DASD A
FIGURE 1 J DIAGRAM OF GEOGRAPHICALLY SPLIT DATA CENTERS.
DATA CENTER A NET
14
HOW TO MANAGE GEOGRAPHICALLY SPLIT
BY ROBERT CRAWFORD B
USINESS AND regulatory requirements constantly push the limits of data center management and recovery. Twenty years ago, trucking tapes to
a remote center for volume restores was good enough. Ten years ago, two data centers within synchronous I/O distance fit the bill. Now, with e-commerce being the primary workload driver, a good recovery plan involves data centers split over geographic distances with little or no recovery time.
Concept and Management Conceptually, the geographically split idea starts with
two data centers, as shown above (Figure 1). Note that this concept may be extended to any number of sites. In Figure 1, the two data centers are separated by a
distance too great for synchronous disk I/O, resulting in several requirements. First, each data center must have its own Direct Access Storage Device (DASD) farm to manage. Second, synchronous hardware replication will not work because of network latency. Lastly, the distance also means the logical partitions (LPARs) in each data center can’t be in the same Sysplex. The network cloud bears an important role in data center management as a switch between the two data centers. With proper internal plumbing, incoming requests can be routed to either data center site based on diverse criteria. In fact, with today’s browser-based applications, a user or customer may be switched between the data centers without interruption. Because hardware replication is unavailable, data must
be captured and applied at the logical database or access method level. There are several products available to help with this task. Some products look for updates by reading database or Virtual Storage Access Method (VSAM) logs
for updates. Any interesting changes are hurled to the other data center via communication links using various transport protocols. At the receiving end, another piece of software issues the database or access method command to complete the remote update.
Find the Right
Configuration for You Geographically split data centers may be configured
in several ways: • HOT-WARM—In the enterprise, one data center is designated as the target of all network traffic. Updates in the primary data center are replicated to the secondary site, which receives and applies the changes to its local DASD farm. In the event of a primary data center failure, the secondary site comes online with minimum fuss. • UPDATE-INQUIRY—In the update-inquiry scenario, one data center fields all updates while the other only allows inquiries. The update site sends changes to the read-only Sysplex in a timely fashion. If the update data center fails, the inquiry Sysplex assumes full responsibilities. The network is crucial to this setup as it must be able
to query message content to distinguish between inquiry and update transactions. The shop may also use the network for workload balancing so that each data center carries its share of the read-only traffic.
• UPDATE-UPDATE—This is the real deal. Each data center supports full updates to all data. 2-way replication flows over the communication links to keep the databases in synch. In the event of failure, the surviving data center takes on all incoming traffic.
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