Turns out that I did not have all of the information regarding this parameter.

For deployments where the clients are mostly IMAP and most folders are not modified frequently, you do not want to set dbcachesize to the sum. Setting dbcachesize equal to the sum of *.db files can actually have negative impact on performance. This would result in the caching of data which is mostly stale. This would cause the process which syncs the in-memory database cache to disk to run longer due to the increased cache size.

The ideal setting of dbcachesize can be found in the following way:

1. Run 'db_stat -m' or 'db_stat -m -h /tmp/msg-<instance>'
(if store.dbtmpdir is set to /tmp/msg-<instance>).

2. The 3rd line tells you your current dbcachesize.

3. The 4th line shows the "hit rate", if it's close to 99%, your dbcachesize is correctly sized. Note: right after the messaging server starts up, the "hit rate" will be low; therefore you should let messaging server run for a period of time before assessing your "hit rate".

Sites have reported that the default of 16384K shows a "hit rate" of 99-100% even on our biggest installations (with over 200,000 folders).