Directory Server lets you replicate directory data for availability and load balancing between the servers in your deployment. Directory Server allows you to have multiple read-write (master) replicas deployed together.
Internally, the server makes this possible by keeping track of changes to directory data. When the same data are modified on more than one read-write replica Directory Server can resolve the changes correctly on all replicas. The data to track these changes, must be retained until they are no longer needed for replication. By default changes are retained for seven days. If your directory data undergoes much modification, especially of large multi-valued attributes, this data can grow quite large.
For a detailed discussion of Directory Server replication, read Chapter 4, Directory Server Replication, in Sun Java System Directory Server Enterprise Edition 6.1 Reference.