Memory mapped I/O can be used for read/writes to every file in the Neo4j storage. The best performance will be obtained if complete memory mapping of the file can occur, but if there is a shortage of memory for that, then Neo4j tries to optimize the memory use.
Note
Neo4j makes extensive use of the java.nio native Java package. Use of the native I/O package allows the allocation of memory external to the Java heap, which has to be handled separately. It will also depend on other system processes using memory. Neo4j allocates memory, which is a total of the JVM heap memory and the memory mapping needs, leaving the remaining memory for system processes.
It is not a great idea to use the complete available system memory for heap memory. The Neo4j data store (in the Neo4j database directory) stores the data in separate files which are outlined as follows: