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Depending on the jurisdiction in which the DMS is being operated, you may be obliged by law to (a) retain audit log information up to a certain date or, alternatively, (b) remove all auditing data after a certain retention period. Please make sure to observe the applicable legal frameworks.
Purging the nuxeo audit log, incl. fully vacuuming the database and rebuilding its indexes is very time consuming (up to and potentially beyond 1hr per 30GB of database size, depending on disk i/o). It is important that you schedule the audit log purging appropriately.