Table of Contents
How do I put NameNode in safe mode?
A safe mode for NameNode is essentially a read-only mode for the HDFS cluster, it does not allow any modifications to file system or blocks. Normally, NameNode disables safe mode automatically at the beginning. If required, HDFS can be placed in safe mode explicitly using bin/hadoop dfsadmin -safemode command.
How do I get my NameNode out of safe mode?
NameNode leaves Safemode after the DataNodes have reported that most blocks are available.
- Use below command to know status of Safemode. hadoop dfsadmin –safemode get.
- Use below command to know enter in Safemode. bin/hadoop dfsadmin –safemode enter.
- Use below command to leave Safemode. hadoop dfsadmin -safemode leave.
How many Namenodes are in HDFS?
You can have only a single name node in a cluster. Detail – In Yarn / Hadoop 2.0 they have come with a concept of active name node and standby name node. ( This is where most of the people get confused. They consider them to be 2 nodes in a cluster).
How do I know if NameNode is in safe mode?
NameNode leaves Safemode after the DataNodes have reported that most blocks are available.
- To know the status of Safemode, use command: hadoop dfsadmin –safemode get.
- To enter Safemode, use command: bin/hadoop dfsadmin –safemode enter.
- To come out of Safemode, use command: hadoop dfsadmin -safemode leave.
What happens when NameNode fails?
Whenever the active NameNode fails, the passive NameNode or the standby NameNode replaces the active NameNode, to ensure that the Hadoop cluster is never without a NameNode. The passive NameNode takes over the responsibility of the failed NameNode and keep the HDFS up and running.
Why is NameNode in safe mode?
Safemode for the NameNode is essentially a read-only mode for the Hadoop Distributed File System (HDFS) cluster. NameNode might enter into Safemode for different reasons, such as the following: Available space is less than the amount of space required for the NameNode storage directory.
What is secondary NameNode?
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.
What helps with multiple NameNode?
Multiple Namenodes/Namespaces. In order to scale the name service horizontally, federation uses multiple independent Namenodes/namespaces. The Namenodes are federated; the Namenodes are independent and do not require coordination with each other. The Datanodes are used as common storage for blocks by all the Namenodes.
What is a secondary NameNode?
The Secondary namenode is a helper node in hadoop, To understand the functionality of the secondary namenodelet’s understand how the namenode works. Name node stores metadata like file system namespace information, blockinformation etc in the memory.It also stores the persistent copy of the same on the disk.
What is namespace and Blockpool?
A Namespace and its block pool together are called Namespace Volume. It is a self-contained unit of management. When a Namenode/namespace is deleted, the corresponding block pool at the Datanodes is deleted. Each namespace volume is upgraded as a unit, during cluster upgrade. ClusterID.
What is the difference between standby and secondary NameNode?
So a Standby Node comes in action which is nothing but a backup for the Name Node . Secondary NameNode : It is one of the poorest named part of the hadoop ecosystem usually beginners get confused thinking of it as a backup.
What is the purpose of the secondary NameNode?
What if NameNode goes down?
When the NameNode goes down, the file system goes offline. There is an optional SecondaryNameNode that can be hosted on a separate machine. It only creates checkpoints of the namespace by merging the edits file into the fsimage file and does not provide any real redundancy.
What happens when namenode is in SafeMode?
When namenode is in safemode, namenode will bring the FSIMAGE file, which has the metadata of HDFS, to memory. It will wait for all the blocks for all the files in HDFS to be available.
What is SafeMode in HDFS?
Safemode: Safemode is a state where no changes can be made to the blocks. HDFS cluster is in safemode state during start up because the cluster needs to validate all the blocks and their locations. Once validated, safemode is then disabled.
What happens when namenode is started or restarted?
When namenode is started or restarted, namenode will be in safemode for a period of time. At this time you will not be able to use your Hadoop cluster fully. Write operations to HDFS will fail and because of that your MapReduce jobs will also fail.