RDF/IMP, IMPX, and ZLT System Management Manual

HP NonStop RDF/IMP, IMPX, and ZLT System Management Manual—524388-002
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Zero Lost Transactions (ZLT)
Zero Lost Transactions (ZLT), functionality that is available only with the RDF/ZLT
product, ensures that no transactions that commit on the primary system are lost on
the RDF backup system if that primary system is downed by an unplanned outage.
RDF achieves this though the use of remote mirroring for the relevant TMF audit-trail
volume(s). That is, one mirror of an audit-trail volume remains local to the primary
system, but the other mirror is located at a remote site.
When a primary system is downed by some unplanned outage or disaster, there may
be some audit data that the extractor on the primary system was unable to send to the
backup system before the outage. With ZLT functionality, RDF fetches all remaining
audit data from the remote mirror, thereby guaranteeing no loss of committed data
during the RDF takeover operation.
The ZLT guarantee can only be maintained if the remote mirror of each TMF audit-trail
volume is up. If any remote mirror fails, RDF falls back to non-ZLT replication for all
audit trails until the downed mirror comes back up and is fully revived.
ZLT with remote mirror has no impact on normal RDF operations. If you must perform
an RDF takeover operation, however, there are additional steps involved that can
lengthen the time to perform the overall operation. In return, you get the ZLT
guarantee of not losing any transactions that committed on the primary system.
ZLT functionality is only available on S-series systems, although RDF 1.6 runs
correctly on K-series systems without using ZLT. As a result, RDF users on K-series
systems can upgrade to RDF 1.6, but ZLT functionality is not available to them.
How It Works
One mirror of each audit-trail disk volume is removed to a remote location from the
local mirror. The distance is limited by the chosen disk technology and acceptable
communications latency. Thus, each audit-trail volume is still mapped to a mirrored
pair of disks, but one of the disks is physically removed. For the remote mirror, an
alternate cable must be present so that this mirror can be attached to a standby
system in the event of a takeover. That standby system can be either the backup
system itself or a separate system geographically removed from the primary and
backup systems. If you are using a separate standby system, it must be connected to
the backup system by way of Expand lines.
Note. Because writes take longer to complete as the distance from the primary system
to the remote mirror increases, commit response times may increase and transaction
throughput may decrease. HP Professional services are equipped with the tools to
help the customer understand this impact.