HDFS Overview

HDFS is the primary distributed storage used by Hadoop applications. A HDFS cluster primarily consists of a NameNode that manages the file system metadata and DataNodes that store the actual data. The HDFS Architecture Guide describes HDFS in detail. 

The HDFS architecture diagram depicts basic interactions among NameNode, the DataNodes, and the clients. Clients contact NameNode for file metadata or file modifications and perform actual file I/O directly with the DataNodes.

The following are some of the salient features that could be of interest to many users.
  • Hadoop, including HDFS, is well suited for distributed storage and distributed processing using commodity hardware. It is fault tolerant, scalable, and extremely simple to expand. MapReduce, well known for its simplicity and applicability for large set of distributed applications, is an integral part of Hadoop.
  • HDFS is highly configurable with a default configuration well suited for many installations. Most of the time, configuration needs to be tuned only for very large clusters.
  • Hadoop is written in Java and is supported on all major platforms.
  • Hadoop supports shell-like commands to interact with HDFS directly.
  • The NameNode and Datanodes have built in web servers that makes it easy to check current status of the cluster.
  • New features and improvements are regularly implemented in HDFS. The following is a subset of useful features in HDFS:
    • File permissions and authentication.
    • Rack awareness: to take a node's physical location into account while scheduling tasks and allocating storage.
    • Safemode: an administrative mode for maintenance.
    • fsck: a utility to diagnose health of the file system, to find missing files or blocks.
    • fetchdt: a utility to fetch DelegationToken and store it in a file on the local system.
    • Rebalancer: tool to balance the cluster when the data is unevenly distributed among DataNodes.
    • Upgrade and rollback: after a software upgrade, it is possible to rollback to HDFS' state before the upgrade in case of unexpected problems.
    • Secondary NameNode: performs periodic checkpoints of the namespace and helps keep the size of file containing log of HDFS modifications within certain limits at the NameNode.
    • Checkpoint node: performs periodic checkpoints of the namespace and helps minimize the size of the log stored at the NameNode containing changes to the HDFS. Replaces the role previously filled by the Secondary NameNode, though is not yet battle hardened. The NameNode allows multiple Checkpoint nodes simultaneously, as long as there are no Backup nodes registered with the system.
    • Backup node: An extension to the Checkpoint node. In addition to checkpointing it also receives a stream of edits from the NameNode and maintains its own in-memory copy of the namespace, which is always in sync with the active NameNode namespace state. Only one Backup node may be registered with the NameNode at once.


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