rbd: create+truncate for whole-object layered discards (2bb1e56e) · Commits · 戴 / test

A whole-object layered discard is implemented as a truncate rather than a delete: a dummy object is needed to prevent the CoW machinery from kicking in. However, a truncate on a non-existent object is a no-op. If the object doesn't exist in HEAD, a discard request is effectively ignored, which violates our "discard zeroes data" promise and breaks REQ_OP_WRITE_ZEROES implementation. A non-exclusive create on an existing object is also a no-op, so the fix is to do a compound create+truncate instead. Signed-off-by:

Ilya Dryomov <idryomov@gmail.com>

drivers/block/rbd.c

+6 −1

Original line number	Diff line number	Diff line
		@@ -1884,6 +1884,8 @@ static void __rbd_obj_setup_discard(struct rbd_obj_request *obj_req,

		if (rbd_obj_is_entire(obj_req)) {
		if (obj_req->num_img_extents) {
		osd_req_op_init(obj_req->osd_req, which++,
		CEPH_OSD_OP_CREATE, 0);
		opcode = CEPH_OSD_OP_TRUNCATE;
		} else {
		osd_req_op_init(obj_req->osd_req, which++,
		@@ -1917,7 +1919,10 @@ static int rbd_obj_setup_discard(struct rbd_obj_request *obj_req)

		if (rbd_obj_is_entire(obj_req)) {
		obj_req->write_state = RBD_OBJ_WRITE_FLAT;
		num_osd_ops = 1; /* truncate/delete */
		if (obj_req->num_img_extents)
		num_osd_ops = 2; /* create + truncate */
		else
		num_osd_ops = 1; /* delete */
		} else {
		if (obj_req->num_img_extents) {
		obj_req->write_state = RBD_OBJ_WRITE_GUARD;

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