MongoDB Documentation, Release 3.0.4 db.fsyncLock() is not Compatible with WiredTiger With WiredTiger the db.fsyncLock() and db.fsyncUnlock() operations cannot guarantee that the data files do not change. As a result, do not use these methods to ensure consistency for the purposes of creating backups. Support for touch Command If a storage engine does not support the touch , then the touch command will return an error. Dynamic Record Allocation MongoDB 3.0 no longer supports dynamic record allocation and deprecates padding- Factor . MongoDB 3.0 deprecates the newCollectionsUsePowerOf2Sizes parameter such that you can no longer use the parameter to disable the power of 2 sizes allocation for a collection. Instead, use the collMod command with the noPadding flag or the db.createCollection() method with the noPadding option. Only set noPadding for collections with workloads that consist only of inserts or in-place updates (such as incrementing counters). Warning: Only set noPadding to true for collections whose workloads have no update operations that cause documents to grow, such as for collections with workloads that are insert-only. For more information, see No Padding Allocation Strategy (page 91). For more information, see MMAPv1 Record Allocation Behavior Changes (page 800). Replica Set Oplog Format Change MongoDB 3.0 is not compatible with oplog entries generated by versions of MongoDB before 2.2.1. If you upgrade from one of these versions, you must wait for new oplog entries to overwrite all old oplog entries generated by one of these versions before upgrading to 3.0.0 or earlier. Secondaries may abort if they replay a pre-2.6 oplog with an index build operation that would fail on a 2.6 or later primary. Replica Set Configuration Validation MongoDB 3.0 provides a stricter validation of replica set configuration settings (page 652) and replica sets invalid replica set configurations. Stricter validations include: 805 12.1. Current Stable Release • Arbiters can only have 1 vote. Previously, arbiters could also have a value of 0 for votes . If an arbiter has any value other than 1 for votes , you must fix the setting. • Non-arbiter members can only have value of 0 or 1 for votes . If a non-arbiter member has any other value for votes , you must fix the setting. • _id in the Replica Set Configuration (page 652) must specify the same name as that specified by --replSet or replication.replSetName . Otherwise, you must fix the setting. • Disallows 0 for getLastErrorDefaults value. If getLastErrorDefaults value is 0 , you must fix the setting. • settings can only contain the recognized settings. Previously, MongoDB ignored unrecognized settings. If settings contains unrecognized settings, you must remove the unrecognized settings. Replication Changes • The MMAPv1 storage engine supports touch . • The WiredTiger storage engine does not support touch .