Audit log not migrated when using MergeContentDBS and old entries remain in old database
Recently I migrated a site collection from one database to another using the StsAdm command MergeContentDBS. After this migration I found out that audit entries, stored in the audit log table of the database, were still present in the old database. In total this was occupying about 20GB of space. In one of the previous updates of SharePoint, Microsoft introduced the stsadm command trimauditlog, but unfortunately this command cannot be used to trim entries in the old database if the site collection in not in there anymore.
It would be a possiblity to manually delete the entries in the audit log, however with SharePoint you will end up with an unsupported environment after manually changing databases.
The only workaround I would currently have is to move out all remaining site collections to other databases and then delete the old database.