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. This user guide primarily deals with the interaction of users and administrators with HDFS clusters. 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.
SHELL COMMANDS:
Hadoop includes various shell-like commands that directly interact with HDFS and other file systems that Hadoop supports. The command bin/hdfs dfs -help lists the commands supported by Hadoop shell. Furthermore, the command bin/hdfs dfs -help command-name displays more detailed help for a command. These commands support most of the normal files system operations like copying files, changing file permissions, etc. It also supports a few HDFS specific operations like changing replication of files. For more information see .DFSAdmin Command
The bin/hadoop dfsadmin command supports a few HDFS administration related operations. The bin/hadoop dfsadmin -help command lists all the commands currently supported. For e.g.:- -report: reports basic statistics of HDFS. Some of this information is also available on the NameNode front page.
- -safemode: though usually not required, an administrator can manually enter or leave Safemode.
- -finalizeUpgrade: removes previous backup of the cluster made during last upgrade.
- -refreshNodes: Updates the namenode with the set of datanodes allowed to connect to the namenode. Namenodes re-read datanode hostnames in the file defined bydfs.hosts, dfs.hosts.exclude. Hosts defined in dfs.hosts are the datanodes that are part of the cluster. If there are entries in dfs.hosts, only the hosts in it are allowed to register with the namenode. Entries in dfs.hosts.exclude are datanodes that need to be decommissioned. Datanodes complete decommissioning when all the replicas from them are replicated to other datanodes. Decommissioned nodes are not automatically shutdown and are not chosen for writing for new replicas.
- -printTopology : Print the topology of the cluster. Display a tree of racks and datanodes attached to the tracks as viewed by the NameNode.
SECONDARY NAMENODE
The NameNode stores modifications to the file system as a log appended to a native file system file, edits. When a NameNode starts up, it reads HDFS state from an image file, fsimage, and then applies edits from the edits log file. It then writes new HDFS state to the fsimage and starts normal operation with an empty edits file. Since NameNode merges fsimage and edits files only during start up, the edits log file could get very large over time on a busy cluster. Another side effect of a larger edits file is that next restart of NameNode takes longer.The secondary NameNode merges the fsimage and the edits log files periodically and keeps edits log size within a limit. It is usually run on a different machine than the primary NameNode since its memory requirements are on the same order as the primary NameNode.The start of the checkpoint process on the secondary NameNode is controlled by two configuration parameters. - dfs.namenode.checkpoint.period, set to 1 hour by default, specifies the maximum delay between two consecutive checkpoints, and
- dfs.namenode.checkpoint.txns, set to 40000 default, defines the number of uncheckpointed transactions on the NameNode which will force an urgent checkpoint, even if the checkpoint period has not been reached.
The secondary NameNode stores the latest checkpoint in a directory which is structured the same way as the primary NameNode's directory. So that the check pointed image is always ready to be read by the primary NameNode if necessary.
CHECKPOINT NODE
NameNode persists its namespace using two files: fsimage, which is the latest checkpoint of the namespace and edits, a journal (log) of changes to the namespace since the checkpoint. When a NameNode starts up, it merges the fsimage and edits journal to provide an up-to-date view of the file system metadata. The NameNode then overwrites fsimage with the new HDFS state and begins a new edits journal.
The Checkpoint node periodically creates checkpoints of the namespace. It downloads fsimage and edits from the active NameNode, merges them locally, and uploads the new image back to the active NameNode. The Checkpoint node usually runs on a different machine than the NameNode since its memory requirements are on the same order as the NameNode. The Checkpoint node is started by bin/hdfs namenode -checkpoint on the node specified in the configuration file.
The location of the Checkpoint (or Backup) node and its accompanying web interface are configured via the dfs.namenode.backup.address and dfs.namenode.backup.http-address configuration variables.
The start of the checkpoint process on the Checkpoint node is controlled by two configuration parameters.
- dfs.namenode.checkpoint.period, set to 1 hour by default, specifies the maximum delay between two consecutive checkpoints
- dfs.namenode.checkpoint.txns, set to 40000 default, defines the number of uncheckpointed transactions on the NameNode which will force an urgent checkpoint, even if the checkpoint period has not been reached.
The Checkpoint node stores the latest checkpoint in a directory that is structured the same as the NameNode's directory. This allows the checkpointed image to be always available for reading by the NameNode if necessary. See Import checkpoint.
Multiple checkpoint nodes may be specified in the cluster configuration file.
BACKUP NODE
The Backup node provides the same checkpointing functionality as the Checkpoint node, as well as maintaining an in-memory, up-to-date copy of the file system namespace that is always synchronized with the active NameNode state. Along with accepting a journal stream of file system edits from the NameNode and persisting this to disk, the Backup node also applies those edits into its own copy of the namespace in memory, thus creating a backup of the namespace.
The Backup node does not need to download fsimage and edits files from the active NameNode in order to create a checkpoint, as would be required with a Checkpoint node or Secondary NameNode, since it already has an up-to-date state of the namespace state in memory. The Backup node checkpoint process is more efficient as it only needs to save the namespace into the local fsimage file and reset edits.
As the Backup node maintains a copy of the namespace in memory, its RAM requirements are the same as the NameNode.
The NameNode supports one Backup node at a time. No Checkpoint nodes may be registered if a Backup node is in use. Using multiple Backup nodes concurrently will be supported in the future.
The Backup node is configured in the same manner as the Checkpoint node. It is started with bin/hdfs namenode -backup.
The location of the Backup (or Checkpoint) node and its accompanying web interface are configured via the dfs.namenode.backup.address and dfs.namenode.backup.http-address configuration variables.
Use of a Backup node provides the option of running the NameNode with no persistent storage, delegating all responsibility for persisting the state of the namespace to the Backup node. To do this, start the NameNode with the -importCheckpoint option, along with specifying no persistent storage directories of type edits dfs.namenode.edits.dirfor the NameNode configuration.
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