MongoDB Documentation, Release 3.0.4 Step 1: Shutdown the secondary member. Stop the mongod instance for the secondary member. Step 2: Prepare data directory for WiredTiger. mongod with WiredTiger will not start if the --dbpath direc- tory contains data files created with a different storage engine. For the stopped secondary member, either delete the content of the data directory or create a new data directory. If creating a new directory, ensure that the user account running mongod has read and write permissions for the new directory. Step 3: Restart the secondary member with WiredTiger. Restart the 3.0 mongod , specifying wiredTiger as the --storageEngine and the data directory for WiredTiger as the --dbpath . Specify additional options as appropriate for the member. mongod --storageEngine wiredTiger --dbpath <newWiredTigerDBPath> Since no data exists in the --dbpath , the mongod will perform an initial sync. The length of the initial sync process depends on the size of the database and network connection between members of the replica set. You can also specify the options in a configuration file . To specify the storage engine, use the new storage.engine setting. To update all members of the replica set to use WiredTiger, update the secondary members first. Then step down the primary, and update the stepped-down member. Upgrade a Sharded Cluster to 3.0 Only upgrade sharded clusters to 3.0 if all members of the cluster are currently running instances of 2.6. The only supported upgrade path for sharded clusters running 2.4 is via 2.6. The upgrade process checks all components of the cluster and will produce warnings if any component is running version 2.4. Considerations The upgrade process does not require any downtime. However, while you upgrade the sharded cluster, ensure that clients do not make changes to the collection meta-data. For example, during the upgrade, do not do any of the following: Upgrade Sharded Clusters Optional but Recommended. As a precaution, take a backup of the config database before upgrading the sharded cluster. Step 1: Disable the Balancer. Turn off the balancer (page 684) in the sharded cluster, as described in Disable the Balancer (page 717). 813 12.1. Current Stable Release • any other operation that modifies the cluster metadata in any way. See Sharding Reference (page 736) for a com- plete list of sharding commands. Note, however, that not all commands on the Sharding Reference (page 736) page modifies the cluster meta-data. • any operation that creates a database • db.dropDatabase() • db.collection.drop() • db.createCollection() • sh.addShard() • sh.shardCollection() • sh.enableSharding()