Question: Which NameNode Is Used When The Primary NameNode Goes Down?

Which files deal with small file problems?

A HAR file is created using the hadoop archive command, which runs a MapReduce job to pack the files being archived into a small number of HDFS files.

To a client using the HAR filesystem nothing has changed: all of the original files are visible and accessible (albeit using a har:// URL)..

What happens failing DataNode?

If a DataNode fails to heartbeat for reasons other than disk failure, it needs to be recommissioned to be added back to the cluster. If a DataNode rejoins the cluster, there is a possibility for surplus replicas of blocks that were on that DataNode.

What happens if secondary NameNode fails?

If NameNode is failed, File System metadata can be recovered from the last saved FsImage on the Secondary NameNode but Secondary NameNode can’t take the primary NameNode’s functionality.

What is DataNode in Hadoop?

DataNode: DataNodes are the slave nodes in HDFS. Unlike NameNode, DataNode is a commodity hardware, that is, a non-expensive system which is not of high quality or high-availability. The DataNode is a block server that stores the data in the local file ext3 or ext4.

What kind of information is stored in NameNode?

NameNode is the centerpiece of HDFS. NameNode only stores the metadata of HDFS – the directory tree of all files in the file system, and tracks the files across the cluster. NameNode does not store the actual data or the dataset. The data itself is actually stored in the DataNodes.

How do you recover NameNode if it is down?

Recover Hadoop NameNode FailureStart the namenode in a different host with a empty dfs. name. dir.Point the dfs. name. … Use –importCheckpoint option while starting namenode after pointing fs. checkpoint. … Change the fs.default.name to the backup host name URI and restart the cluster with all the slave IP’s in slaves file.

When NameNode fails which node takes the responsibility of active node?

Passive nodeWhen active node fails, then Passive node takes the responsibility of active node. Passive node is also called standby namenode that takes the responsibility to remove the problem of single point of failure (SPOF).

What is the order of the three steps to MapReduce?

MapReduce program executes in three stages, namely map stage, shuffle stage, and reduce stage. Map stage − The map or mapper’s job is to process the input data. Generally the input data is in the form of file or directory and is stored in the Hadoop file system (HDFS).

Which serves as the master and there is only one NameNode per cluster?

HDFS works in a master-slave/master-worker fashion. All the metadata related to HDFS including the information about data nodes, files stored on HDFS, and Replication, etc. are stored and maintained on the NameNode. A NameNode serves as the master and there is only one NameNode per cluster.

What is NameNode and DataNode?

The NameNode keeps an image of the entire file system namespace and file Blockmap in memory. … 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.

How does Hadoop work when a DataNode fails?

What happens if one of the Datanodes gets failed in HDFS? Namenode periodically receives a heartbeat and a Block report from each Datanode in the cluster. Every Datanode sends heartbeat message after every 3 seconds to Namenode.

What was Hadoop named after?

What was Hadoop named after? Explanation: Doug Cutting, Hadoop creator, named the framework after his child’s stuffed toy elephant. Explanation: Apache Hadoop is an open-source software framework for distributed storage and distributed processing of Big Data on clusters of commodity hardware. 8.

What if NameNode fails in Hadoop?

The single point of failure in Hadoop v1 is NameNode. If NameNode gets fail the whole Hadoop cluster will not work. Actually, there will not any data loss only the cluster work will be shut down, because NameNode is only the point of contact to all DataNodes and if the NameNode fails all communication will stop.

How is Hdfs tolerant?

HDFS is highly fault-tolerant. … It creates a replica of users’ data on different machines in the HDFS cluster. So whenever if any machine in the cluster goes down, then data is accessible from other machines in which the same copy of data was created.

What happens when the NameNode on the Hadoop cluster goes down?

The NameNode is the centerpiece of an HDFS file system. It keeps the directory tree of all files in the file system, and tracks where across the cluster the file data is kept. … HDFS is not currently a High Availability system. When the NameNode goes down, the file system goes offline.

Can you access cluster and data if NameNode is down?

So if Namenode is down (Master node), then the data remains as is in the cluster, BUT you will not be able to access it at all. Because, Name node holds the meta data of the data nodes. … So you can still access the data from other two nodes.

Which has the world’s largest Hadoop cluster?

FacebookFacebook has the world’s largest Hadoop Cluster. Facebook is using Hadoop for data warehousing and they are having the largest Hadoop storage cluster in the world. Some of the properties of the HDFS cluster of Facebook is: HDFS cluster of 21 PB storage capacity.

How does NameNode tackle DataNode failures and what will you do when NameNode is down?

This is how Namenode handles datanode failures. HDFS works in Master/Slave mode where NameNode act as a Master and DataNodes act as a Slave. NameNode periodically receives a Heartbeat and a Data Blocks report from each of the DataNodes in the cluster in an interval of specified time.