RDF System Management Manual for J-series and H-series RVUs (RDF 1.10)

Figure 15 ZLT Configuration With Standby and Backup Systems Located at Separate Sites
If the standby and backup systems are not one and the same, you must remember to set up remote
passwords between the standby and backup systems. You must do so with the same user ID that
has control over starting and stopping RDF.
If you lose your primary system due to an unplanned outage, you connect the remote mirrors to
the standby system, and then initiate a takeover operation on the backup system. Before performing
the takeover, RDF reads the remaining audit records from the remote mirrors, and processes those
audit records. Thus, RDF can read absolutely all of the audit records that were generated on the
primary system prior to the system failure, and no committed data is lost.
NOTE: You must connect the remote mirrors to the standby system before starting the RDF takeover
operation; otherwise, the takeover aborts because RDF cannot find the disks you configured in
RDF for remote mirroring. In such a case, you should connect the disks and then restart the RDF
takeover operation.
If you lose the primary system to a disaster and that disaster does not affect the standby and backup
systems, no committed transactions are lost because RDF on the backup system can fetch all missing
audit records from the remote mirrors. If a regional disaster takes down both the primary and the
standby systems, however, you can still resume business on the backup system but without the ZLT
guarantee. Some transactions committed on the primary system might be lost.
Using CommitHoldMode
If you want absolute ZLT protection, you must configure your audit trails with the
COMMITHOLDMODE attribute set to on. Doing so causes each write to the audit trail to be directed
to the remote audit trail disk first. If that write fails for any reason, TMF activates commit-hold mode.
322 Zero Lost Transactions (ZLT)