Before migrating from a 5.1 server instance, ensure that the following prerequisites are met:
Directory Server 6.0 must be installed. The new server can be installed on the same machine as the existing server or on a different machine.
Ensure that the new machine has sufficient local disk space to house binaries and databases for both the old and new servers, and also enough extra space to hold LDIF files containing the entries in all existing suffixes. You can estimate the local disk space required as somewhat larger than the following calculation.
local space required = 2 * (space for existing server) + (space for LDIF files)