Chapter 6. Designing the Replication Process88Fractional replication is enabled and configured per replication agreement. The exclusion of attributesis applied equally to all entries. As far as the consumer server is concerned, the excluded attributesalways have no value. Therefore, a client performing a search against the consumer server neversees the excluded attributes. Similarly, should it perform a search that specifies those attributes in itsfilter, no entries match.Fractional replication is particularly useful in the following situations:• Where the consumer server is connected via a slow network, excluding infrequently changedattributes or larger attributes such as jpegPhoto results in less network traffic.• Where the consumer server is placed on an untrusted network such as the public Internet, excludingsensitive attributes such as telephone numbers provides an extra level of protection that guaranteesno access to those attributes even if the server's access control measures are defeated or themachine is compromised by an attacker.Configuring fractional replication is described in the replication agreement and supplier configurationsections in chapter 8, "Managing Replication," in the Administrator's Guide.6.3.3. Replication Resource RequirementsUsing replication requires more resources. Consider the following resource requirements whendefining the replication strategy:• Disk usage — On supplier servers, the changelog is written after each update operation. Supplierservers that receive many update operations may experience higher disk usage.NOTEEach supplier server uses a single changelog. If a supplier contains multiple replicateddatabases, the changelog is used more frequently, and the disk usage is even higher.• Server threads — Each replication agreement consumes one server thread. So, the number ofthreads available to client applications is reduced, possibly affecting the server performance for theclient applications.• File descriptors — The number of file descriptors available to the server is reduced by thechangelog (one file descriptor) and each replication agreement (one file descriptor per agreement).6.3.4. Managing Disk Space Required for Multi-Master ReplicationMulti-master replicas maintain additional logs, including the changelog of directory edits, stateinformation for update entries, and tombstone entries for deleted entries. This information is requiredfor multi-master replication to be performed. Because these log files can get very large, periodicallycleaning up these files is necessary to keep from wasting disk space.There are four attributes which can configure the changelog maintenance for the multi-master replica.Two are under cn=changelog5 and relate directly to trimming the changelog:• nsslapd-changelogmaxage sets the maximum age that the entries in the changelog canbe; once an entry is older than that limit, it is deleted. This keeps the changelog from growingindefinitely.
