Ceph » rbd

On an image without fast-diff feature:

$ rbd create -s 1G --image-shared foo
$ sudo rbd device map foo
/dev/rbd0
$ sudo xfs_io -d -c 'pwrite 0 2M' /dev/rbd0
wrote 2097152/2097152 bytes at offset 0
$ rbd snap create foo@snap
Creating snap: 100% complete...done.
$ sudo blkdiscard -o 1M -l 2M /dev/rbd0
$ sudo rbd device unmap /dev/rbd0

Running a diff from the beginning of time to HEAD:

$ rbd diff foo
Offset  Length   Type
0       2097152  data

0~1M extent is expected instead of 0~2M extent, because the size of the object in question is 1M:

$ rbd info foo | grep block_name_prefix
        block_name_prefix: rbd_data.81db3b85b87e
$ rados -p rbd stat rbd_data.81db3b85b87e.0000000000000000
rbd/rbd_data.81db3b85b87e.0000000000000000 mtime 2023-12-08T12:26:57.000000+0100, size 1048576

This appears to be yet another bug in calc_snap_set_diff() which only ever appends to the diff:

calc_snap_set_diff start 0 end 18446744073709551614, snap_set seq 24
 clone 24 snaps [24] -> [24,24] size 2097152 overlap to next [0,1048576]
  start, after 0
  diff_to_next [1048576~1048576]
  diff now [0~2097152]
 clone 18446744073709551614 snaps [] -> [25,18446744073709551614] size 1048576 overlap to next []
 end