 1. Lock the {@link FSNamesystem} lock in {@link FSNamesystem} before calling
 * into {@link SnapshotManager} methods.<br>
 * 2. Lock the {@link FSDirectory} lock for the {@link SnapshotManager} methods
 * if necessary.
 1. Lock the {@link FSNamesystem} lock in {@link FSNamesystem} before calling
 * into {@link SnapshotManager} methods.<br>
 * 2. Lock the {@link FSDirectory} lock for the {@link SnapshotManager} methods
 * if necessary. | SnapshotManager.java