Review the cache documentation at:
Review the note on memcached use in config-dist.php
Review the note on memcached use in cache/README.md
MDL-45724Detect the use of same memcached instance for both sessions and muc and warn about it
will be (partly) resolved by
MDL-45810Useless cache warning - "This store doesn't meet the requirements for all known definitions. Definitions for which this store is inadequate will be given the original default store instead of the selected store"
will help resolve
MDL-45375Allow multiple MUC instances on one memcached server using cachestore_memcache