Support have recently received a number of cases where customers have gotten themselves into a spot of trouble because they enabled mirror mode in the CM IDOL Service's configuration file.
Unfortunately people seem to be following the rather simplistic suggestion in CM's standard documentation "IDOL and Document Content Indexing Installation and Configuration", which suggests enabling MirrorMode=True might be a good thing to do, without properly considering the effect of doing so. This option tells the CM IDOL Service to make every content engine an exact mirror of the others. So for example if you have 10 content engines in mirror mode, you have 10 copies of the same index data. Each OEM engine has a licensed limit of 10M documents. If they're all mirrored, the maximum # of docs you'll be able to add to your IDOL index is 10 million. If they were in the default non-mirror mode you could fit 100M docs in your IDOL index.
Micro Focus published a knowledgebase article many years ago explaining the right way to go about mirroring your CM IDOL environment:
What seems to have happened at a number of sites is the recommendations in the above article were followed, AND MirrorMode=True was also set in the IDOL Service's configuration. This is bad. Don't do both.