MongoDB Documentation, Release 3.0.4 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. Repeat for the remaining the secondary members. Once all the secondary members have switched to MMAPv1, step down the primary, and update the stepped-down member. Downgrade Binaries Once upgraded to MongoDB 3.0, you cannot downgrade to a version lower than 2.6.5 . The following steps outline a “rolling” downgrade process for the replica set. The “rolling” downgrade process minimizes downtime by downgrading the members individually while the other members are available: Step 1: Downgrade secondary members of the replica set. Downgrade each secondary member of the replica set, one at a time: Step 2: Step down the primary. Use rs.stepDown() in the mongo shell to step down the primary and force the normal failover (page 580) procedure. rs.stepDown() rs.stepDown() expedites the failover procedure and is preferable to shutting down the primary directly. Step 3: Replace and restart former primary mongod . When rs.status() shows that the primary has stepped down and another member has assumed PRIMARY state, shut down the previous primary and replace the mongod binary with the 2.6 binary and start the new instance. Replica set failover is not instant but will render the set unavailable to writes and interrupt reads until the failover pro- cess completes. Typically this takes 10 seconds or more. You may wish to plan the downgrade during a predetermined maintenance window. Requirements While the downgrade is in progress, you cannot make changes to the collection meta-data. For example, during the downgrade, do not do any of the following: 823 12.1. Current Stable Release Downgrade a 3.0 Sharded Cluster • sh.enableSharding() • sh.shardCollection() • sh.addShard() • db.createCollection() • db.collection.drop() • db.dropDatabase() • any operation that creates a database 1. Shut down the mongod . See Stop mongod Processes (page 225) for instructions on safely terminating mongod processes. 2. Replace the 3.0 binary with the 2.6 binary and restart. 3. Wait for the member to recover to SECONDARY state before downgrading the next secondary. To check the member’s state, use the rs.status() method in the mongo shell.