MongoDB Documentation, Release 3.0.4 815 12.1. Current Stable Release Step 5: Upgrade the config servers. After you have successfully upgraded all mongos instances, upgrade all 3 mongod config server instances, leaving the first config server listed in the mongos --configdb argument to upgrade last . Step 6: Upgrade the shards. Upgrade each shard, one at a time, upgrading the mongod secondaries before running replSetStepDown and upgrading the primary of each shard. Step 7: Re-enable the balancer. Once the upgrade of sharded cluster components is complete, Re-enable the bal- ancer (page 718). Change Sharded Cluster Storage Engine to WiredTiger For a sharded cluster in MongoDB 3.0, you can choose to update the shards to use WiredTiger storage engine and have the config servers use MMAPv1. If you update the config servers to use WiredTiger, you must update all three config servers to use WiredTiger. Considerations Before enabling the new WiredTiger storage engine, ensure that all replica set/sharded cluster mem- bers are running at least MongoDB version 2.6.8, and preferably version 3.0.0 or newer. Note: You may safely continue to use MMAPv1 for the config servers even if the rest of your cluster is using WiredTiger Change Shards to Use WiredTiger To change the storage engine for the shards to WiredTiger, refer to the proce- dures in Change Storage Engine to WiredTiger for replica sets (page 812) and Change Storage Engine to WiredTiger for standalone mongod (page 811) as appropriate for your shards. Change Config Server to Use WiredTiger For a sharded cluster in MongoDB 3.0, you may continue to use MMAPv1 for the config servers even if the shards use WiredTiger; if, however, you decide to change the config servers to use the WiredTiger storage engine, you must change all three config servers to use WiredTiger. Note: During this process, only two config servers will be running at any given time to ensure that the sharded cluster’s metadata is read only . As a reminder, you may safely continue to use MMAPv1 for the config servers even if the rest of your cluster is using WiredTiger. Step 1: Disable the Balancer. Turn off the balancer (page 684) in the sharded cluster, as described in Disable the Balancer (page 717). Step 2: Stop the last config server listed in the mongos ‘ configDB setting. Step 3: Export data of the second config server listed in the mongos ‘ configDB setting. Specify additional options as appropriate, such as username and password if running with authorization enabled. See mongodump for available options. mongodump --out <exportDataDestination>