6.3.3 Replication resource requirements

Using replication requires more resources. Consider the following resource requirements when defining the replication strategy:

•Disk usage — On supplier servers, the changelog is written after each update operation. Supplier servers that receive many update operations may experience higher disk usage.

NOTE:

Each supplier server uses a single changelog. If a supplier contains multiple replicated databases, 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 of threads available to client applications is reduced, possibly affecting the server performance for the client applications.

•File descriptors — The number of file descriptors available to the server is reduced by the changelog (one file descriptor) and each replication agreement (one file descriptor per agreement).

6.3.4Managing disk space required for multi-master replication

Multi-master replicas maintain additional logs, including the changelog of directory edits, state information for update entries, and tombstone entries for deleted entries. This information is required for multi-master replication to be performed. Because these log files can get very large, periodically cleaning 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-changelogmaxagesets the maximum age that the entries in the changelog can be; once an entry is older than that limit, it is deleted. This keeps the changelog from growing indefinitely.

•nsslapd-changelogmaxentriessets the maximum number of entries that are allowed in the changelog. Like nsslapd-changelogmaxage, this also trims the changelog, but be careful about the setting. This must be large enough to allow a complete set of directory information or multi-master replication may not function properly.

The other two attributes are under the replication agreement entry in cn=replica,

cn="suffixDN", cn=mapping tree, cn=config. These two attributes relate to maintenance information kept in the changelog, the tombstone and state information, rather than the directory edits information.

•nsDS5ReplicaPurgeDelay sets the maximum age that tombstone (deleted) entries and state information can be in the changelog. Once a tombstone or state information entry is older than that age, it is deleted. This differs from the nsslapd-changelogmaxageattribute in that the nsDS5ReplicaPurgeDelay value applies only to tombstone and state information entries; nsslapd-changelogmaxageapplies to every entry in the changelog, including directory modifications.

•nsDS5ReplicaTombstonePurgeInterval sets the frequency which the server runs a purge operation. At this interval, the Directory Server runs an internal operation to clean the tombstone and state entries out of the changelog. Make sure that the maximum age is longer than the longest replication update schedule or multi-master replication may not be able to update replicas properly.

The parameters for managing replication and the changelog are described in chapter 2, "Core Configuration Attributes," in the Configuration, Command, and File Reference.

84 Designing the replication process