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  ---
  HDFS Architecture
  ---
  Dhruba Borthakur
  ---
  ${maven.build.timestamp}

%{toc|section=1|fromDepth=0}

HDFS Architecture

Introduction

   The Hadoop Distributed File System (HDFS) is a distributed file system
   designed to run on commodity hardware. It has many similarities with
   existing distributed file systems. However, the differences from other
   distributed file systems are significant. HDFS is highly fault-tolerant
   and is designed to be deployed on low-cost hardware. HDFS provides high
   throughput access to application data and is suitable for applications
   that have large data sets. HDFS relaxes a few POSIX requirements to
   enable streaming access to file system data. HDFS was originally built
   as infrastructure for the Apache Nutch web search engine project. HDFS
   is part of the Apache Hadoop Core project. The project URL is
   {{http://hadoop.apache.org/}}.

Assumptions and Goals

Hardware Failure

   Hardware failure is the norm rather than the exception. An HDFS
   instance may consist of hundreds or thousands of server machines, each
   storing part of the file system’s data. The fact that there are a huge
   number of components and that each component has a non-trivial
   probability of failure means that some component of HDFS is always
   non-functional. Therefore, detection of faults and quick, automatic
   recovery from them is a core architectural goal of HDFS.

Streaming Data Access

   Applications that run on HDFS need streaming access to their data sets.
   They are not general purpose applications that typically run on general
   purpose file systems. HDFS is designed more for batch processing rather
   than interactive use by users. The emphasis is on high throughput of
   data access rather than low latency of data access. POSIX imposes many
   hard requirements that are not needed for applications that are
   targeted for HDFS. POSIX semantics in a few key areas has been traded
   to increase data throughput rates.

Large Data Sets

   Applications that run on HDFS have large data sets. A typical file in
   HDFS is gigabytes to terabytes in size. Thus, HDFS is tuned to support
   large files. It should provide high aggregate data bandwidth and scale
   to hundreds of nodes in a single cluster. It should support tens of
   millions of files in a single instance.

Simple Coherency Model

   HDFS applications need a write-once-read-many access model for files. A
   file once created, written, and closed need not be changed. This
   assumption simplifies data coherency issues and enables high throughput
   data access. A Map/Reduce application or a web crawler application fits
   perfectly with this model. There is a plan to support appending-writes
   to files in the future.

“Moving Computation is Cheaper than Moving Data”

   A computation requested by an application is much more efficient if it
   is executed near the data it operates on. This is especially true when
   the size of the data set is huge. This minimizes network congestion and
   increases the overall throughput of the system. The assumption is that
   it is often better to migrate the computation closer to where the data
   is located rather than moving the data to where the application is
   running. HDFS provides interfaces for applications to move themselves
   closer to where the data is located.

Portability Across Heterogeneous Hardware and Software Platforms

   HDFS has been designed to be easily portable from one platform to
   another. This facilitates widespread adoption of HDFS as a platform of
   choice for a large set of applications.

NameNode and DataNodes

   HDFS has a master/slave architecture. An HDFS cluster consists of a
   single NameNode, a master server that manages the file system namespace
   and regulates access to files by clients. In addition, there are a
   number of DataNodes, usually one per node in the cluster, which manage
   storage attached to the nodes that they run on. HDFS exposes a file
   system namespace and allows user data to be stored in files.
   Internally, a file is split into one or more blocks and these blocks
   are stored in a set of DataNodes. The NameNode executes file system
   namespace operations like opening, closing, and renaming files and
   directories. It also determines the mapping of blocks to DataNodes. The
   DataNodes are responsible for serving read and write requests from the
   file system’s clients. The DataNodes also perform block creation,
   deletion, and replication upon instruction from the NameNode.


[images/hdfsarchitecture.png] HDFS Architecture

   The NameNode and DataNode are pieces of software designed to run on
   commodity machines. These machines typically run a GNU/Linux operating
   system (OS). HDFS is built using the Java language; any machine that
   supports Java can run the NameNode or the DataNode software. Usage of
   the highly portable Java language means that HDFS can be deployed on a
   wide range of machines. A typical deployment has a dedicated machine
   that runs only the NameNode software. Each of the other machines in the
   cluster runs one instance of the DataNode software. The architecture
   does not preclude running multiple DataNodes on the same machine but in
   a real deployment that is rarely the case.

   The existence of a single NameNode in a cluster greatly simplifies the
   architecture of the system. The NameNode is the arbitrator and
   repository for all HDFS metadata. The system is designed in such a way
   that user data never flows through the NameNode.

The File System Namespace

   HDFS supports a traditional hierarchical file organization. A user or
   an application can create directories and store files inside these
   directories. The file system namespace hierarchy is similar to most
   other existing file systems; one can create and remove files, move a
   file from one directory to another, or rename a file. HDFS does not yet
   implement user quotas or access permissions. HDFS does not support hard
   links or soft links. However, the HDFS architecture does not preclude
   implementing these features.

   The NameNode maintains the file system namespace. Any change to the
   file system namespace or its properties is recorded by the NameNode. An
   application can specify the number of replicas of a file that should be
   maintained by HDFS. The number of copies of a file is called the
   replication factor of that file. This information is stored by the
   NameNode.

Data Replication

   HDFS is designed to reliably store very large files across machines in
   a large cluster. It stores each file as a sequence of blocks; all
   blocks in a file except the last block are the same size. The blocks of
   a file are replicated for fault tolerance. The block size and
   replication factor are configurable per file. An application can
   specify the number of replicas of a file. The replication factor can be
   specified at file creation time and can be changed later. Files in HDFS
   are write-once and have strictly one writer at any time.

   The NameNode makes all decisions regarding replication of blocks. It
   periodically receives a Heartbeat and a Blockreport from each of the
   DataNodes in the cluster. Receipt of a Heartbeat implies that the
   DataNode is functioning properly. A Blockreport contains a list of all
   blocks on a DataNode.

[images/hdfsdatanodes.png] HDFS DataNodes

Replica Placement: The First Baby Steps

   The placement of replicas is critical to HDFS reliability and
   performance. Optimizing replica placement distinguishes HDFS from most
   other distributed file systems. This is a feature that needs lots of
   tuning and experience. The purpose of a rack-aware replica placement
   policy is to improve data reliability, availability, and network
   bandwidth utilization. The current implementation for the replica
   placement policy is a first effort in this direction. The short-term
   goals of implementing this policy are to validate it on production
   systems, learn more about its behavior, and build a foundation to test
   and research more sophisticated policies.

   Large HDFS instances run on a cluster of computers that commonly spread
   across many racks. Communication between two nodes in different racks
   has to go through switches. In most cases, network bandwidth between
   machines in the same rack is greater than network bandwidth between
   machines in different racks.

   The NameNode determines the rack id each DataNode belongs to via the
   process outlined in {{{../hadoop-common/ClusterSetup.html#Hadoop+Rack+Awareness}Hadoop Rack Awareness}}. A simple but non-optimal policy
   is to place replicas on unique racks. This prevents losing data when an
   entire rack fails and allows use of bandwidth from multiple racks when
   reading data. This policy evenly distributes replicas in the cluster
   which makes it easy to balance load on component failure. However, this
   policy increases the cost of writes because a write needs to transfer
   blocks to multiple racks.

   For the common case, when the replication factor is three, HDFS’s
   placement policy is to put 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. This policy cuts the inter-rack
   write traffic which generally improves write performance. The chance of
   rack failure is far less than that of node failure; this policy does
   not impact data reliability and availability guarantees. However, it
   does reduce the aggregate network bandwidth used when reading data
   since a block is placed in only two unique racks rather than three.
   With this policy, the replicas of a file do not evenly distribute
   across the racks. One third of replicas are on one node, two thirds of
   replicas are on one rack, and the other third are evenly distributed
   across the remaining racks. This policy improves write performance
   without compromising data reliability or read performance.

   The current, default replica placement policy described here is a work
   in progress.

Replica Selection

   To minimize global bandwidth consumption and read latency, HDFS tries
   to satisfy a read request from a replica that is closest to the reader.
   If there exists a replica on the same rack as the reader node, then
   that replica is preferred to satisfy the read request. If angg/ HDFS
   cluster spans multiple data centers, then a replica that is resident in
   the local data center is preferred over any remote replica.

Safemode

   On startup, the NameNode enters a special state called Safemode.
   Replication of data blocks does not occur when the NameNode is in the
   Safemode state. The NameNode receives Heartbeat and Blockreport
   messages from the DataNodes. A Blockreport contains the list of data
   blocks that a DataNode is hosting. Each block has a specified minimum
   number of replicas. A block is considered safely replicated when the
   minimum number of replicas of that data block has checked in with the
   NameNode. After a configurable percentage of safely replicated data
   blocks checks in with the NameNode (plus an additional 30 seconds), the
   NameNode exits the Safemode state. It then determines the list of data
   blocks (if any) that still have fewer than the specified number of
   replicas. The NameNode then replicates these blocks to other DataNodes.

The Persistence of File System Metadata

   The HDFS namespace is stored by the NameNode. The NameNode uses a
   transaction log called the EditLog to persistently record every change
   that occurs to file system metadata. For example, creating a new file
   in HDFS causes the NameNode to insert a record into the EditLog
   indicating this. Similarly, changing the replication factor of a file
   causes a new record to be inserted into the EditLog. The NameNode uses
   a file in its local host OS file system to store the EditLog. The
   entire file system namespace, including the mapping of blocks to files
   and file system properties, is stored in a file called the FsImage. The
   FsImage is stored as a file in the NameNode’s local file system too.

   The NameNode keeps an image of the entire file system namespace and
   file Blockmap in memory. This key metadata item is designed to be
   compact, such that a NameNode with 4 GB of RAM is plenty to support a
   huge number of files and directories. When the NameNode starts up, it
   reads the FsImage and EditLog from disk, applies all the transactions
   from the EditLog to the in-memory representation of the FsImage, and
   flushes out this new version into a new FsImage on disk. It can then
   truncate the old EditLog because its transactions have been applied to
   the persistent FsImage. This process is called a checkpoint. In the
   current implementation, a checkpoint only occurs when the NameNode
   starts up. Work is in progress to support periodic checkpointing in the
   near future.

   The DataNode stores HDFS data in files in its local file system. The
   DataNode has no knowledge about HDFS files. It stores each block of
   HDFS data in a separate file in its local file system. The DataNode
   does not create all files in the same directory. Instead, it uses a
   heuristic to determine the optimal number of files per directory and
   creates subdirectories appropriately. It is not optimal to create all
   local files in the same directory because the local file system might
   not be able to efficiently support a huge number of files in a single
   directory. When a DataNode starts up, it scans through its local file
   system, generates a list of all HDFS data blocks that correspond to
   each of these local files and sends this report to the NameNode: this
   is the Blockreport.

The Communication Protocols

   All HDFS communication protocols are layered on top of the TCP/IP
   protocol. A client establishes a connection to a configurable TCP port
   on the NameNode machine. It talks the ClientProtocol with the NameNode.
   The DataNodes talk to the NameNode using the DataNode Protocol. A
   Remote Procedure Call (RPC) abstraction wraps both the Client Protocol
   and the DataNode Protocol. By design, the NameNode never initiates any
   RPCs. Instead, it only responds to RPC requests issued by DataNodes or
   clients.

Robustness

   The primary objective of HDFS is to store data reliably even in the
   presence of failures. The three common types of failures are NameNode
   failures, DataNode failures and network partitions.

Data Disk Failure, Heartbeats and Re-Replication

   Each DataNode sends a Heartbeat message to the NameNode periodically. A
   network partition can cause a subset of DataNodes to lose connectivity
   with the NameNode. The NameNode detects this condition by the absence
   of a Heartbeat message. The NameNode marks DataNodes without recent
   Heartbeats as dead and does not forward any new IO requests to them.
   Any data that was registered to a dead DataNode is not available to
   HDFS any more. DataNode death may cause the replication factor of some
   blocks to fall below their specified value. The NameNode constantly
   tracks which blocks need to be replicated and initiates replication
   whenever necessary. The necessity for re-replication may arise due to
   many reasons: a DataNode may become unavailable, a replica may become
   corrupted, a hard disk on a DataNode may fail, or the replication
   factor of a file may be increased.

Cluster Rebalancing

   The HDFS architecture is compatible with data rebalancing schemes. A
   scheme might automatically 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 rebalance other data in the
   cluster. These types of data rebalancing schemes are not yet
   implemented.

Data Integrity

   It is possible that a block of data fetched from a DataNode arrives
   corrupted. This corruption can occur because of faults in a storage
   device, network faults, or buggy software. The HDFS client software
   implements checksum checking on the contents of HDFS files. When a
   client creates an HDFS file, it computes a checksum of each block of
   the file and stores these checksums in a separate hidden file in the
   same HDFS namespace. When a client retrieves file contents it verifies
   that the data it received from each DataNode matches the checksum
   stored in the associated checksum file. If not, then the client can opt
   to retrieve that block from another DataNode that has a replica of that
   block.

Metadata Disk Failure

   The FsImage and the EditLog are central data structures of HDFS. A
   corruption of these files can cause the HDFS instance to be
   non-functional. For this reason, the NameNode can be configured to
   support maintaining multiple copies of the FsImage and EditLog. Any
   update to either the FsImage or EditLog causes each of the FsImages and
   EditLogs to get updated synchronously. This synchronous updating of
   multiple copies of the FsImage and EditLog may degrade the rate of
   namespace transactions per second that a NameNode can support. However,
   this degradation is acceptable because even though HDFS applications
   are very data intensive in nature, they are not metadata intensive.
   When a NameNode restarts, it selects the latest consistent FsImage and
   EditLog to use.

   The NameNode machine is a single point of failure for an HDFS cluster.
   If the NameNode machine fails, manual intervention is necessary.
   Currently, automatic restart and failover of the NameNode software to
   another machine is not supported.

Snapshots

   Snapshots support storing a copy of data at a particular instant of
   time. One usage of the snapshot feature may be to roll back a corrupted
   HDFS instance to a previously known good point in time. HDFS does not
   currently support snapshots but will in a future release.

Data Organization

Data Blocks

   HDFS is designed to support very large files. Applications that are
   compatible with HDFS are those that deal with large data sets. These
   applications write their data only once but they read it one or more
   times and require these reads to be satisfied at streaming speeds. HDFS
   supports write-once-read-many semantics on files. A typical block size
   used by HDFS is 64 MB. Thus, an HDFS file is chopped up into 64 MB
   chunks, and if possible, each chunk will reside on a different
   DataNode.

Staging

   A client request to create a file does not reach the NameNode
   immediately. In fact, initially the HDFS client caches the file data
   into a temporary local file. Application writes are transparently
   redirected to this temporary local file. When the local file
   accumulates data worth over one HDFS block size, the client contacts
   the NameNode. The NameNode inserts the file name into the file system
   hierarchy and allocates a data block for it. The NameNode responds to
   the client request with the identity of the DataNode and the
   destination data block. Then the client flushes the block of data from
   the local temporary file to the specified DataNode. When a file is
   closed, the remaining un-flushed data in the temporary local file is
   transferred to the DataNode. The client then tells the NameNode that
   the file is closed. At this point, the NameNode commits the file
   creation operation into a persistent store. If the NameNode dies before
   the file is closed, the file is lost.

   The above approach has been adopted after careful consideration of
   target applications that run on HDFS. These applications need streaming
   writes to files. If a client writes to a remote file directly without
   any client side buffering, the network speed and the congestion in the
   network impacts throughput considerably. This approach is not without
   precedent. Earlier distributed file systems, e.g. AFS, have used client
   side caching to improve performance. A POSIX requirement has been
   relaxed to achieve higher performance of data uploads.

Replication Pipelining

   When a client is writing data to an HDFS file, its data is first
   written to a local file as explained in the previous section. Suppose
   the HDFS file has a replication factor of three. When the local file
   accumulates a full block of user data, the client retrieves a list of
   DataNodes from the NameNode. This list contains the DataNodes that will
   host a replica of that block. The client then flushes the data block to
   the first DataNode. The first DataNode starts receiving the data in
   small portions (4 KB), writes each portion to its local repository and
   transfers that portion to the second DataNode in the list. The second
   DataNode, in turn starts receiving each portion of the data block,
   writes that portion to its repository and then flushes that portion to
   the third DataNode. Finally, the third DataNode writes the data to its
   local repository. Thus, a DataNode can be receiving data from the
   previous one in the pipeline and at the same time forwarding data to
   the next one in the pipeline. Thus, the data is pipelined from one
   DataNode to the next.

Accessibility

   HDFS can be accessed from applications in many different ways.
   Natively, HDFS provides a
   {{{http://hadoop.apache.org/docs/current/api/}FileSystem Java API}}
   for applications to use. A C language wrapper for this Java API is also
   available. In addition, an HTTP browser can also be used to browse the files
   of an HDFS instance. Work is in progress to expose HDFS through the WebDAV
   protocol.

FS Shell

   HDFS allows user data to be organized in the form of files and
   directories. It provides a commandline interface called FS shell that
   lets a user interact with the data in HDFS. The syntax of this command
   set is similar to other shells (e.g. bash, csh) that users are already
   familiar with. Here are some sample action/command pairs:

*---------+---------+
|| Action | Command
*---------+---------+
| Create a directory named <<</foodir>>> | <<<bin/hadoop dfs -mkdir /foodir>>>
*---------+---------+
| Remove a directory named <<</foodir>>> | <<<bin/hadoop dfs -rmr /foodir>>>
*---------+---------+
| View the contents of a file named <<</foodir/myfile.txt>>> | <<<bin/hadoop dfs -cat /foodir/myfile.txt>>>
*---------+---------+

   FS shell is targeted for applications that need a scripting language to
   interact with the stored data.

DFSAdmin

   The DFSAdmin command set is used for administering an HDFS cluster.
   These are commands that are used only by an HDFS administrator. Here
   are some sample action/command pairs:

*---------+---------+
|| Action | Command
*---------+---------+
|Put the cluster in Safemode              | <<<bin/hadoop dfsadmin -safemode enter>>>
*---------+---------+
|Generate a list of DataNodes             | <<<bin/hadoop dfsadmin -report>>>
*---------+---------+
|Recommission or decommission DataNode(s) | <<<bin/hadoop dfsadmin -refreshNodes>>>
*---------+---------+

Browser Interface

   A typical HDFS install configures a web server to expose the HDFS
   namespace through a configurable TCP port. This allows a user to
   navigate the HDFS namespace and view the contents of its files using a
   web browser.

Space Reclamation

File Deletes and Undeletes

   When a file is deleted by a user or an application, it is not
   immediately removed from HDFS. Instead, HDFS first renames it to a file
   in the <<</trash>>> directory. The file can be restored quickly as long as it
   remains in <<</trash>>>. A file remains in <<</trash>>> for a configurable amount
   of time. After the expiry of its life in <<</trash>>>, the NameNode deletes
   the file from the HDFS namespace. The deletion of a file causes the
   blocks associated with the file to be freed. Note that there could be
   an appreciable time delay between the time a file is deleted by a user
   and the time of the corresponding increase in free space in HDFS.

   A user can Undelete a file after deleting it as long as it remains in
   the <<</trash>>> directory. If a user wants to undelete a file that he/she
   has deleted, he/she can navigate the <<</trash>>> directory and retrieve the
   file. The <<</trash>>> directory contains only the latest copy of the file
   that was deleted. The <<</trash>>> directory is just like any other directory
   with one special feature: HDFS applies specified policies to
   automatically delete files from this directory. The current default
   policy is to delete files from <<</trash>>> that are more than 6 hours old.
   In the future, this policy will be configurable through a well defined
   interface.

Decrease Replication Factor

   When the replication factor of a file is reduced, the NameNode selects
   excess replicas that can be deleted. The next Heartbeat transfers this
   information to the DataNode. The DataNode then removes the
   corresponding blocks and the corresponding free space appears in the
   cluster. Once again, there might be a time delay between the completion
   of the setReplication API call and the appearance of free space in the
   cluster.

References

   Hadoop {{{http://hadoop.apache.org/docs/current/api/}JavaDoc API}}.

   HDFS source code: {{http://hadoop.apache.org/version_control.html}}