The screenshot added below shows the output of hdfs fsck /. It shows that the "/" directory is corrupted. This is the masternode of my Hadoop cluster. What to do?
If you are using Hadoop 2, you can run a Standby namenode to achieve High Availability. Without that, your cluster's master will be a Single Point of Failure.
You can not retrieve the data of Namenode from anywhere else since it is different from the usual data you store. If your namenode goes down, your blocks and files will still be there, but you won't be able to access them since there would be no related metadata in the namenode.
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In my understanding, uploading files to HDFS (the filesystem for Apache Hadoop) follows the below procedures:
client (hdfs shell) asks Namenode which datanodes to put the data chunks
Namenode answers it and saves the files location and some metadata in itself
client puts the data chunks to the given datanodes
Suppose, in 1, Namenode returned which datanode to store the data. However, after that, the datanode gets unavailable because of some reasons (e.g. Network failure or Machine outage). So, the data cannot be saved in datanode but the metadata is stored in Namenode. The data gets inconsistent state.
Can someone explain how HDFS is avoiding this situation? I tried to read hadoop source code but I finally gave up because it's huge.
I couldn't understand the difference between secondary name node and standby name node and backup name node. I am looking for in depth understanding of these terms. Kindly help me out with this.
Secondary namenode is just a helper for Namenode.
It gets the edit logs from the namenode in regular intervals and applies to fsimage.
Once it has new fsimage, it copies back to namenode.
Namenode will use this fsimage for the next restart, which will reduce the startup time.
Secondary Namenode's whole purpose is to have a checkpoint in HDFS. Its just a helper node for namenode. That’s why it also known as checkpoint node.
But, It cant replace namenode on namenode's failure.
So, Namenode still is Single-Point-of-Failure.
To overcome this issue; STANDBY-NAMENODE comes into picture.
It does three things:
merging fsimage and edits-log files. (Secondary-namenode's work)
receive online updates of the file system meta-data, apply them to its memory state and persist them on disks just like the name-node does.
Thus at any time the Backup node contains an up-to-date image of the namespace both in memory and on local disk(s).
Cluster will switch over to the new name-node (this standby-node) if the active namenode dies.
However, the answer explained above is satisfactory but I want to add some points to it.
About Standby-Namenode
Both active and standby Namenode use a shared directory and standby Namenode sync through that directory from time to time so there must be no delay in activating it if the active Namenode goes down.
But the main factor is about the block reports, Block reports are not written in edit-logs, they are stored in local disk space. So syncing with a shared directory is not enough.
To avoid this conflict, data-nodes has the addresses of both the name-nodes,
and they send the block reports to both of them but they only follow the block commands coming from the active Namenode.
Hope this is helpful
Standby Node : In the case of an unplanned event such as a machine crash, the cluster would be unavailable until an operator restarted the NameNode.Planned maintenance events such as software or hardware upgrades on the NameNode machine could result in whole cluster downtime. 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.Secondary NameNode in hadoop is a specially dedicated node in HDFS cluster whose main function is to take checkpoints of the file system metadata present on namenode. It is not a backup namenode. It just checkpoints namenode’s file system namespace. The Secondary NameNode is a helper to the primary NameNode but not replace for primary namenode.
The Secondary namenode maps the fsimage and the edit log transactions periodically stores them in a shared storage location in case of HA enabled HDFS Cluster.
In other hand, Standby node has the ability to transfer the latest built fsimage to the Active NameNode via HTTP Get call .
So the main difference between Secondary and standby namenode is secondary namenode does not upload the merged Fsimage with editlogs to active namenode
where as the standby node uplods the merged new image back to active Namenode.
So the NameNode need to fetch the state from the Secondary NameNode
Does hard restart of agent delete data?
I did a hard restart of my agent and now I do not see data in hadoop fs -ls /user/hue or hadoop fs -ls /user/hive where did it go? I also do not see my other users but only hue and hive. What do I do? Where did it go?
I don't think data in hdfs should go anywhere with that.
If I query my tables in hive, I keep getting
The operation has no results
Help please!
Doing a hard restart of the Cloudera Manager agent will not cause data loss, but will cause all of the Hadoop daemons to be restarted. A normal restart of the agent does not do this, so a hard restart is useful if you need to force a stop of all the running processes.
If you are seeing no data in HDFS following a restart check the status of the HDFS service in Cloudera Manager. It will tell you how much capacity is used in HDFS, the number of files and other metrics. If you're seeing no data it could be that your DataNodes have not been started. Check to see if this is the case and if your NameNode is still in safe mode.
I want to delete datanode from my hadoop cluster, but don't want to lose my data. Is there any technique so that data which are there on the node which I am going to delete may get replicated to the reaming datanodes?
What is the replication factor of your hadoop cluster?
If it is default which is generally 3, you can delete the datanode directly since the data automatically gets replicated. this process is generally controlled by name node.
If you changed the replication factor of the cluster to 1, then if you delete the node, the data in it will be lost. You cannot replicate it further.
Check all the current data nodes are healthy, for these you can go to the Hadoop master admin console under the Data nodes tab, the address is normally something link http://server-hadoop-master:50070
Add the server you want to delete to the files /opt/hadoop/etc/hadoop/dfs.exclude using the full domain name in the Hadoop master and all the current datanodes (your config directory installation can be different, please double check this)
Refresh the cluster nodes configuration running the command hdfs dfsadmin -refreshNodes from the Hadoop name node master
Check the Hadoop master admin home page to check the state of the server to remove at the "Decommissioning" section, this may take from couple of minutes to several hours and even days depending of the volume of data you have.
Once the server is shown as decommissioned complete, you may delete the server.
NOTE: if you have other services like Yarn running on the same server, the process is relative similar but with the file /opt/hadoop/etc/hadoop/yarn.exclude and then running yarn rmadmin -refreshNodes from the Yarn master node
I have installed a hadoop cluster with total 3 machines, with 2 nodes acting as datanodes and 1 node acting as Namenode and as well as a Datanode.
I wanted to clear certain doubts regarding hadoop cluster installation and architecture.
Here is a list of questions I am looking answers for----
I uploaded a data file around 500mb size in the cluster and then checked the hdfs report.
I noticed that the namenode I made is also occupying 500mb size in the hdfs, along with datanodes with a replication factor of 2.
The problem here is that I want the namenode not to store any data on it, in short i dont want it to work as a datanode as it is also storing the file I am uploading. So what is the way of making it only act as a Master Node and not like a datanode?
I tried running the command hadoop -daemon.sh stop on the Namenode to stop the datanode services on it but it wasnt of any help.
How much metadata does a Namenode generate for a filesize typically of 1 GB? Any approximations?
Go to conf directory inside your $HADOOP_HOME directory on your master. Edit the file named slaves and remove the entry corresponding to your name node from it. This way you are only asking the other two nodes to act as slaves and name node as only the master.