Hadoop Distributed Filesystem: Tutorial and Course

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Hadoop Distributed Filesystem

Hadoop Distributed Filesystem: Overview

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Hadoop Distributed Filesystem: Facts and Information

When a dataset outgrows the storage capacity of a single physical machine, it becomes necessary to partition it across a number of separate machines. Filesystems that manage the storage across a network of machines are called distributed filesystems. Since they are network based, all the complications of network programming kick in, thus making distributed filesystems more complex than regular disk filesystems. For example, one of the biggest challenges is making the filesystem tolerate node failure without suffering data loss.

Hadoop comes with a distributed filesystem called HDFS, which stands for Hadoop Distributed Filesystem. HDFS is Hadoop's flagship filesystem and is the focus of Tutorial and Course for Hadoop Distributed Filesystem, but Hadoop actually has a general-purpose filesystem abstraction, so we'll see along the way how Hadoop integrates with other storage systems (such as the local filesystem and Amazon S3).

Hadoop Distributed Filesystem: Tutorial and Course

Hadoop Distributed Filesystem: The Design Of HDFS (Hadoop Distributed Filesystem)

HDFS is a filesystem designed for storing very large files with streaming data access patterns, running on clusters of commodity hardware. Let's examine this statement in more detail:

  • Very Large Files: "Very Large" means files that are hundreds of megabytes, gigabytes, or terabytes in size. There are Hadoop clusters running today that store petabytes of data.
  • Streaming Data Access: HDFS is built around the idea that the most efficient data processing pattern is a write-once, read-many-times pattern. A dataset is typically generated or copied from source, and then various analyses are performed on that dataset over time. Each analysis will involve a large proportion, if not all, of the dataset, so the time to read the whole dataset is more important than the latency in reading the first record.
  • Commodity Hardware: Hadoop doesn't require expensive, highly reliable hardware. It’s designed to run on clusters of commodity hardware (commonly available hardware that can be obtained from multiple vendors) for which the chance of node failure across the cluster is high, at least for large clusters. HDFS is designed to carry on working without a noticeable interruption to the user in the face of such failure.

It is also worth examining the applications for which using HDFS does not work so well. Although this may change in the future, these are areas where HDFS is not a good fit today:

  • Low-Latency Data Access: Applications that require low-latency access to data, in the tens of milliseconds range, will not work well with HDFS. Remember, HDFS is optimized for delivering a high throughput of data, and this may be at the expense of latency. HBase is currently a better choice for low-latency access.
  • Lots Of Small Files: Because the namenode holds filesystem metadata in memory, the limit to the number of files in a filesystem is governed by the amount of memory on the namenode. As a rule of thumb, each file, directory, and block takes about 150 bytes. So, for example, if you had one million files, each taking one block, you would need at least 300 MB of memory. Although storing millions of files is feasible, billions is beyond the capability of current hardware.
  • Multiple Writers, Arbitrary File Modifications: Files in HDFS may be written to by a single writer. Writes are always made at the end of the file, in append-only fashion. There is no support for multiple writers or for modifications at arbitrary offsets in the file. (These might be supported in the future, but they are likely to be relatively inefficient.)

Hadoop Distributed Filesystem: HDFS Concepts

Hadoop Distributed Filesystem: Blocks

A disk has a block size, which is the minimum amount of data that it can read or write. Filesystems for a single disk build on this by dealing with data in blocks, which are an integral multiple of the disk block size. Filesystem blocks are typically a few kilobytes in size, whereas disk blocks are normally 512 bytes. This is generally transparent to the filesystem user who is simply reading or writing a file of whatever length. However, there are tools to perform filesystem maintenance, such as df and fsck, that operate on the filesystem block level.

HDFS, too, has the concept of a block, but it is a much larger unit - 128 MB by default. Like in a filesystem for a single disk, files in HDFS are broken into block-sized chunks, which are stored as independent units. Unlike a filesystem for a single disk, a file in HDFS that is smaller than a single block does not occupy a full block’s worth of underlying storage. (For example, a 1 MB file stored with a block size of 128 MB uses 1 MB of disk space, not 128 MB.) When unqualified, the term “block” in this book refers to a block in HDFS.

Having a block abstraction for a distributed filesystem brings several benefits. The first benefit is the most obvious: a file can be larger than any single disk in the network. There’s nothing that requires the blocks from a file to be stored on the same disk, so they can take advantage of any of the disks in the cluster. In fact, it would be possible, if unusual, to store a single file on an HDFS cluster whose blocks filled all the disks in the cluster.

Second, making the unit of abstraction a block rather than a file simplifies the storage subsystem. Simplicity is something to strive for in all systems, but it is especially important for a distributed system in which the failure modes are so varied. The storage subsystem deals with blocks, simplifying storage management (because blocks are a fixed size, it is easy to calculate how many can be stored on a given disk) and eliminating metadata concerns (because blocks are just chunks of data to be stored, file metadata such as permissions information does not need to be stored with the blocks, so another system can handle metadata separately).

Furthermore, blocks fit well with replication for providing fault tolerance and availability. To insure against corrupted blocks and disk and machine failure, each block is replicated to a small number of physically separate machines (typically three). If a block becomes unavailable, a copy can be read from another location in a way that is transparent to the client. A block that is no longer available due to corruption or machine failure can be replicated from its alternative locations to other live machines to bring the replication factor back to the normal level. (See Data Integrity for more on guarding against corrupt data.) Similarly, some applications may choose to set a high replication factor for the blocks in a popular file to spread the read load on the cluster.

Like its disk filesystem cousin, HDFS’s fsck command understands blocks. For example, running:

 % hdfs fsck / -files -blocks 

will list the blocks that make up each file in the filesystem. (See also Filesystem check (fsck).)

Hadoop Distributed Filesystem: Namenodes and Datanodes

An HDFS cluster has two types of nodes operating in a master−worker pattern: a namenode (the master) and a number of datanodes (workers). The namenode manages the filesystem namespace. It maintains the filesystem tree and the metadata for all the files and directories in the tree. This information is stored persistently on the local disk in the form of two files: the namespace image and the edit log. The namenode also knows the datanodes on which all the blocks for a given file are located; however, it does not store block locations persistently, because this information is reconstructed from datanodes when the system starts.

A client accesses the filesystem on behalf of the user by communicating with the namenode and datanodes. The client presents a filesystem interface similar to a Portable Operating System Interface (POSIX), so the user code does not need to know about the namenode and datanodes to function.

Datanodes are the workhorses of the filesystem. They store and retrieve blocks when they are told to (by clients or the namenode), and they report back to the namenode periodically with lists of blocks that they are storing.

Without the namenode, the filesystem cannot be used. In fact, if the machine running the namenode were obliterated, all the files on the filesystem would be lost since there would be no way of knowing how to reconstruct the files from the blocks on the datanodes. For this reason, it is important to make the namenode resilient to failure, and Hadoop provides two mechanisms for this.

The first way is to back up the files that make up the persistent state of the filesystem metadata. Hadoop can be configured so that the namenode writes its persistent state to multiple filesystems. These writes are synchronous and atomic. The usual configuration choice is to write to local disk as well as a remote NFS mount.

It is also possible to run a secondary namenode, which despite its name does not act as a namenode. Its main role is to periodically merge the namespace image with the edit log to prevent the edit log from becoming too large. The secondary namenode usually runs on a separate physical machine because it requires plenty of CPU and as much memory as the namenode to perform the merge. It keeps a copy of the merged namespace image, which can be used in the event of the namenode failing. However, the state of the secondary namenode lags that of the primary, so in the event of total failure of the primary, data loss is almost certain. The usual course of action in this case is to copy the namenode’s metadata files that are on NFS to the secondary and run it as the new primary.

Hadoop Distributed Filesystem: Block Caching

Normally a datanode reads blocks from disk, but for frequently accessed files the blocks may be explicitly cached in the datanode’s memory, in an off-heap block cache. By default, a block is cached in only one datanode’s memory, although the number is configurable on a per-file basis. Job schedulers (for MapReduce, Spark, and other frameworks) can take advantage of cached blocks by running tasks on the datanode where a block is cached, for increased read performance. A small lookup table used in a join is a good candidate for caching, for example.

Users or applications instruct the namenode which files to cache (and for how long) by adding a cache directive to a cache pool. Cache pools are an administrative grouping for managing cache permissions and resource usage.

Hadoop Distributed Filesystem: HDFS Federation

The namenode keeps a reference to every file and block in the filesystem in memory, which means that on very large clusters with many files, memory becomes the limiting factor for scaling (see How Much Memory Does a Namenode Need?). HDFS federation, introduced in the 2.x release series, allows a cluster to scale by adding namenodes, each of which manages a portion of the filesystem namespace. For example, one namenode might manage all the files rooted under /user, say, and a second namenode might handle files under /share.

Under federation, each namenode manages a namespace volume, which is made up of the metadata for the namespace, and a block pool containing all the blocks for the files in the namespace. Namespace volumes are independent of each other, which means namenodes do not communicate with one another, and furthermore the failure of one namenode does not affect the availability of the namespaces managed by other namenodes. Block pool storage is not partitioned, however, so datanodes register with each namenode in the cluster and store blocks from multiple block pools.

To access a federated HDFS cluster, clients use client-side mount tables to map file paths to namenodes. This is managed in configuration using ViewFileSystem and the viewfs:// URIs.

Hadoop Distributed Filesystem: HDFS High Availability

The combination of replicating namenode metadata on multiple filesystems and using the secondary namenode to create checkpoints protects against data loss, but it does not provide high availability of the filesystem. The namenode is still a single point of failure (SPOF). If it did fail, all clients — including MapReduce jobs — would be unable to read, write, or list files, because the namenode is the sole repository of the metadata and the file-to-block mapping. In such an event, the whole Hadoop system would effectively be out of service until a new namenode could be brought online.

To recover from a failed namenode in this situation, an administrator starts a new primary namenode with one of the filesystem metadata replicas and configures datanodes and clients to use this new namenode. The new namenode is not able to serve requests until it has (i) loaded its namespace image into memory, (ii) replayed its edit log, and (iii) received enough block reports from the datanodes to leave safe mode. On large clusters with many files and blocks, the time it takes for a namenode to start from cold can be 30 minutes or more.

The long recovery time is a problem for routine maintenance, too. In fact, because unexpected failure of the namenode is so rare, the case for planned downtime is actually more important in practice.

Hadoop 2 remedied this situation by adding support for HDFS high availability (HA). In this implementation, there are a pair of namenodes in an active-standby configuration. In the event of the failure of the active namenode, the standby takes over its duties to continue servicing client requests without a significant interruption. A few architectural changes are needed to allow this to happen:

  • The namenodes must use highly available shared storage to share the edit log. When a standby namenode comes up, it reads up to the end of the shared edit log to synchronize its state with the active namenode, and then continues to read new entries as they are written by the active namenode.
  • Datanodes must send block reports to both namenodes because the block mappings are stored in a namenode’s memory, and not on disk.
  • Clients must be configured to handle namenode failover, using a mechanism that is transparent to users.
  • The secondary namenode’s role is subsumed by the standby, which takes periodic checkpoints of the active namenode’s namespace.

There are two choices for the highly available shared storage: an NFS filer, or a quorum journal manager (QJM). The QJM is a dedicated HDFS implementation, designed for the sole purpose of providing a highly available edit log, and is the recommended choice for most HDFS installations. The QJM runs as a group of journal nodes, and each edit must be written to a majority of the journal nodes. Typically, there are three journal nodes, so the system can tolerate the loss of one of them. This arrangement is similar to the way ZooKeeper works, although it is important to realize that the QJM implementation does not use ZooKeeper.

If the active namenode fails, the standby can take over very quickly (in a few tens of seconds) because it has the latest state available in memory: both the latest edit log entries and an up-to-date block mapping. The actual observed failover time will be longer in practice (around a minute or so), because the system needs to be conservative in deciding that the active namenode has failed.

In the unlikely event of the standby being down when the active fails, the administrator can still start the standby from cold. This is no worse than the non-HA case, and from an operational point of view it’s an improvement, because the process is a standard operational procedure built into Hadoop.

The transition from the active namenode to the standby is managed by a new entity in the system called the failover controller. There are various failover controllers, but the default implementation uses ZooKeeper to ensure that only one namenode is active. Each namenode runs a lightweight failover controller process whose job it is to monitor its namenode for failures (using a simple heartbeating mechanism) and trigger a failover should a namenode fail.

Failover may also be initiated manually by an administrator, for example, in the case of routine maintenance. This is known as a graceful failover, since the failover controller arranges an orderly transition for both namenodes to switch roles.

In the case of an ungraceful failover, however, it is impossible to be sure that the failed namenode has stopped running. For example, a slow network or a network partition can trigger a failover transition, even though the previously active namenode is still running and thinks it is still the active namenode.

The HA implementation goes to great lengths to ensure that the previously active namenode is prevented from doing any damage and causing corruption — a method known as fencing.

The QJM only allows one namenode to write to the edit log at one time; however, it is still possible for the previously active namenode to serve stale read requests to clients, so setting up an SSH fencing command that will kill the namenode’s process is a good idea. Stronger fencing methods are required when using an NFS filer for the shared edit log, since it is not possible to only allow one namenode to write at a time (this is why QJM is recommended). The range of fencing mechanisms includes revoking the namenode’s access to the shared storage directory (typically by using a vendor-specific NFS command), and disabling its network port via a remote management command. As a last resort, the previously active namenode can be fenced with a technique rather graphically known as STONITH, or “shoot the other node in the head,” which uses a specialized power distribution unit to forcibly power down the host machine.

Client failover is handled transparently by the client library. The simplest implementation uses client-side configuration to control failover. The HDFS URI uses a logical hostname that is mapped to a pair of namenode addresses (in the configuration file), and the client library tries each namenode address until the operation succeeds.

Hadoop Distributed Filesystem: Command-Line Interface

We’re going to have a look at HDFS by interacting with it from the command line. There are many other interfaces to HDFS, but the command line is one of the simplest and, to many developers, the most familiar.

We are going to run HDFS on one machine, so first follow the instructions for setting up Hadoop in pseudodistributed mode. Later we’ll see how to run HDFS on a cluster of machines to give us scalability and fault tolerance.

There are two properties that we set in the pseudodistributed configuration that deserve further explanation. The first is fs.defaultFS, set to hdfs://localhost/, which is used to set a default filesystem for Hadoop.[29] Filesystems are specified by a URI, and here we have used an hdfs URI to configure Hadoop to use HDFS by default. The HDFS daemons will use this property to determine the host and port for the HDFS namenode. We’ll be running it on localhost, on the default HDFS port, 8020. And HDFS clients will use this property to work out where the namenode is running so they can connect to it.

We set the second property, dfs.replication, to 1 so that HDFS doesn’t replicate filesystem blocks by the default factor of three. When running with a single datanode, HDFS can’t replicate blocks to three datanodes, so it would perpetually warn about blocks being under-replicated. This setting solves that problem.

The filesystem is ready to be used, and we can do all of the usual filesystem operations, such as reading files, creating directories, moving files, deleting data, and listing directories. You can type hadoop fs -help to get detailed help on every command.

Start by copying a file from the local filesystem to HDFS:

 % hadoop fs -copyFromLocal input/docs/quangle.txt \ hdfs://localhost/user/john/quangle.txt 

This command invokes Hadoop’s filesystem shell command fs, which supports a number of subcommands — in this case, we are running -copyFromLocal. The local file quangle.txt is copied to the file /user/john/quangle.txt on the HDFS instance running on localhost. In fact, we could have omitted the scheme and host of the URI and picked up the default, hdfs://localhost, as specified in core-site.xml:

 % hadoop fs -copyFromLocal input/docs/quangle.txt /user/john/quangle.txt 

We also could have used a relative path and copied the file to our home directory in HDFS, which in this case is /user/john:

 % hadoop fs -copyFromLocal input/docs/quangle.txt quangle.txt 

Let’s copy the file back to the local filesystem and check whether it’s the same:

 % hadoop fs -copyToLocal quangle.txt quangle.copy.txt % md5 input/docs/quangle.txt quangle.copy.txt MD5 (input/docs/quangle.txt) = e7891a2627cf263a079fb0f18256ffb2 MD5 (quangle.copy.txt) = e7891a2627cf263a079fb0f18256ffb2 

The MD5 digests are the same, showing that the file survived its trip to HDFS and is back intact.

Finally, let’s look at an HDFS file listing. We create a directory first just to see how it is displayed in the listing:

 % hadoop fs -mkdir books % hadoop fs -ls . Found 2 items drwxr-xr-x - john supergroup 0 2014-10-04 13:22 books -rw-r--r-- 1 john supergroup 119 2014-10-04 13:21 quangle.txt 

The information returned is very similar to that returned by the Unix command ls -l, with a few minor differences. The first column shows the file mode. The second column is the replication factor of the file (something a traditional Unix filesystem does not have). Remember we set the default replication factor in the site-wide configuration to be 1, which is why we see the same value here. The entry in this column is empty for directories because the concept of replication does not apply to them — directories are treated as metadata and stored by the namenode, not the datanodes. The third and fourth columns show the file owner and group. The fifth column is the size of the file in bytes, or zero for directories. The sixth and seventh columns are the last modified date and time. Finally, the eighth column is the name of the file or directory.

Hadoop Distributed Filesystem: Hadoop Filesystems

Hadoop has an abstract notion of filesystems, of which HDFS is just one implementation. The Java abstract class org.apache.hadoop.fs.FileSystem represents the client interface to a filesystem in Hadoop, and there are several concrete implementations.

Hadoop provides many interfaces to its filesystems, and it generally uses the URI scheme to pick the correct filesystem instance to communicate with. For example, the filesystem shell that we met in the previous section operates with all Hadoop filesystems. To list the files in the root directory of the local filesystem, type:

 % hadoop fs -ls file:/// 

Although it is possible (and sometimes very convenient) to run MapReduce programs that access any of these filesystems, when you are processing large volumes of data you should choose a distributed filesystem that has the data locality optimization, notably HDFS.

Hadoop is written in Java, so most Hadoop filesystem interactions are mediated through the Java API. The filesystem shell, for example, is a Java application that uses the Java FileSystem class to provide filesystem operations. The other filesystem interfaces are discussed briefly in this section. These interfaces are most commonly used with HDFS, since the other filesystems in Hadoop typically have existing tools to access the underlying filesystem (FTP clients for FTP, S3 tools for S3, etc.), but many of them will work with any Hadoop filesystem.

Hadoop Distributed Filesystem: HTTP

By exposing its filesystem interface as a Java API, Hadoop makes it awkward for non-Java applications to access HDFS. The HTTP REST API exposed by the WebHDFS protocol makes it easier for other languages to interact with HDFS. Note that the HTTP interface is slower than the native Java client, so should be avoided for very large data transfers if possible.

There are two ways of accessing HDFS over HTTP: directly, where the HDFS daemons serve HTTP requests to clients; and via a proxy (or proxies), which accesses HDFS on the client’s behalf using the usual DistributedFileSystem API. The two ways are both use the WebHDFS protocol.

In the first case, the embedded web servers in the namenode and datanodes act as WebHDFS endpoints. (WebHDFS is enabled by default, since dfs.webhdfs.enabled is set to true.) File metadata operations are handled by the namenode, while file read (and write) operations are sent first to the namenode, which sends an HTTP redirect to the client indicating the datanode to stream file data from (or to).

The second way of accessing HDFS over HTTP relies on one or more standalone proxy servers. (The proxies are stateless, so they can run behind a standard load balancer.) All traffic to the cluster passes through the proxy, so the client never accesses the namenode or datanode directly. This allows for stricter firewall and bandwidth-limiting policies to be put in place. It’s common to use a proxy for transfers between Hadoop clusters located in different data centers, or when accessing a Hadoop cluster running in the cloud from an external network.

The HttpFS proxy exposes the same HTTP (and HTTPS) interface as WebHDFS, so clients can access both using webhdfs (or swebhdfs) URIs. The HttpFS proxy is started independently of the namenode and datanode daemons, using the httpfs.sh script, and by default listens on a different port number (14000).

Hadoop Distributed Filesystem: C

Hadoop provides a C library called libhdfs that mirrors the Java FileSystem interface (it was written as a C library for accessing HDFS, but despite its name it can be used to access any Hadoop filesystem). It works using the Java Native Interface (JNI) to call a Java filesystem client. There is also a libwebhdfs library that uses the WebHDFS interface described in the previous section.

The C API is very similar to the Java one, but it typically lags the Java one, so some newer features may not be supported. You can find the header file, hdfs.h, in the include directory of the Apache Hadoop binary tarball distribution.

The Apache Hadoop binary tarball comes with prebuilt libhdfs binaries for 64-bit Linux, but for other platforms you will need to build them yourself by following the BUILDING.txt instructions at the top level of the source tree.

Hadoop Distributed Filesystem: NFS

It is possible to mount HDFS on a local client’s filesystem using Hadoop’s NFSv3 gateway. You can then use Unix utilities (such as ls and cat) to interact with the filesystem, upload files, and in general use POSIX libraries to access the filesystem from any programming language. Appending to a file works, but random modifications of a file do not, since HDFS can only write to the end of a file.

Consult the Hadoop documentation for how to configure and run the NFS gateway and connect to it from a client.

Hadoop Distributed Filesystem: FUSE

Filesystem in Userspace (FUSE) allows filesystems that are implemented in user space to be integrated as Unix filesystems. Hadoop’s Fuse-DFS contrib module allows HDFS (or any Hadoop filesystem) to be mounted as a standard local filesystem. Fuse-DFS is implemented in C using libhdfs as the interface to HDFS. At the time of writing, the Hadoop NFS gateway is the more robust solution to mounting HDFS, so should be preferred over Fuse-DFS.

Hadoop Distributed Filesystem: Java Interface

In this section, we dig into the Hadoop FileSystem class: the API for interacting with one of Hadoop’s filesystems. Although we focus mainly on the HDFS implementation, DistributedFileSystem, in general you should strive to write your code against the FileSystem abstract class, to retain portability across filesystems. This is very useful when testing your program, for example, because you can rapidly run tests using data stored on the local filesystem.

One of the simplest ways to read a file from a Hadoop filesystem is by using a java.net.URL object to open a stream to read the data from. The general idiom is:

 InputStream in = null; try { in = new URL("hdfs://host/path").openStream(); // process in } finally { IOUtils.closeStream(in); } 

There’s a little bit more work required to make Java recognize Hadoop’s hdfs URL scheme. This is achieved by calling the setURLStreamHandlerFactory() method on URL with an instance of FsUrlStreamHandlerFactory. This method can be called only once per JVM, so it is typically executed in a static block. This limitation means that if some other part of your program — perhaps a third-party component outside your control — sets a URLStreamHandlerFactory, you won’t be able to use this approach for reading data from Hadoop. The next section discusses an alternative.

The following example shows a program for displaying files from Hadoop filesystems on standard output, like the Unix cat command.

Displaying files from a Hadoop filesystem on standard output using a URLStreamHandler:

 public class URLCat { static { URL.setURLStreamHandlerFactory(new FsUrlStreamHandlerFactory()); } public static void main(String[] args) throws Exception { InputStream in = null; try { in = new URL(args[0]).openStream(); IOUtils.copyBytes(in, System.out, 4096, false); } finally { IOUtils.closeStream(in); } } } 

We make use of the handy IOUtils class that comes with Hadoop for closing the stream in the finally clause, and also for copying bytes between the input stream and the output stream (System.out, in this case). The last two arguments to the copyBytes() method are the buffer size used for copying and whether to close the streams when the copy is complete. We close the input stream ourselves, and System.out doesn’t need to be closed.

Here’s a sample run:

 % export HADOOP_CLASSPATH=hadoop-examples.jar % hadoop URLCat hdfs://localhost/user/john/quangle.txt On the top of the Crumpetty Tree The Quangle Wangle sat, But his face you could not see, On account of his Beaver Hat. 

As the previous section explained, sometimes it is impossible to set a URLStreamHandlerFactory for your application. In this case, you will need to use the FileSystem API to open an input stream for a file.

A file in a Hadoop filesystem is represented by a Hadoop Path object (and not a java.io.File object, since its semantics are too closely tied to the local filesystem). You can think of a Path as a Hadoop filesystem URI, such as hdfs://localhost/user/john/quangle.txt.

FileSystem is a general filesystem API, so the first step is to retrieve an instance for the filesystem we want to use — HDFS, in this case. There are several static factory methods for getting a FileSystem instance:

 public static FileSystem get(Configuration conf) throws IOException public static FileSystem get(URI uri, Configuration conf) throws IOException public static FileSystem get(URI uri, Configuration conf, String user) throws IOException 

A Configuration object encapsulates a client or server’s configuration, which is set using configuration files read from the classpath, such as etc/hadoop/core-site.xml. The first method returns the default filesystem (as specified in core-site.xml, or the default local filesystem if not specified there). The second uses the given URI’s scheme and authority to determine the filesystem to use, falling back to the default filesystem if no scheme is specified in the given URI. The third retrieves the filesystem as the given user, which is important in the context of security.

In some cases, you may want to retrieve a local filesystem instance. For this, you can use the convenience method getLocal():

 public static LocalFileSystem getLocal(Configuration conf) throws IOException 

With a FileSystem instance in hand, we invoke an open() method to get the input stream for a file:

 public FSDataInputStream open(Path f) throws IOException public abstract FSDataInputStream open(Path f, int bufferSize) throws IOException 

The first method uses a default buffer size of 4 KB.

Putting this together, we can rewrite the above example as shown in the following example:

 public class FileSystemCat { public static void main(String[] args) throws Exception { String uri = args[0]; Configuration conf = new Configuration(); FileSystem fs = FileSystem.get(URI.create(uri), conf); InputStream in = null; try { in = fs.open(new Path(uri)); IOUtils.copyBytes(in, System.out, 4096, false); } finally { IOUtils.closeStream(in); } } } 

The program runs as follows:

 % hadoop FileSystemCat hdfs://localhost/user/john/quangle.txt On the top of the Crumpetty Tree The Quangle Wangle sat, But his face you could not see, On account of his Beaver Hat. 

The open() method on FileSystem actually returns an FSDataInputStream rather than a standard java.io class. This class is a specialization of java.io.DataInputStream with support for random access, so you can read from any part of the stream:

 package org.apache.hadoop.fs; public class FSDataInputStream extends DataInputStream implements Seekable, PositionedReadable { // implementation elided } 

The Seekable interface permits seeking to a position in the file and provides a query method for the current offset from the start of the file (getPos()):

 public interface Seekable { void seek(long pos) throws IOException; long getPos() throws IOException; } 

Calling seek() with a position that is greater than the length of the file will result in an IOException. Unlike the skip() method of java.io.InputStream, which positions the stream at a point later than the current position, seek() can move to an arbitrary, absolute position in the file.

A simple extension of the above example, which writes a file to standard output twice: after writing it once, it seeks to the start of the file and streams through it once again.

Displaying files from a Hadoop filesystem on standard output twice, by using seek()

 public class FileSystemDoubleCat { public static void main(String[] args) throws Exception { String uri = args[0]; Configuration conf = new Configuration(); FileSystem fs = FileSystem.get(URI.create(uri), conf); FSDataInputStream in = null; try { in = fs.open(new Path(uri)); IOUtils.copyBytes(in, System.out, 4096, false); in.seek(0); // go back to the start of the file IOUtils.copyBytes(in, System.out, 4096, false); } finally { IOUtils.closeStream(in); } } } 

Here’s the result of running it on a small file:

 % hadoop FileSystemDoubleCat hdfs://localhost/user/john/quangle.txt On the top of the Crumpetty Tree The Quangle Wangle sat, But his face you could not see, On account of his Beaver Hat. On the top of the Crumpetty Tree The Quangle Wangle sat, But his face you could not see, On account of his Beaver Hat. 

FSDataInputStream also implements the PositionedReadable interface for reading parts of a file at a given offset:

 public interface PositionedReadable { public int read(long position, byte[] buffer, int offset, int length) throws IOException; public void readFully(long position, byte[] buffer, int offset, int length) throws IOException; public void readFully(long position, byte[] buffer) throws IOException; } 

The read() method reads up to length bytes from the given position in the file into the buffer at the given offset in the buffer. The return value is the number of bytes actually read; callers should check this value, as it may be less than length. The readFully() methods will read length bytes into the buffer (or buffer.length bytes for the version that just takes a byte array buffer), unless the end of the file is reached, in which case an EOFException is thrown.

All of these methods preserve the current offset in the file and are thread safe (although FSDataInputStream is not designed for concurrent access; therefore, it’s better to create multiple instances), so they provide a convenient way to access another part of the file — metadata, perhaps — while reading the main body of the file.

Finally, bear in mind that calling seek() is a relatively expensive operation and should be done sparingly. You should structure your application access patterns to rely on streaming data (by using MapReduce, for example) rather than performing a large number of seeks.

The FileSystem class has a number of methods for creating a file. The simplest is the method that takes a Path object for the file to be created and returns an output stream to write to:

 public FSDataOutputStream create(Path f) throws IOException 

There are overloaded versions of this method that allow you to specify whether to forcibly overwrite existing files, the replication factor of the file, the buffer size to use when writing the file, the block size for the file, and file permissions.

There’s also an overloaded method for passing a callback interface, Progressable, so your application can be notified of the progress of the data being written to the datanodes:

 package org.apache.hadoop.util; public interface Progressable { public void progress(); } 

As an alternative to creating a new file, you can append to an existing file using the append() method (there are also some other overloaded versions):

 public FSDataOutputStream append(Path f) throws IOException 

The append operation allows a single writer to modify an already written file by opening it and writing data from the final offset in the file. With this API, applications that produce unbounded files, such as logfiles, can write to an existing file after having closed it. The append operation is optional and not implemented by all Hadoop filesystems. For example, HDFS supports append, but S3 filesystems don’t.

The following example shows how to copy a local file to a Hadoop filesystem. We illustrate progress by printing a period every time the progress() method is called by Hadoop, which is after each 64 KB packet of data is written to the datanode pipeline. (Note that this particular behavior is not specified by the API, so it is subject to change in later versions of Hadoop. The API merely allows you to infer that “something is happening.”)

Copying a local file to a Hadoop filesystem:

 public class FileCopyWithProgress { public static void main(String[] args) throws Exception { String localSrc = args[0]; String dst = args[1]; InputStream in = new BufferedInputStream(new FileInputStream(localSrc)); Configuration conf = new Configuration(); FileSystem fs = FileSystem.get(URI.create(dst), conf); OutputStream out = fs.create(new Path(dst), new Progressable() { public void progress() { System.out.print("."); } }); IOUtils.copyBytes(in, out, 4096, true); } } 

Typical usage:

 % hadoop FileCopyWithProgress input/docs/1400-8.txt hdfs://localhost/user/john/1400-8.txt ................. 

Currently, none of the other Hadoop filesystems call progress() during writes. Progress is important in MapReduce applications.

The create() method on FileSystem returns an FSDataOutputStream, which, like FSDataInputStream, has a method for querying the current position in the file:

 package org.apache.hadoop.fs; public class FSDataOutputStream extends DataOutputStream implements Syncable { public long getPos() throws IOException { // implementation elided } // implementation elided } 

However, unlike FSDataInputStream, FSDataOutputStream does not permit seeking. This is because HDFS allows only sequential writes to an open file or appends to an already written file. In other words, there is no support for writing to anywhere other than the end of the file, so there is no value in being able to seek while writing.

FileSystem provides a method to create a directory:

 public boolean mkdirs(Path f) throws IOException 

This method creates all of the necessary parent directories if they don’t already exist, just like the java.io.File’s mkdirs() method. It returns true if the directory (and all parent directories) was (were) successfully created.

Often, you don’t need to explicitly create a directory, because writing a file by calling create() will automatically create any parent directories.

An important feature of any filesystem is the ability to navigate its directory structure and retrieve information about the files and directories that it stores. The FileStatus class encapsulates filesystem metadata for files and directories, including file length, block size, replication, modification time, ownership, and permission information.

The method getFileStatus() on FileSystem provides a way of getting a FileStatus object for a single file or directory. The following example shows an example of its use.

 public class ShowFileStatusTest { private MiniDFSCluster cluster; // use an in-process HDFS cluster for testing private FileSystem fs; @Before public void setUp() throws IOException { Configuration conf = new Configuration(); if (System.getProperty("test.build.data") == null) { System.setProperty("test.build.data", "/tmp"); } cluster = new MiniDFSCluster.Builder(conf).build(); fs = cluster.getFileSystem(); OutputStream out = fs.create(new Path("/dir/file")); out.write("content".getBytes("UTF-8")); out.close(); } @After public void tearDown() throws IOException { if (fs != null) { fs.close(); } if (cluster != null) { cluster.shutdown(); } } @Test(expected = FileNotFoundException.class) public void throwsFileNotFoundForNonExistentFile() throws IOException { fs.getFileStatus(new Path("no-such-file")); } @Test public void fileStatusForFile() throws IOException { Path file = new Path("/dir/file"); FileStatus stat = fs.getFileStatus(file); assertThat(stat.getPath().toUri().getPath(), is("/dir/file")); assertThat(stat.isDirectory(), is(false)); assertThat(stat.getLen(), is(7L)); assertThat(stat.getModificationTime(), is(lessThanOrEqualTo(System.currentTimeMillis()))); assertThat(stat.getReplication(), is((short) 1)); assertThat(stat.getBlockSize(), is(128 * 1024 * 1024L)); assertThat(stat.getOwner(), is(System.getProperty("user.name"))); assertThat(stat.getGroup(), is("supergroup")); assertThat(stat.getPermission().toString(), is("rw-r--r--")); } @Test public void fileStatusForDirectory() throws IOException { Path dir = new Path("/dir"); FileStatus stat = fs.getFileStatus(dir); assertThat(stat.getPath().toUri().getPath(), is("/dir")); assertThat(stat.isDirectory(), is(true)); assertThat(stat.getLen(), is(0L)); assertThat(stat.getModificationTime(), is(lessThanOrEqualTo(System.currentTimeMillis()))); assertThat(stat.getReplication(), is((short) 0)); assertThat(stat.getBlockSize(), is(0L)); assertThat(stat.getOwner(), is(System.getProperty("user.name"))); assertThat(stat.getGroup(), is("supergroup")); assertThat(stat.getPermission().toString(), is("rwxr-xr-x")); } } 

If no file or directory exists, a FileNotFoundException is thrown. However, if you are interested only in the existence of a file or directory, the exists() method on FileSystem is more convenient:

 public boolean exists(Path f) throws IOException 

Finding information on a single file or directory is useful, but you also often need to be able to list the contents of a directory. That’s what FileSystem’s listStatus() methods are for:

 public FileStatus[] listStatus(Path f) throws IOException public FileStatus[] listStatus(Path f, PathFilter filter) throws IOException public FileStatus[] listStatus(Path[] files) throws IOException public FileStatus[] listStatus(Path[] files, PathFilter filter) throws IOException 

When the argument is a file, the simplest variant returns an array of FileStatus objects of length 1. When the argument is a directory, it returns zero or more FileStatus objects representing the files and directories contained in the directory.

Overloaded variants allow a PathFilter to be supplied to restrict the files and directories to match. You will see an example of this in the section PathFilter. Finally, if you specify an array of paths, the result is a shortcut for calling the equivalent single-path listStatus() method for each path in turn and accumulating the FileStatus object arrays in a single array. This can be useful for building up lists of input files to process from distinct parts of the filesystem tree. The following example is a simple demonstration of this idea. Note the use of stat2Paths() in Hadoop’s FileUtil for turning an array of FileStatus objects into an array of Path objects.

Showing the file statuses for a collection of paths in a Hadoop filesystem

 public class ListStatus { public static void main(String[] args) throws Exception { String uri = args[0]; Configuration conf = new Configuration(); FileSystem fs = FileSystem.get(URI.create(uri), conf); Path[] paths = new Path[args.length]; for (int i = 0; i < paths.length; i++) { paths[i] = new Path(args[i]); } FileStatus[] status = fs.listStatus(paths); Path[] listedPaths = FileUtil.stat2Paths(status); for (Path p : listedPaths) { System.out.println(p); } } } 

We can use this program to find the union of directory listings for a collection of paths:

 % hadoop ListStatus hdfs://localhost/ hdfs://localhost/user/john hdfs://localhost/user hdfs://localhost/user/john/books hdfs://localhost/user/john/quangle.txt 

It is a common requirement to process sets of files in a single operation. For example, a MapReduce job for log processing might analyze a month’s worth of files contained in a number of directories. Rather than having to enumerate each file and directory to specify the input, it is convenient to use wildcard characters to match multiple files with a single expression, an operation that is known as globbing. Hadoop provides two FileSystem methods for processing globs:

 public FileStatus[] globStatus(Path pathPattern) throws IOException public FileStatus[] globStatus(Path pathPattern, PathFilter filter) throws IOException 

The globStatus() methods return an array of FileStatus objects whose paths match the supplied pattern, sorted by path. An optional PathFilter can be specified to restrict the matches further.

Hadoop supports the same set of glob characters as the Unix bash shell. Imagine that logfiles are stored in a directory structure organized hierarchically by date. So, logfiles for the last day of 2015 would go in a directory named /2015/11/15.

Glob patterns are not always powerful enough to describe a set of files you want to access. For example, it is not generally possible to exclude a particular file using a glob pattern. The listStatus() and globStatus() methods of FileSystem take an optional PathFilter, which allows programmatic control over matching:

 package org.apache.hadoop.fs; public interface PathFilter { boolean accept(Path path); } 

PathFilter is the equivalent of java.io.FileFilter for Path objects rather than File objects.

The following example shows a PathFilter for excluding paths that match a regular expression.

 public class RegexExcludePathFilter implements PathFilter { private final String regex; public RegexExcludePathFilter(String regex) { this.regex = regex; } public boolean accept(Path path) { return !path.toString().matches(regex); } } 

The filter passes only those files that don’t match the regular expression. After the glob picks out an initial set of files to include, the filter is used to refine the results. For example:

 fs.globStatus(new Path("/2015/*/*"), new RegexExcludeFilter("^.*/2015/11/31$")) 

will expand to /2007/12/30.

Filters can act only on a file’s name, as represented by a Path. They can’t use a file’s properties, such as creation time, as their basis. Nevertheless, they can perform matching that neither glob patterns nor regular expressions can achieve. For example, if you store files in a directory structure that is laid out by date (like in the previous section), you can write a PathFilter to pick out files that fall in a given date range.

Use the delete() method on FileSystem to permanently remove files or directories:

 public boolean delete(Path f, boolean recursive) throws IOException 

If f is a file or an empty directory, the value of recursive is ignored. A nonempty directory is deleted, along with its contents, only if recursive is true (otherwise, an IOException is thrown).

Hadoop Distributed Filesystem: Data Flow

To get an idea of how data flows between the client interacting with HDFS, the namenode, and the datanodes, consider the following figure, which shows the main sequence of events when reading a file.

Tutorial and Course for Hadoop Distributed Filesystem

The main sequence of events when reading a file. Tutorial and Course for Hadoop Distributed Filesystem by SEO University, including facts and information about Hadoop Distributed Filesystem.

The client opens the file it wishes to read by calling open() on the FileSystem object, which for HDFS is an instance of DistributedFileSystem (step 1 in Figure). DistributedFileSystem calls the namenode, using remote procedure calls (RPCs), to determine the locations of the first few blocks in the file (step 2). For each block, the namenode returns the addresses of the datanodes that have a copy of that block. Furthermore, the datanodes are sorted according to their proximity to the client (according to the topology of the cluster’s network; see Network Topology and Hadoop). If the client is itself a datanode (in the case of a MapReduce task, for instance), the client will read from the local datanode if that datanode hosts a copy of the block.

The DistributedFileSystem returns an FSDataInputStream (an input stream that supports file seeks) to the client for it to read data from. FSDataInputStream in turn wraps a DFSInputStream, which manages the datanode and namenode I/O.

The client then calls read() on the stream (step 3). DFSInputStream, which has stored the datanode addresses for the first few blocks in the file, then connects to the first (closest) datanode for the first block in the file. Data is streamed from the datanode back to the client, which calls read() repeatedly on the stream (step 4). When the end of the block is reached, DFSInputStream will close the connection to the datanode, then find the best datanode for the next block (step 5). This happens transparently to the client, which from its point of view is just reading a continuous stream.

Blocks are read in order, with the DFSInputStream opening new connections to datanodes as the client reads through the stream. It will also call the namenode to retrieve the datanode locations for the next batch of blocks as needed. When the client has finished reading, it calls close() on the FSDataInputStream (step 6).

During reading, if the DFSInputStream encounters an error while communicating with a datanode, it will try the next closest one for that block. It will also remember datanodes that have failed so that it doesn’t needlessly retry them for later blocks. The DFSInputStream also verifies checksums for the data transferred to it from the datanode. If a corrupted block is found, the DFSInputStream attempts to read a replica of the block from another datanode; it also reports the corrupted block to the namenode.

One important aspect of this design is that the client contacts datanodes directly to retrieve data and is guided by the namenode to the best datanode for each block. This design allows HDFS to scale to a large number of concurrent clients because the data traffic is spread across all the datanodes in the cluster. Meanwhile, the namenode merely has to service block location requests (which it stores in memory, making them very efficient) and does not, for example, serve data, which would quickly become a bottleneck as the number of clients grew.

Next we’ll look at how files are written to HDFS. Although quite detailed, it is instructive to understand the data flow because it clarifies HDFS’s coherency model.

We’re going to consider the case of creating a new file, writing data to it, then closing the file. This is illustrated in the following figure.

Tutorial and Course for Hadoop Distributed Filesystem

The main sequence of events when writing data to HDFS. Tutorial and Course for Hadoop Distributed Filesystem by SEO University, including facts and information about Hadoop Distributed Filesystem.

The client creates the file by calling create() on DistributedFileSystem. DistributedFileSystem makes an RPC call to the namenode to create a new file in the filesystem’s namespace, with no blocks associated with it (step 2). The namenode performs various checks to make sure the file doesn’t already exist and that the client has the right permissions to create the file. If these checks pass, the namenode makes a record of the new file; otherwise, file creation fails and the client is thrown an IOException. The DistributedFileSystem returns an FSDataOutputStream for the client to start writing data to. Just as in the read case, FSDataOutputStream wraps a DFSOutputStream, which handles communication with the datanodes and namenode.

As the client writes data (step 3), the DFSOutputStream splits it into packets, which it writes to an internal queue called the data queue. The data queue is consumed by the DataStreamer, which is responsible for asking the namenode to allocate new blocks by picking a list of suitable datanodes to store the replicas. The list of datanodes forms a pipeline, and here we’ll assume the replication level is three, so there are three nodes in the pipeline. The DataStreamer streams the packets to the first datanode in the pipeline, which stores each packet and forwards it to the second datanode in the pipeline. Similarly, the second datanode stores the packet and forwards it to the third (and last) datanode in the pipeline (step 4).

The DFSOutputStream also maintains an internal queue of packets that are waiting to be acknowledged by datanodes, called the ack queue. A packet is removed from the ack queue only when it has been acknowledged by all the datanodes in the pipeline (step 5).

If any datanode fails while data is being written to it, then the following actions are taken, which are transparent to the client writing the data. First, the pipeline is closed, and any packets in the ack queue are added to the front of the data queue so that datanodes that are downstream from the failed node will not miss any packets. The current block on the good datanodes is given a new identity, which is communicated to the namenode, so that the partial block on the failed datanode will be deleted if the failed datanode recovers later on. The failed datanode is removed from the pipeline, and a new pipeline is constructed from the two good datanodes. The remainder of the block’s data is written to the good datanodes in the pipeline. The namenode notices that the block is under-replicated, and it arranges for a further replica to be created on another node. Subsequent blocks are then treated as normal.

It’s possible, but unlikely, for multiple datanodes to fail while a block is being written. As long as dfs.namenode.replication.min replicas (which defaults to 1) are written, the write will succeed, and the block will be asynchronously replicated across the cluster until its target replication factor is reached (dfs.replication, which defaults to 3).

When the client has finished writing data, it calls close() on the stream (step 6). This action flushes all the remaining packets to the datanode pipeline and waits for acknowledgments before contacting the namenode to signal that the file is complete (step 7). The namenode already knows which blocks the file is made up of (because DataStreamer asks for block allocations), so it only has to wait for blocks to be minimally replicated before returning successfully.

A coherency model for a filesystem describes the data visibility of reads and writes for a file. HDFS trades off some POSIX requirements for performance, so some operations may behave differently than you expect them to.

After creating a file, it is visible in the filesystem namespace, as expected:

 Path p = new Path("p"); fs.create(p); assertThat(fs.exists(p), is(true)); 

However, any content written to the file is not guaranteed to be visible, even if the stream is flushed. So, the file appears to have a length of zero:

 Path p = new Path("p"); OutputStream out = fs.create(p); out.write("content".getBytes("UTF-8")); out.flush(); assertThat(fs.getFileStatus(p).getLen(), is(0L)); 

Once more than a block’s worth of data has been written, the first block will be visible to new readers. This is true of subsequent blocks, too: it is always the current block being written that is not visible to other readers.

HDFS provides a way to force all buffers to be flushed to the datanodes via the hflush() method on FSDataOutputStream. After a successful return from hflush(), HDFS guarantees that the data written up to that point in the file has reached all the datanodes in the write pipeline and is visible to all new readers:

 Path p = new Path("p"); FSDataOutputStream out = fs.create(p); out.write("content".getBytes("UTF-8")); out.hflush(); assertThat(fs.getFileStatus(p).getLen(), is(((long) "content".length()))); 

Note that hflush() does not guarantee that the datanodes have written the data to disk, only that it’s in the datanodes’ memory (so in the event of a data center power outage, for example, data could be lost). For this stronger guarantee, use hsync() instead.

The behavior of hsync() is similar to that of the fsync() system call in POSIX that commits buffered data for a file descriptor. For example, using the standard Java API to write a local file, we are guaranteed to see the content after flushing the stream and synchronizing:

 FileOutputStream out = new FileOutputStream(localFile); out.write("content".getBytes("UTF-8")); out.flush(); // flush to operating system out.getFD().sync(); // sync to disk assertThat(localFile.length(), is(((long) "content".length()))); 

Closing a file in HDFS performs an implicit hflush(), too:

 Path p = new Path("p"); OutputStream out = fs.create(p); out.write("content".getBytes("UTF-8")); out.close(); assertThat(fs.getFileStatus(p).getLen(), is(((long) "content".length()))); 

This coherency model has implications for the way you design applications. With no calls to hflush() or hsync(), you should be prepared to lose up to a block of data in the event of client or system failure. For many applications, this is unacceptable, so you should call hflush() at suitable points, such as after writing a certain number of records or number of bytes. Though the hflush() operation is designed to not unduly tax HDFS, it does have some overhead (and hsync() has more), so there is a trade-off between data robustness and throughput. What constitutes an acceptable trade-off is application dependent, and suitable values can be selected after measuring your application’s performance with different hflush() (or hsync()) frequencies.

Hadoop Distributed Filesystem: Parallel Copying With Distcp

The HDFS access patterns that we have seen so far focus on single-threaded access. It’s possible to act on a collection of files - by specifying file globs, for example - but for efficient parallel processing of these files, you would have to write a program yourself. Hadoop comes with a useful program called distcp for copying data to and from Hadoop filesystems in parallel.

One use for distcp is as an efficient replacement for hadoop fs -cp. For example, you can copy one file to another with:

 % hadoop distcp file1 file2 

You can also copy directories:

 % hadoop distcp dir1 dir2 

If dir2 does not exist, it will be created, and the contents of the dir1 directory will be copied there. You can specify multiple source paths, and all will be copied to the destination.

If dir2 already exists, then dir1 will be copied under it, creating the directory structure dir2/dir1. If this isn’t what you want, you can supply the -overwrite option to keep the same directory structure and force files to be overwritten. You can also update only the files that have changed using the -update option. This is best shown with an example. If we changed a file in the dir1 subtree, we could synchronize the change with dir2 by running:

 % hadoop distcp -update dir1 dir2 

distcp is implemented as a MapReduce job where the work of copying is done by the maps that run in parallel across the cluster. There are no reducers. Each file is copied by a single map, and distcp tries to give each map approximately the same amount of data by bucketing files into roughly equal allocations. By default, up to 20 maps are used, but this can be changed by specifying the -m argument to distcp.

A very common use case for distcp is for transferring data between two HDFS clusters. For example, the following creates a backup of the first cluster’s /foo directory on the second:

 % hadoop distcp -update -delete -p hdfs://namenode1/foo hdfs://namenode2/foo 

The -delete flag causes distcp to delete any files or directories from the destination that are not present in the source, and -p means that file status attributes like permissions, block size, and replication are preserved. You can run distcp with no arguments to see precise usage instructions.

If the two clusters are running incompatible versions of HDFS, then you can use the webhdfs protocol to distcp between them:

 % hadoop distcp webhdfs://namenode1:50070/foo webhdfs://namenode2:50070/foo 

Another variant is to use an HttpFs proxy as the distcp source or destination (again using the webhdfs protocol), which has the advantage of being able to set firewall and bandwidth controls.

When copying data into HDFS, it’s important to consider cluster balance. HDFS works best when the file blocks are evenly spread across the cluster, so you want to ensure that distcp doesn’t disrupt this. For example, if you specified -m 1, a single map would do the copy, which — apart from being slow and not using the cluster resources efficiently — would mean that the first replica of each block would reside on the node running the map (until the disk filled up). The second and third replicas would be spread across the cluster, but this one node would be unbalanced. By having more maps than nodes in the cluster, this problem is avoided. For this reason, it’s best to start by running distcp with the default of 20 maps per node.

However, it’s not always possible to prevent a cluster from becoming unbalanced. Perhaps you want to limit the number of maps so that some of the nodes can be used by other jobs. In this case, you can use the balancer tool to subsequently even out the block distribution across the cluster.

Hadoop Distributed Filesystem: References

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