Purging a deduplication device is different that purging a staging FSD. When a staging FSD is purged, Arcserve Backup immediately removes the session file. But for a deduplication session purge, Arcserve Backup renames the session hash file to .hash_ToPurge and updates the reference counter. In other words, the session is merely "marked" as purged, but not actually removed because there may be other sessions that still point to the original data.
The reference counter stored in the index files is decreased. When the reference counter reaches 0, no more hashes referencing the original data exist and the data chunk is now considered to be a "hole." When Arcserve Backup finds data files with holes greater than 25%, disk space is reclaimed by a purge thread that runs every 6 hours.
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