MongoDB Documentation, Release 3.0.4 Step 2: Upgrade the cluster’s meta data. Start a single 3.0 mongos instance with the configDB pointing to the cluster’s config servers and with the --upgrade option. To run a mongos with the --upgrade option, you can upgrade an existing mongos instance to 3.0, or if you need to avoid reconfiguring a production mongos instance, you can use a new 3.0 mongos that can reach all the config servers. To upgrade the meta data, run: mongos --configdb <configDB string> --upgrade You can include the --logpath option to output the log messages to a file instead of the standard output. Also include any other options required to start mongos instances in your cluster, such as --sslOnNormalPorts or --sslPEMKeyFile . The 3.0 mongos will output informational log messages. The mongos will exit upon completion of the --upgrade process. The upgrade will prevent any chunk moves or splits from occurring during the upgrade process. If the data files have many sharded collections or if failed processes hold stale locks, acquiring the locks for all collections can take seconds or minutes. Watch the log for progress updates. Step 3: Ensure mongos --upgrade process completes successfully. The mongos will exit upon completion of the meta data upgrade process. If successful, the process will log the following messages: After a successful upgrade, restart the mongos instance. If mongos fails to start, check the log for more information. If the mongos instance loses its connection to the config servers during the upgrade or if the upgrade is otherwise unsuccessful, you may always safely retry the upgrade. <timestamp> I SHARDING [mongosMain] upgrade of config server to v6 successful ... <timestamp> I - [mongosMain] Config database is at version v6 <timestamp> I SHARDING [mongosMain] MongoS version 3.0.0 starting: ... ... <timestamp> I SHARDING [mongosMain] starting upgrade of config server from v5 to v6 <timestamp> I SHARDING [mongosMain] starting next upgrade step from v5 to v6 <timestamp> I SHARDING [mongosMain] about to log new metadata event: ... <timestamp> I SHARDING [mongosMain] checking that version of host ... is compatible with 2.6 ... <timestamp> I SHARDING [mongosMain] upgrade of config server to v6 successful ... <timestamp> I SHARDING [mongosMain] distributed lock 'configUpgrade/...' unlocked. <timestamp> I - [mongosMain] Config database is at version v6 Step 4: Upgrade the remaining mongos instances to 3.0. Upgrade and restart without the --upgrade option the other mongos instances in the sharded cluster. After you have successfully upgraded all mongos instances, you can proceed to upgrade the other components in your sharded cluster. Warning: Do not upgrade the mongod instances until after you have upgraded all the mongos instances. Chapter 12. Release Notes 814