If we are mapping a snapshot, we must read in the parent_overlap value
of that snapshot instead of that of the base image. Not doing so may
in particular result in us returning zeros instead of user data:
# cat overlap-snap.sh
#!/bin/bash
rbd create --size 10 --image-format 2 foo
FOO_DEV=$(rbd map foo)
dd if=/dev/urandom of=$FOO_DEV bs=1M &>/dev/null
echo "Base image"
dd if=$FOO_DEV bs=1 count=16 skip=$(((4 << 20) - 8)) 2>/dev/null | xxd
rbd snap create foo@snap
rbd snap protect foo@snap
rbd clone foo@snap bar
rbd snap create bar@snap
BAR_DEV=$(rbd map bar@snap)
echo "Snapshot"
dd if=$BAR_DEV bs=1 count=16 skip=$(((4 << 20) - 8)) 2>/dev/null | xxd
rbd resize --allow-shrink --size 4 bar
echo "Snapshot after base image resize"
dd if=$BAR_DEV bs=1 count=16 skip=$(((4 << 20) - 8)) 2>/dev/null | xxd
# ./overlap-snap.sh
Base image
0000000: e781 e33b d34b 2225 6034 2845 a2e3 36ed ...;.K"%`4(E..6.
Snapshot
0000000: e781 e33b d34b 2225 6034 2845 a2e3 36ed ...;.K"%`4(E..6.
Resizing image: 100% complete...done.
Snapshot after base image resize
0000000: e781 e33b d34b 2225 0000 0000 0000 0000 ...;.K"%........
Even though bar@snap is taken with the old bar parent_overlap (8M),
reads from bar@snap beyond the new bar parent_overlap (4M) return
zeroes. Fix it.
Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>
4d9b67cddd