When the index.recovery.mode property is set to "VALIDATE" or "AUTO", Alfresco will check during startup whether the Lucene indexes are up to date or not (it does this by comparing transactions in the repo with transactions that have been indexed). If the indexes are out of date, Alfresco will incrementally rebuild just the missing index entries.
In contrast, if index.recovery.mode=FULL, during startup Alfresco will assume the indexes (if any) that are currently on disk are completely out of date, and rebuild the entire Lucene index from scratch (which can take a long time if there's a lot of content in the repo).
This is relevant to backup/restore in that a backup set (particularly a hot backup set) may contain a copy of the Lucene indexes that are somewhat stale (out of date) compared to the backup of the repo itself. This means that in the event of a restore Alfresco will need to bring the indexes back up to date by reindexing the missing transactions - this is the behaviour if index.recovery.mode=AUTO.
You can also skip backing up the Lucene indexes entirely and set index.recovery.mode=FULL, however this may dramatically increase the time it takes for a restore to occur.
Cheers,
Peter
