Stubbing is an automated operation within HDI which provides the 'Elastic' capability to the File System. It is important to note that stubbing operates independently from Migration to HCP although with the introduction of Automatic File Migration (AFM), a migration to HCP event can trigger stubbing to operate.
In normal operation stubbing will run every 30 minutes to check how full a file system is. By default a 'high water mark' is set at 90% of the file system capacity. If the file system is found to be filled to above the high water mark during one of the 30 minute checks then this will trigger a stubbing operation to take place. The stubbing operation will then take some of the oldest files in the file system (based on m-time) and stub those files down to a 4K stub which points to the full data object in HCP. By stubbing the files the capacity of the file system will be reduced to below the high water mark, allowing more files to be ingested into HDI.