Scaling Namespaces and Optimizing Data Storage
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Balancing data across disks of a DataNode

The HDFS Disk Balancer is a command line tool that evenly distributes data across all the disks of a DataNode to ensure that the disks are effectively utilized. Unlike the HDFS Balancer that balances data across DataNodes of a cluster, the Disk Balancer balances data at the level of individual DataNodes.

Disks on a DataNode can have an uneven spread of data because of reasons such as large amount of writes and deletes or disk replacement. To ensure uniformity in the distribution of data across disks of a specified DataNode, the Disk Balancer operates against the DataNode and moves data blocks from one disk to another.

For a specified DataNode, the Disk Balancer determines the ideal amount of data to store per disk based on its total capacity and used space, and computes the amount of data to redistribute between disks. The Disk Balancer captures this information about data movement across the disks of a DataNode in a plan. On executing the plan, the Disk Balancer moves the data as specified.