Hadoop High Availability

In this chapter

This chapter explains High Availability feature supported by QueryIO. Various aspects of high availability explained are:


High Availability(HA) feature of QueryIO will make sure that QueryIO agent service on host is always available. To make it possible, in every two minutes(configurable), host checks itself whether the QueryIO agent process is up or not and if it is down, then host will start QueryIO agent process on itself. This is made possible through crontab, which is a time-based job scheduler in Unix-like computer operating systems.

HDFS has always had a well-known single point of failure which impacts HDFS' availability: The system relies on a single NameNode to coordinate access to the file system data and if NameNode is down, then the whole cluster is unavailable. Hadoop introduced its own High Availability feature to ease this problem. The HA-Enabled cluster comprises of two redundant NameNodes, one is in Active state and the other is in StandBy state. These two nodes access a shared directory(example: NFS Mounted disk) to synchronize their FileSystem Image through Audit Logs. Now the single point of failure is reduced to availability of this shared directory. This feature provides a hot-standby between these two nodes and in case of failure, manual failover can be performed through QueryIO UI to switch states between these nodes.

NameNode federation does not support HA feature. There must be single active NameNode in the cluster for HA feature to work.

HA feature with QJM (Quorum Journal Manager)

For HA feature to use QJM, Add Standby NameNode with QJM configured.
For High availability, Standby NameNode needs to be synchronized with active NameNode, so both NameNodes communicate with each other through Journal Node. When any namespace modification is performed by the Active node, it durably logs a record of the modification to a majority of these JNs. Standby NameNode constantly monitors edit logs at journal nodes and updates its namespace accordingly.
In the event of failover, standby NameNode will ensure that its namespace is completely updated according to edit logs before it is changes to Active state. Thus namespace is fully synchronized before a failover.

Click on FailOver to initiate HA feature.

HA feature with NFS Mount

How to use HA Feature

For HA feature to use QJM, Add Standby NameNode with NFS mount configured.

Use following guidelines to configure HA feature:

NFS Mount Steps

Both the NFS server and NFS client must have parts of the NFS package installed and running. NFS is installed by default, and also by default NFS is activated when the system boots. If you want to install NFS package manually, use following command:

Configuring NFS on The Server

Configuring NFS on The Client

Mounting the NFS Directory

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