Best practices for disk space management

You can maintain the high performance of decision services in your application by following best practices for allocating disk space to the Decision Data Store (DDS) nodes.

• For DDS nodes, perform the following actions:
  • Assign a maximum of 1 TB of DDS data per Cassandra node, with a maximum of 100 GB per node for a single table.
    Important: To avoid very long compaction procedures and, in effect, a build-up of SSTables, you can configure the compaction settings for SSTables. For more information, see Configuring compaction settings for SSTables.
  • Facilitate compaction by ensuring at least 2 TB of disk space.

    Use an HDD with a maximum capacity of 1 TB, or an SSD with a maximum capacity of between 2 and 5 TB.

    Note: To avoid issues when compacting the largest SSTables, ensure that the disk space that you provide for Cassandra is at least double the size of your Cassandra cluster. One single DDS node running out of disk space does not affect service availability, but might cause performance degradation and eventually result in failure. For more information, see Sizing a Cassandra cluster.
  • Ensure that all DDS nodes have the same disk capacity.
  • Store the commit log and caches on separate disks by configuring the following properties: dnode/yaml/commitlog_directory and dnode/yaml/saved_caches_directory.
• For application data, perform the following actions:
  • Avoid distributing data unequally across nodes by limiting the size of a single data record to less than 100 MB.

    For DDS data sets, when the size of the data record exceeds the threshold limit, Pega Platform triggers the PEGA0079 alert. For more information, see PEGA0079 alert.

  • Avoid splitting records across nodes by writing short rows.

    For example, do not write to a table as a ping test by using the same partition key repeatedly.

• Monitor the available disk space on a regular basis.
  For more information, see Verifying the available disk space.