.
IIRC the only way to control it from userspace is through fallocate(2):
FALLOC_FL_PUNCH_HOLE can unmap, while FALLOC_FL_ZERO_RANGE is supposed
to not unmap. Given that fallocate(2) on block devices is fairly new,
I'm curious if you have an application that actually cares in mind?
Thanks,
Ilya
