How do I keep my HDFS cluster balanced?

How do I keep my HDFS cluster balanced?

However, it’s not always possible to prevent a cluster from becoming unbalanced. Perhaps you want to limit the number of maps so that some of the nodes can be used by other jobs. In this case, you can use the balancer tool (see Balancer) to subsequently even out the block distribution across the cluster.

How can I check my HDFS balance?

You can run the balancer manually from the command line by invoking the balancer command. The start-balancer.sh command invokes the balancer. You can also run it by issuing the command hdfs –balancer.

What is balancing in HDFS?

HDFS provides a balancer utility. This utility analyzes block placement and balances data across the DataNodes. It keeps on moving blocks until the cluster is deemed to be balanced, which means that the utilization of every DataNode is uniform.

What is rebalancing in Hadoop?

The Balancer tool available in Hadoop is used to balance the data blocks across all the datanodes when a new datanode is added or when an existing datanode reaches full capacity.

How does HDFS balancer work?

The HDFS balancer re-balances data across the DataNodes, moving blocks from overutilized to underutilized nodes. As the system administrator, you can run the balancer from the command-line as necessary — for example, after adding new DataNodes to the cluster.

Why we need disk balancer and what is HDFS disk balancer?

Disk Balancer is a command-line tool introduced in Hadoop HDFS for Intra-DataNode balancing. HDFS diskbalancer spread data evenly across all disks of a DataNode. HDFS Disk Balancer operates against a given DataNode and moves blocks from one disk to another.

What is expunge in HDFS?

13. expunge: This command is used to empty the trash available in an HDFS system.

What is rack awareness in Hadoop HDFS?

Hadoop components are rack-aware. For example, HDFS block placement will use rack awareness for fault tolerance by placing one block replica on a different rack. This provides data availability in the event of a network switch failure or partition within the cluster.

How are file systems checked in HDFS?

The fsck Hadoop command is used to check the health of the HDFS. It moves a corrupted file to the lost+found directory. It deletes the corrupted files present in HDFS. It prints the files being checked.

How do I fix a corrupt block in HDFS?

Missing mount points, datanode not running, file system reformatted/reprovisioned. If you can find a problem in that way and bring the block back online that file will be healthy again. Lather rinse and repeat until all files are healthy or you exhaust all alternatives looking for the blocks.

What is rack in cluster?

Most of us are familiar with the term Rack. The rack is a physical collection of nodes in our Hadoop cluster (maybe 30 to 40). A large Hadoop cluster is consists of many Racks. The Hadoop itself is so smart that it will automatically write a particular file block in 2 different Data nodes in Rack.

What is yarn cluster?

YARN is an Apache Hadoop technology and stands for Yet Another Resource Negotiator. The technology is designed for cluster management and is one of the key features in the second generation of Hadoop, the Apache Software Foundation’s open source distributed processing framework.

How does HDFS balancer work in Hadoop cluster?

The aforementioned HDFS balancer is a tool provided by Hadoop to balance the data spread across the DataNodes in a cluster by moving data blocks from the over-utilized to the under-utilized DataNodes. Figure 9.8 shows the idea behind the balancer tool.

What happens when data is added to a HDFS cluster?

Whenever a new data node is added to the existing HDFS cluster or a data node is removed from the cluster then some of the data nodes in the cluster will have more/less blocks compared to other data nodes. In this unbalanced cluster, data read/write requests become very busy on some data nodes and some data nodes are under utilized.

How does the HDFS disk balancer measure data density?

In order to define a way to measure which machines in the cluster suffer from the uneven data distribution, the HDFS disk balancer defines the HDFS Volume Data Density metric and the Node Data Density metric. HDFS Volume data density metric allows us to compare how well the data is spread across different volumes of a given node.

What is the default balancer threshold for HDFS?

This balancer command uses the default threshold of 10 percent. This means that the balancer will balance data by moving blocks from over-utilized to under-utilized nodes, until each DataNode’s disk usage differs by no more than plus or minus 10 percent of the average disk usage in the cluster.