There was a bug  in the release notes  which had incorrect
commands for upgrading the snapshot format of an existing CephFS file
system which has had snapshots enabled at some point. The correction
is here :
diff --git a/doc/releases/mimic.rst b/doc/releases/mimic.rst
index 137d56311c..3a3345bbc0 100644
@@ -346,8 +346,8 @@ These changes occurred between the Luminous and
previous max_mds" step in above URL to fail. To re-enable the feature,
either delete all old snapshots or scrub the whole filesystem:
- - ``ceph daemon <mds of rank 0> scrub_path /``
- - ``ceph daemon <mds of rank 0> scrub_path '~mdsdir'``
+ - ``ceph daemon <mds of rank 0> scrub_path / force recursive repair``
+ - ``ceph daemon <mds of rank 0> scrub_path '~mdsdir' force
- Support has been added in Mimic for quotas in the Linux kernel
client as of v4.17.
The release notes on the blog have already been updated.
If you executed the wrong commands already, it should be sufficient to
run the correct commands once more to fix the file system.