The relinker function can be enabled in the realtime mode by setting the configutil parameter local.store.relinker.enabled to yes. Using relinker in the realtime mode will compute the digest of every message delivered (or restored, IMAP appended, and so forth) which matches the configured relinker criteria (18.104.22.168 Configuring Relinker), then look in the repository to see if that digest is already present. If the digest is present, it creates a link to it in the destination mailbox instead of creating a new copy of the message. If there is no digest, it creates the message and adds a link to it in the repository afterwards.
stored scans the digest repositories of each partition and purges the messages having a link count of 1, or which don’t match the relinker criteria. The scan is done one directory at a time over a configurable time period. This is so that the I/O load is evenly distributed and doesn’t noticeably impact other server operations. By default the purge cycle is 24 hours, which means messages can still be present on the disk for up to 24 hours after they’ve been deleted from the store or have exceeded the configured maximum age. This task is enabled when the relinker realtime mode is enabled.