What happens if NameNode fails in Hadoop?
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.
Can you recover a NameNode when it is down?
When a name node fails, it is possible to recover from a previous checkpoint generated by Secondary Namenode. Secondary Namenode performs periodic checkpoint process.
How the NameNode failure is handled in the HDFS?
As soon as the data node is declared dead/non-functional all the data blocks it hosts are transferred to the other data nodes with which the blocks are replicated initially. 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.
Related Question What would happen if NameNode fails How do you bring it up?
What is NameNode recovery process?
The lease recovery process is triggered on the NameNode to recover leases for a given client, either by the monitor thread upon hard limit expiry, or when a client tries to take over lease from another client when the soft limit expires.
What is NameNode?
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. It does not store the data of these files itself. When the NameNode goes down, the file system goes offline.
Can you access cluster and data if NameNode is down?
There are daemons in Hadoop, Namenode, data node, Resource manager, AppMaster and etc. 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.
How does NameNode tackle DataNode failures and what will you do when NameNode is down?
How are failures handled in Mapreduce?
After the task is failed, the application master will try to avoid rescheduling the task on a node manager. It will not be retried again if a task fails four times. This value is configurable to control the maximum number of the task. It is controlled by the mapreduce.
What will happen when NameNode is down and a user submits a new job?
By Hadoop job, you probably mean MapReduce job. If your NN is down, and you don't have spare one (in HA setup) your HDFS will not be working and every component dependent on this HDFS namespace will be either stuck or crashed.
What are the different ways to restart NameNode?
By following methods we can restart the NameNode:
When NameNode is down what happens to Job Tracker?
What happens to job tracker when Namenode is down? When Namenode is down, your cluster is OFF, this is because Namenode is the single point of failure in HDFS.
Which of the following feature overcomes the single point of failure issue of NameNode?
Hadoop High Availability feature tackles the namenode failure problem for all the components in the hadoop stack.
Does NameNode high availability solves single point of failure?
The single point of failure in a Hadoop cluster is the NameNode. While the loss of any other machine (intermittently or permanently) does not result in data loss, NameNode loss results in cluster unavailability. The permanent loss of NameNode data would render the cluster's HDFS inoperable.
Why is MapReduce needed?
MapReduce facilitates concurrent processing by splitting petabytes of data into smaller chunks, and processing them in parallel on Hadoop commodity servers. In the end, it aggregates all the data from multiple servers to return a consolidated output back to the application.
How do I fix a corrupted block in HDFS?
Possible remedies. Bring up the failed DataNodes with missing or corrupt blocks. Identify the files associated with the missing or corrupt blocks by running the Hadoop fsck command. Delete the corrupt files and recover them from backup, if it exists.
What is a checkpoint in Hadoop?
Checkpointing is a process that takes an fsimage and edit log and compacts them into a new fsimage. This way, instead of replaying a potentially unbounded edit log, the NameNode can load the final in-memory state directly from the fsimage. This is a far more efficient operation and reduces NameNode startup time.
What does NameNode format do?
When we format namenode(bin/hadoop namenode -format) it formats the meta-data related to data-nodes. By doing that, all the information on the datanodes are lost and the datanodes becomes reusable for new data.
Where is NameNode stored?
NameNode service stores its metadata on the configured "dfs. namenode. name. dir" tag available on hdfs-site.
What is the role of NameNode in HDFS architecture?
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.
What is NameNode and data node?
The main difference between NameNode and DataNode in Hadoop is that the NameNode is the master node in Hadoop Distributed File System (HDFS) that manages the file system metadata while the DataNode is a slave node in Hadoop distributed file system that stores the actual data as instructed by the NameNode.
What happens when NameNode fails to receive heartbeat from a DataNode?
When NameNode notices that it has not received a heartbeat message from a data node after a certain amount of time, the data node is marked as dead. Since blocks will be under-replicated the system begins replicating the blocks that were stored on the dead DataNode.
What happens if DataNode fails while writing a file in the HDFS?
I read from hadoop operations that if a datanode fails during writing process, A new replication pipeline containing the remaining datanodes is opened and the write resumes. The namenode will notice that one of the blocks in the file is under-replicated and will arrange for a new replica to be created asynchronously.
What happens when a DataNode fails?
When NameNode notices that it has not received a heartbeat message from a datanode after a certain amount of time (usually 10 minutes by default), the data node is marked as dead. Since blocks will be under-replicated, the system begins replicating the blocks that were stored on the dead DataNode.
What if node Manager fails?
If any health check fails, the NodeManager marks the node as unhealthy and communicates this to the ResourceManager, which then stops assigning containers to the node. Communication of the node status is done as part of the heartbeat between the NodeManager and the ResourceManager.
How do HDFS cope up with node failure?
As soon as the datanodes are declared dead. Data blocks on the failed Datanode are replicated on other Datanodes based on the specified replication factor in hdfs-site. Once the failed datanodes comes back the Name node will manage the replication factor again. This is how Namenode handles the failure of data node.
How NameNode failure is handled in HDFS explain failover & fencing?
Failure Detection- Zookeeper in Hadoop maintains a session with the namenodes. During a failure, the session will expire. So, the Zookeeper will inform other namenodes to start the failover process. Active NameNode Election- A simple mechanism provided by Zookeeper to only elect a node as active.
What happens when a user submits a Hadoop job when the job tracker is down does the job get in to hold or does it fail?
When a user submits a job, the client get a new application id for this particular job. It will check for the output directory that was passed and will copy the jar and other resources to HDFS. It wont be able to connect to HDFS since the Namenode is down , the job will not be submitted and it will fail.
What happens when Namenode restarts?
Only in the restart of namenode , edit logs are applied to fsimage to get the latest snapshot of the file system. But namenode restart are rare in production clusters which means edit logs can grow very large for the clusters where namenode runs for a long period of time.
How can we check whether Namenode is working and how do you restart?
To check whether NameNode is working or not, use the jps command, this will show all the running Hadoop daemons and there you can check whether NameNode daemon is running or not.
What if a Namenode has Nodata?
What happens to a NameNode that has no data? Answer:There does not exist any NameNode without data. If it is a NameNode then it should have some sort of data in it.
What will happen when NameNode is down and a user submits a new job?
By Hadoop job, you probably mean MapReduce job. If your NN is down, and you don't have spare one (in HA setup) your HDFS will not be working and every component dependent on this HDFS namespace will be either stuck or crashed.
Is NameNode also a commodity hardware?
Is Namenode also a commodity? No. Namenode can never be a commodity hardware because the entire HDFS rely on it. It is the single point of failure in HDFS.
What is Namenode?
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. It does not store the data of these files itself. When the NameNode goes down, the file system goes offline.
How do you get Namenode?
List the namenode and datanodes of a cluster from any node? - Stack Overflow.
Which NameNode is used when the primary NameNode fails?
4. ________ NameNode is used when the Primary NameNode goes down. Explanation: Secondary namenode is used for all time availability and reliability.