Since 2.2 version of OrientDB, we have migrated from the usage of sun.misc.Unsafe for direct memory management to the usage of direct ByteBuffer-s. But there is a small side effect which affects all users of embedded databases. The amount of direct memory which can be allocated in total, inside of all ByteBuffer-s, limited either to the maximum allowed heap size or to size specified in -XX:MaxDirectMemorySize parameter.
What would it be if you had not set this parameter for embedded database ? Well, nothing dramatical, OrientDB will detect the absence of this parameter and decrease the size of disk cache. As the result, all database operation would be much slower and overall system performance would suffer.
If there are no any specific requirements for your application we recommend to set this parameter to 512g for 64-bit JVMs and to 2g for 32-bit JVMs as we do inside of server startup script.
The setting of this parameter to 512g does not mean that OrientDB will consume all 512GB memory. The size of direct memory consumed by OrientDB is limited by the size of disc cache, more details on this topic may be found there.