Following are the physical capabilities of Directory Server that specify its scalability:
Process size. Depending on the operating system, the 32-bit versions of Directory Server supports 2GB – 4GB process size. The process size on 64-bit versions of Directory Server is defined by the amount of physical memory available on the machine. It is tested with 128GB process size.
Number of LDAP entries. The total number of LDAP entries that can be created on a single server instance is 2^32 -1, that is, 4G entries.
Size of each entry. The size of a single record in LDAP server is 4GB as per the DB itself. The size of an entry also depends on maximum size of the LDAP request (maxbersize). Its maximum value is 2 GB.
Number of LDAP connections. The number of LDAP connections depends on the number of file descriptors that a process can open. Note that too many open connections tend to degrade performances.
Size of LDAP Server (Berkery DB). The size of an LDAP server is defined by the size of your filesystem.