This advice only applies to users with filesystems created using versions of Ceph older than Firefly (0.80). Users creating new filesystems may disregard this advice.
Pre-firefly versions of Ceph used a now-deprecated format for storing CephFS directory objects, called TMAPs. Support for reading these in RADOS will be removed after the Jewel release of Ceph, so for upgrading CephFS users it is important to ensure that any old directory objects have been converted.
After installing Jewel on all your MDS and OSD servers, and restarting the services, run the following command:
cephfs-data-scan tmap_upgrade <metadata pool name>
This only needs to be run once, and it is not necessary to stop any other services while it runs. The command may take some time to execute, as it iterates overall objects in your metadata pool. It is safe to continue using your filesystem as normal while it executes. If the command aborts for any reason, it is safe to simply run it again.
If you are upgrading a pre-Firefly CephFS filesystem to a newer Ceph version than Jewel, you must first upgrade to Jewel and run the tmap_upgrade command before completing your upgrade to the latest version.