Godeps/_workspace/src/gopkg.in/mgo.v2/session.go
func (s *Session) FsyncLock() error {
	return s.Run(bson.D{{"fsync", 1}, {"lock", true}}, nil)
}
func (s *Session) FsyncLock() error {
	return s.Run(bson.D{{"fsync", 1}, {"lock", true}}, nil)
}
// FsyncLock locks all writes in the specific server the session is
// established with and returns. Any writes attempted to the server
// after it is successfully locked will block until FsyncUnlock is
// called for the same server.
//
// This method works on secondaries as well, preventing the oplog from
// being flushed while the server is locked, but since only the server
// connected to is locked, for locking specific secondaries it may be
// necessary to establish a connection directly to the secondary (see
// Dial's connect=direct option).
//
// As an important caveat, note that once a write is attempted and
// blocks, follow up reads will block as well due to the way the
// lock is internally implemented in the server. More details at:
//
//     https://jira.mongodb.org/browse/SERVER-4243
//
// FsyncLock is often used for performing consistent backups of
// the database files on disk.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/fsync+Command
//     http://www.mongodb.org/display/DOCS/Backups
//