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Disk cache cleanup and tuning
Objects that are in the disk cache are cleaned up when they are explicitly invalidated through either programmatic or policy-based invalidations, or when the objects expire. When the disk cache is cleaned up the tables that host the dependency ID to cache ID mappings and template ID to cache ID mappings are updated and the disk space occupied by the cache entries is returned to the internal storage manager. The available space on the file system does not increase after the objects are deleted from the cache because the space is claimed back by the internal storage manager and reused by other objects that are cached to the disk.
The cleanup is done in the background as a low priority thread to reduce contention for the disk from active request and response threads. The time to perform this cleanup, as reported in the logs, tracks the duration of the scan. With the low priority of the scan, it can take several minutes.