MongoDB Documentation, Release 3.0.4 Step 2: Upgrade the cluster’s meta data. Start a single 2.6 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 2.6, or if you need to avoid reconfiguring a production mongos instance, you can use a new 2.6 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 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: upgrade of config server to v5 successful Config database is at version v5 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. Step 4: Upgrade the remaining mongos instances to v2.6. Upgrade and restart without the --upgrade option the other mongos instances in the sharded cluster. After upgrading all the mongos , see Complete Sharded Cluster Upgrade (page 869) for information on upgrading the other cluster components. Complete Sharded Cluster Upgrade After you have successfully upgraded all mongos instances, you can upgrade the other instances in your MongoDB deployment. Warning: Do not upgrade mongod instances until after you have upgraded all mongos instances. While the balancer is still disabled, upgrade the components of your sharded cluster in the following order: When this process is complete, re-enable the balancer (page 718). Upgrade Procedure Once upgraded to MongoDB 2.6, you cannot downgrade to any version earlier than MongoDB 2.4. If you have text or 2dsphere indexes, you can only downgrade to MongoDB 2.4.10 or later. Except as described on this page, moving between 2.4 and 2.6 is a drop-in replacement: • Upgrade all 3 mongod config server instances, leaving the first system in the mongos --configdb argu- ment to upgrade last . • Upgrade each shard, one at a time, upgrading the mongod secondaries before running replSetStepDown and upgrading the primary of each shard. 869 12.2. Previous Stable Releases