Manardb provides persistent classes (integrated into the object system via the meta-object protocol), stored efficiently in memory mapped regions. It features transactions, two different garbage collection mechanisms (to clean up unreferenced objects in the database), in-place modification of arrays, efficient in-place numeric typed slots, and the ability to transparently serialise Lisp objects of many types (lists, vectors, floats, integers, symbols, strings).
It allows Common Lisp programs to efficiently access in-memory representations of large numbers of persistent objects without putting pressure on the Lisp garbage collector.
2020 review of manardb for a particular use case
Repository: https://github.com/danlentz/manardb
ObjectStore persistence