Hadoop Distributed File System

Hadoop is:

  • An open source, Java-based software framework

  • Supports the processing of large data sets in a distributed computing environment

  • Designed to scale up from a single server to thousands of machines

  • Has a very high degree of fault tolerance

  • Possible to run application on systems with thousands of nodes involving thousands of terabytes

  • Consists of two main layers:
    • Hadoop Distributed File Systems(HDFS): Distributes data

    • Map/Reduce: Distributes application


  • File system component of Hadoop

  • Master/Slave architecture

  • Separately stores file system metadata and application data

  • Stores metadata on a dedicated server(Master), called the NameNode

  • Stores application data on a number of other servers(Slave), called DataNodes

  • Single NameNode and many DataNodes(100s or 1000s of nodes)


  • Comprised of interconnected clusters of nodes where files and directories reside

  • A single NameNode, that manages the file system namespace

  • DataNodes store data as blocks within files.

High-level architecture of HDFS

(High-level view)

HDFS architecture

(HDFS Architecture)

NameNode and DataNodes

  • NameNode:

    • Bookkeeper of HDFS. Repository for all HDFS metadata

    • Its function is memory and I/O intensive. So the server hosting the NameNode typically does not store any user data

    • Executes file system namespace operations; like; opening, closing, and renaming files and directories

    • Maintains the namespace tree and the mapping of file blocks to DataNodes

    • Files and directories are represented by inodes

    • Regulates client access to files

  • Secondary NameNode:

    • An assistant daemon for monitoring the state of the cluster HDFS

    • Like the NameNode, each cluster has one Secondary NameNode

    • Differs from the NameNode:
      • It does not receive or record any real-time changes to HDFS

      • Instead, it communicates with the NameNode to take snapshots of HDFS metadata at intervals defined by the cluster configuration

Secondary NameNode
  • DataNodes:

    • Manage storage attached to the nodes that they run on

    • A file is splitted into one or more blocks (typically 128MB). These blocks are stored in a set of DataNodes

    • Performs block creation, deletion, and replication upon instruction from the NameNode

    • Data blocks are replicated for fault tolerance and fast access

    • DataNodes send heartbeats to the NameNode to confirm that the DataNode is working and the block replicas it hosts are available

    • Responsible for serving read and write requests from the file system’s client

File System Namespace

  • Traditional hierarchical file organization

  • User/Application can:
    • Create directories

    • Create/delete/remove/rename a file

    • Maintains the file system

    • Any meta information changes to the file system recorded by the NameNode

File Read and Write

  • Single-writer, multiple-reader model

  • An HDFS file consists of blocks

  • When there is a need for a new block, the NameNode allocates a block with a unique block ID

  • It also determines a list of DataNodes to host replicas of the block

  • The DataNodes forms a pipeline, the order of which minimizes the total network distance from the client to the last DataNode

HDFS Client

  • User applications access the file system using this client

  • Supports operations to read, write and delete files, and operations to create and delete directories

Read Operation

When an application reads a file, the HDFS client first asks the NameNode for the list of DataNodes that host replicas of the blocks of the file. It then contacts a DataNode directly and requests the transfer of the desired block

File Read

Write Operation

When a client writes, it first asks the NameNode to choose DataNodes to host replicas of the first block of the file. The client organizes a pipeline from node-to-node and sends the data. When the first block is filled, the client requests new DataNodes to be chosen to host replicas of the next block. A new pipeline is organized, and client sends the further bytes of the file

Client Interaction

(An HDFS client creates a new file by giving its path to the NameNode. For each block of the file, the NameNode returns a list of DataNodes to host its replicas. The client then pipelines data to the chosen DataNodes, which eventually confirm the creation of the block replicas to the NameNode)

HDFS provides an API that exposes the locations of a file blocks. This allows applications like the MapReduce framework to schedule a task to where the data are located

4. Data Replication

  • Blocks of a file are replicated for fault tolerance

  • An application can specify the number of replicas of a file

  • The NameNode makes all decisions regarding replication of blocks

  • NameNode periodically receives a Heartbeat and a Block-report from each of the DataNodes in the cluster

  • Heartbeat implies that the DataNode is functioning properly. A Block-report contains a list of all blocks on a DataNode

Replica Placement

  • The placement of replicas are critical to HDFS reliability and performance

  • Rack-aware replica placement model is used to improve reliability, availability and network bandwidth utilization

  • Communication between two nodes in different racks has to go through switches

  • Network bandwidth between machines in the same rack is greater than network bandwidth between machines in different racks

  • NameNode determines the rack id for each DataNode

  • Replicas are typically placed on unique racks. This prevents losing data when an entire rack fails and allows use of bandwidth from multiple racks when reading
    • easy to balance load on component failure, but not an optimal process

    • increase the cost of writes because a write needs to transfer blocks to multiple racks

  • To minimize cost, HDFS put (replication factor is 3) one replica on one node in the local rack, another on a different node in the local rack, and the last on a different node in a different rack

  • Thus 1/3 of replicas are on one node, 2/3 of replicas are on one rack, and the last 1/3 are evenly distributed across the remaining racks

Replica Selection

  • To minimize global bandwidth consumption and read latency, HDFS tries to read a block from a replica that is closest to the reader

  • If there is a replica on the reader node, then that replica is referred

  • Else if there is a replica on the same rack as the reader node, then that one is preferred

  • For a HDFS cluster, that may span multiple data centers, replica in the local data center is preferred over the remote one

File System Metadata

File System Meta-Data
  • HDFS namespace is stored by NameNode

  • NameNode uses a transaction log called the EditLog to record every change that occurs to the file system meta data
    • creating a new file

    • change replication factor of a file

  • Editlog is stored in the NameNode’s local filesystem

  • Entire filesystem namespace including mapping of blocks to files and file system properties is stored in a file FsImage, stored in NameNode’s local filesystem


  • Primary objective of HDFS is to store data reliably even in the presence of failure

  • Three common types of failures:
    • NameNode failures

    • DataNode failures

    • Network partitions

DataNode failure and Heartbeat

  • A network partition can cause a subset of DataNodes to lose connectivity with the NameNode

  • NameNode detects this condition by the absence of a Heartbeat message

  • NameNode marks DataNodes without Heartbeat and does not send any IO requests to them

  • Any data registered to the failed DataNode is not available to the HDFS

  • Also the death of DataNode may cause replication factor of some data blocks to fall below their specified value


  • NameNode constantly tracks which blocks need to be replicated and initiates replication whenever necessary

  • Necessity of replication may arise due to:
    • A DataNode may become unavailable

    • A replica may become corrupted

    • A hard disk on a DataNode may fail

    • The replication factor of a file may be increased

Cluster Re-balancing

  • HDFS architecture is compatible with data re-balancing schemes

  • A scheme might move data from one DataNode to another if the free space on a DataNode falls below a certain threshold

  • In the event of a sudden high demand for a particular file, a scheme might dynamically create additional replicas and re-balance other data in the cluster

  • These types of data re-balancing are not yet implemented

Data Integrity

  • Consider a situation, a block of data fetched from DataNode arrives corrupted

  • This corruption may occur because of faults in a storage device, network faults, or buggy software

  • A HDFS client creates the checksum of every block of its file and stores it in hidden files in the HDFS namespace

  • When a clients retrieves the contents of file, it verifies that the corresponding checksums match

  • If does not match, the client can retrieve the block from a replica

Metadata Disk Failure

  • FsImage and EditLog are central data structures of HDFS

  • A corruption of these files can cause a HDFS instance to be non-functional

  • For this reason, a NameNode can be configured to maintain multiple copies of the FsImage and EditLog

  • Multiple copies of the FsImage and EditLog files are updated synchronously

  • Meta-data is not data-intensive

  • The NameNode machine is a single point of failure for a Hadoop cluster

  • If the NameNode fails, manual intervention is necessary

  • The Secondary NameNode help minimize the downtime and loss of data

  • Automatic restart and fail over of NameNode is NOT supported yet! Still a research topic!


HDFS can be accessed from applications in many different ways. Natively HDFS provides a Java API for applications to use. In addition a HTTP browser can also be used to browse the files of an HDFS instance.

FS Shell

  • HDFS allows user data to be organized in the form of files and directories

  • Provides a command line interface called FS shell that lets a user interact with the data in HDFS

  • Syntax is similar to other shells(e.g. bash, csh)

  • Some commands:
    • Create a directory named “/testdir”: bin/hadoop dfs -mkdir /usr/testdir

    • Create a file named “myfile.txt”: bin/hadoop dfs -touchz /usr/testdir/myfile.txt

    • View the contents of the file: bin/hadoop dfs -cat /usr/testdir/myfile.txt


  • The DFSAdmin command set is used for administering an HDFS cluster

  • Used only by an HDFS administrator

  • Sample commands:
    • Generate a list of DataNodes: bin/hadoop dfsadmin -report

    • Decommission DataNode: bin/hadoop dfsadmin -decommission datanodename

Browser Interface

  • Default HDFS install configures a web server to expose the HDFS namespace through a configurable TCP port

  • Allows the user to navigate the HDFS namespace and view the contents of its files using a web browser