9.2 Providing time consistent data

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Since you cannot rely on volume dumps to restore the System Logger environment at a recovery site, you must use some mechanism that provides for time consistent data across all the components. However, simply mirroring the data from one site to another is not sufficient as you are still exposed to the possible inconsistency of data across multiple control units.

One option for providing time consistent data is IBM's GDPS service offering, and there are two variations of GDPS: GDPS/PPRC and GDPS/XRC. Using either of these products allows the disk mirroring of staging data sets, the System Logger policy (couple data set), the MVS catalogs, all System Logger exploiters' data and all the System Logger data sets to consistent state to the disaster recovery site. Only when the secondary site has the data in a consistent state can the exploiters use their log streams in the recovery site.

9.2.1 Peer to Peer Remote Copy - PPRC

PPRC is a synchronous data mirroring technology: data is committed to both primary and secondary disk subsystems before the write is confirmed to be complete. As a consequence, this technology is sensitive to performance impact as the distance between primary and secondary disk subsystems increases.

At least one system in the recovery site is a member of the same parallel sysplex with the systems in the primary site. The maximum distance between the two sites is 40 kilometers, and this limiting factor is imposed by the Sysplex timer links.

With a synchronous mirroring technology secondary data consistency does not come automatically: it needs to be managed. When GDPS/PPRC detects a freeze trigger (an indication that the disk mirroring is inoperative), it issues the PPRC commands to freeze all the disk mirroring to the secondary disk subsystems. As a result of the freeze, all the data on disk is time consistent.

To support System Logger in a GDPS/PPRC environment, you must:

Ensure that all CF-Structure log streams are defined to unconditionally use staging data sets and the volumes that house the staging data sets must be mirrored to the recovery site.
the System Logger policy primary Couple Data Set must reside on a volume that is being mirrored to the recovery site

Unfortunately, including the System Logger Couple Data Set on volumes in the PPRC configuration makes the recovery a little more complicated.

See Appendix A in GDPS/PPRC V2R8 Installation and Customization Guide, ZG24-6703 for more details.

9.2.2 Extended Remote Copy - XRC

GDPS also supports asynchronous disk mirroring technology with the GDPS/XRC version of the offering. With GDPS/XRC, there is not distance limitation on the distance between the two sites are the systems in the two sites are not part of the same sysplex.

Since GDPS/XRC does not impose any distance limitations, it can provide coverage for more regional type disasters such as earthquakes, blizzards, hurricanes and flooding.

Propagation delays resulting from long distances mean that in many cases synchronous data mirroring would have too much of a performance impact. That's the main reason for the existence of extended Remote Copy (XRC), a software centric data mirroring solution which operates asynchronously: updating of secondary devices is not time-coordinated with the updating of the primary devices.

It is important to remember that since the copy technology is asynchronous, some data loss is to be expected in a failover.

In this environment, you must again ensure that all production disk is included in the mirroring configuration.

See GDPS/XRC V2R8 Installation and Customization Guide, ZG24-6704 for more details on GDPS/XRC.

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