Upgrading the MDS Cluster

Currently the MDS cluster does not have built-in versioning or file system flags to support seamless upgrades of the MDSs without potentially causing assertions or other faults due to incompatible messages or other functional differences. For this reason, it’s necessary during any cluster upgrade to reduce the number of active MDS for a file system to one first so that two active MDS do not communicate with different versions.

The proper sequence for upgrading the MDS cluster is:

  1. For each file system, disable and stop standby-replay daemons.

ceph fs set <fs_name> allow_standby_replay false

In Pacific, the standby-replay daemons are stopped for you after running this command. Older versions of Ceph require you to stop these daemons manually.

ceph fs dump # find standby-replay daemons
ceph mds fail mds.<X>

  1. For each file system, reduce the number of ranks to 1:

ceph fs set <fs_name> max_mds 1

  1. Wait for cluster to stop non-zero ranks where only rank 0 is active and the rest are standbys.

ceph status # wait for MDS to finish stopping

  1. For each MDS, upgrade packages and restart. Note: to reduce failovers, it is recommended – but not strictly necessary – to first upgrade standby daemons.

# use package manager to update cluster
systemctl restart ceph-mds.target

  1. For each file system, restore the previous max_mds and allow_standby_replay settings for your cluster:

ceph fs set <fs_name> max_mds <old_max_mds>
ceph fs set <fs_name> allow_standby_replay <old_allow_standby_replay>

Upgrading pre-Firefly file systems past Jewel

Tip

This advice only applies to users with file systems created using versions of Ceph older than Firefly (0.80). Users creating new file systems 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 file system 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 file system 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.