| Interface | Description | 
|---|---|
| BPServiceActorAction | Base class for BPServiceActor class
 Issued by BPOfferSerivce class to tell BPServiceActor 
 to take several actions. | 
| DiskBalancer.BlockMover | BlockMover supports moving blocks across Volumes. | 
| ReplicaInPipeline | This defines the interface of a replica in Pipeline that's being written to | 
| ShortCircuitRegistry.Visitor | 
| Class | Description | 
|---|---|
| ChunkChecksum | holder class that holds checksum bytes and the length in a block at which
 the checksum bytes end
 
 ex: length = 1023 and checksum is 4 bytes which is for 512 bytes, then
     the checksum applies for the last chunk, or bytes 512 - 1023 | 
| DirectoryScanner.BlockPoolReport | Helper class for compiling block info reports per block pool. | 
| DirectoryScanner.ScanInfoVolumeReport | Helper class for compiling block info reports from report compiler threads. | 
| DiskBalancer.DiskBalancerMover | Actual DataMover class for DiskBalancer. | 
| DiskBalancer.VolumePair | Holds source and dest volumes UUIDs and their BasePaths
 that disk balancer will be operating against. | 
| ErrorReportAction | A ErrorReportAction is an instruction issued by BPOfferService to 
 BPServiceActor about a particular block encapsulated in errorMessage. | 
| FinalizedReplica | This class describes a replica that has been finalized. | 
| FSCachingGetSpaceUsed.Builder | The builder class. | 
| LocalReplica | This class is used for all replicas which are on local storage media
 and hence, are backed by files. | 
| LocalReplica.ReplicaDirInfo | |
| LocalReplicaInPipeline | This class defines a replica in a pipeline, which
 includes a persistent replica being written to by a dfs client or
 a temporary replica being replicated by a source datanode or
 being copied for the balancing purpose. | 
| ReplicaBeingWritten | This class represents replicas being written. | 
| ReplicaBuilder | This class is to be used as a builder for  ReplicaInfoobjects. | 
| ReplicaHandler | This class includes a replica being actively written and the reference to
 the fs volume where this replica is located. | 
| ReplicaUnderRecovery | This class represents replicas that are under block recovery
 It has a recovery id that is equal to the generation stamp 
 that the replica will be bumped to after recovery
 The recovery id is used to handle multiple concurrent block recoveries. | 
| ReplicaWaitingToBeRecovered | This class represents a replica that is waiting to be recovered. | 
| ReportBadBlockAction | ReportBadBlockAction is an instruction issued by {{BPOfferService}} to
 {{BPServiceActor}} to report bad block to namenode | 
| SecureDataNodeStarter | Utility class to start a datanode in a secure cluster, first obtaining 
 privileged resources before main startup and handing them to the datanode. | 
| SecureDataNodeStarter.SecureResources | Stash necessary resources needed for datanode operation in a secure env. | 
| ShortCircuitRegistry | Manages client short-circuit memory segments on the DataNode. | 
| ShortCircuitRegistry.NewShmInfo | |
| ShortCircuitRegistry.RegisteredShm | |
| StorageLocation.CheckContext | Class to hold the parameters for running a  StorageLocation.check(org.apache.hadoop.hdfs.server.datanode.StorageLocation.CheckContext). | 
| VolumeScanner | VolumeScanner scans a single volume. | 
| Enum | Description | 
|---|---|
| DataNodeLayoutVersion.Feature | Enums for features that change the layout version. | 
| FileIoProvider.OPERATION | Lists the types of file system operations. | 
| Exception | Description | 
|---|---|
| BPServiceActorActionException | |
| DiskFileCorruptException | When kernel report a "Input/output error", we use this exception to
 represents some corruption(e.g. | 
| ReplicaAlreadyExistsException | Exception indicating that the target block already exists 
 and is not set to be recovered/overwritten. | 
| UnexpectedReplicaStateException | Exception indicating that the replica is in an unexpected state | 
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