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rbd: create+truncate for whole-object layered discards 

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>
This commit is contained in:
Ilya Dryomov 2018-02-06 19:26:34 +01:00
parent 86bd7998fa
commit 2bb1e56ec6
1 changed files with 6 additions and 1 deletions
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7
drivers/block/rbd.c
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@ -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;