Merge pull request #55427 from idryomov/wip-63341-reef

reef: librbd: improve rbd_diff_iterate2() performance in fast-diff mode

Reviewed-by: Mykola Golub <mgolub@suse.com>

diff --cc PendingReleaseNotes
index 6722b1f8fe1c768e880ba16803530bffa4e9e998,d7a8ed26d25bccd6d4e9158c23376178b07b2100..2c82b13311d10a4d97ee7bc5ee4a650890ffb208
--- 1/PendingReleaseNotes
--- 2/PendingReleaseNotes
+++ b/PendingReleaseNotes
@@@ -1,5 -1,16 +1,14 @@@
+ >=18.2.2
+ --------
+ 
+ * RBD: When diffing against the beginning of time (`fromsnapname == NULL`) in
+   fast-diff mode (`whole_object == true` with `fast-diff` image feature enabled
+   and valid), diff-iterate is now guaranteed to execute locally if exclusive
+   lock is available.  This brings a dramatic performance improvement for QEMU
+   live disk synchronization and backup use cases.
+ 
  >=19.0.0
  
 -* The cephfs-shell utility is now packaged for RHEL 9 / CentOS 9 as required
 -  python dependencies are now available in EPEL9.
  * RGW: S3 multipart uploads using Server-Side Encryption now replicate correctly in
    multi-site. Previously, the replicas of such objects were corrupted on decryption.
    A new tool, ``radosgw-admin bucket resync encrypted multipart``, can be used to