diff --git a/qa/suites/krbd/rbd-nomount/tasks/krbd_fallocate.yaml b/qa/suites/krbd/rbd-nomount/tasks/krbd_fallocate.yaml
new file mode 100644
index 00000000000..a7286982409
--- /dev/null
+++ b/qa/suites/krbd/rbd-nomount/tasks/krbd_fallocate.yaml
@@ -0,0 +1,5 @@
+tasks:
+- workunit:
+    clients:
+      all:
+        - rbd/krbd_fallocate.sh
diff --git a/qa/workunits/rbd/krbd_fallocate.sh b/qa/workunits/rbd/krbd_fallocate.sh
new file mode 100755
index 00000000000..05fc8a98c64
--- /dev/null
+++ b/qa/workunits/rbd/krbd_fallocate.sh
@@ -0,0 +1,124 @@
+#!/bin/bash
+
+# This documents the state of things as of 4.12-rc4.
+#
+# - fallocate -z deallocates because BLKDEV_ZERO_NOUNMAP hint is ignored by
+# krbd
+#
+# - unaligned fallocate -z/-p appear to not deallocate -- see caveat #2 in
+# linux.git commit 6ac56951dc10 ("rbd: implement REQ_OP_WRITE_ZEROES")
+
+set -ex
+
+# no blkdiscard(8) in trusty
+function py_blkdiscard() {
+    local offset=$1
+
+    python <<EOF
+import fcntl, struct
+BLKDISCARD = 0x1277
+with open('$DEV', 'w') as dev:
+    fcntl.ioctl(dev, BLKDISCARD, struct.pack('QQ', $offset, $IMAGE_SIZE - $offset))
+EOF
+}
+
+# fallocate(1) in trusty doesn't support -z/-p
+function py_fallocate() {
+    local mode=$1
+    local offset=$2
+
+    python <<EOF
+import os, ctypes, ctypes.util
+FALLOC_FL_KEEP_SIZE = 0x01
+FALLOC_FL_PUNCH_HOLE = 0x02
+FALLOC_FL_ZERO_RANGE = 0x10
+libc = ctypes.CDLL(ctypes.util.find_library('c'), use_errno=True)
+with open('$DEV', 'w') as dev:
+    if libc.fallocate(dev.fileno(), ctypes.c_int($mode), ctypes.c_long($offset), ctypes.c_long($IMAGE_SIZE - $offset)):
+        err = ctypes.get_errno()
+        raise OSError(err, os.strerror(err))
+EOF
+}
+
+function allocate() {
+    xfs_io -c "pwrite -b $OBJECT_SIZE -W 0 $IMAGE_SIZE" $DEV
+    cmp <(od -xAx $DEV) - <<EOF
+000000 cdcd cdcd cdcd cdcd cdcd cdcd cdcd cdcd
+*
+$(printf %x $IMAGE_SIZE)
+EOF
+    [[ $(rados -p rbd ls | grep -c rbd_data.$IMAGE_ID) -eq $NUM_OBJECTS ]]
+}
+
+function assert_deallocated() {
+    cmp <(od -xAx $DEV) - <<EOF
+000000 0000 0000 0000 0000 0000 0000 0000 0000
+*
+$(printf %x $IMAGE_SIZE)
+EOF
+    [[ $(rados -p rbd ls | grep -c rbd_data.$IMAGE_ID) -eq 0 ]]
+}
+
+function assert_deallocated_unaligned() {
+    local num_objects_expected=$1
+
+    cmp <(od -xAx $DEV) - <<EOF
+000000 cdcd cdcd cdcd cdcd cdcd cdcd cdcd cdcd
+*
+$(printf %x $((OBJECT_SIZE / 2))) 0000 0000 0000 0000 0000 0000 0000 0000
+*
+$(printf %x $IMAGE_SIZE)
+EOF
+    [[ $(rados -p rbd ls | grep -c rbd_data.$IMAGE_ID) -eq $num_objects_expected ]]
+    for ((i = 0; i < $num_objects_expected; i++)); do
+        rados -p rbd stat rbd_data.$IMAGE_ID.$(printf %016x $i) | grep "size $((OBJECT_SIZE / 2))"
+    done
+}
+
+IMAGE_NAME="fallocate-test"
+
+rbd create --size 200 $IMAGE_NAME
+
+IMAGE_SIZE=$(rbd info --format=json $IMAGE_NAME | python -c 'import sys, json; print json.load(sys.stdin)["size"]')
+OBJECT_SIZE=$(rbd info --format=json $IMAGE_NAME | python -c 'import sys, json; print json.load(sys.stdin)["object_size"]')
+NUM_OBJECTS=$((IMAGE_SIZE / OBJECT_SIZE))
+[[ $((IMAGE_SIZE % OBJECT_SIZE)) -eq 0 ]]
+
+IMAGE_ID="$(rbd info --format=json $IMAGE_NAME |
+    python -c "import sys, json; print json.load(sys.stdin)['block_name_prefix'].split('.')[1]")"
+
+DEV=$(sudo rbd map $IMAGE_NAME)
+
+# blkdev_issue_discard
+allocate
+py_blkdiscard 0
+assert_deallocated
+
+# blkdev_issue_zeroout w/ BLKDEV_ZERO_NOUNMAP
+allocate
+py_fallocate FALLOC_FL_ZERO_RANGE\|FALLOC_FL_KEEP_SIZE 0
+assert_deallocated
+
+# blkdev_issue_zeroout w/ BLKDEV_ZERO_NOFALLBACK
+allocate
+py_fallocate FALLOC_FL_PUNCH_HOLE\|FALLOC_FL_KEEP_SIZE 0
+assert_deallocated
+
+# unaligned blkdev_issue_discard
+allocate
+py_blkdiscard $((OBJECT_SIZE / 2))
+assert_deallocated_unaligned 1
+
+# unaligned blkdev_issue_zeroout w/ BLKDEV_ZERO_NOUNMAP
+allocate
+py_fallocate FALLOC_FL_ZERO_RANGE\|FALLOC_FL_KEEP_SIZE $((OBJECT_SIZE / 2))
+assert_deallocated_unaligned $NUM_OBJECTS
+
+# unaligned blkdev_issue_zeroout w/ BLKDEV_ZERO_NOFALLBACK
+allocate
+py_fallocate FALLOC_FL_PUNCH_HOLE\|FALLOC_FL_KEEP_SIZE $((OBJECT_SIZE / 2))
+assert_deallocated_unaligned $NUM_OBJECTS
+
+sudo rbd unmap $DEV
+
+echo OK