Hitachi TrueCopy for IBM z/OS User and Reference Guide (T5211-96001, June 2007)

C-2 TrueCopy for z/OS Load Balancing and Sidefile Management and Control
Hitachi Universal Storage Platform V TrueCopy for IBM z/OS User’s Guide
Load Balancing and Sidefile Management and Control
Asynchronous Remote Copy solutions, as applicable to Disaster Recovery (DR),
protect operational environments by assuring data integrity and rapid recovery
at an alternate location should the operational environment at the home site
be disrupted. As with any implemented application or solution, impact to the
operational environment must be minimal.
In an environment with a stable workload and a network (paths, channel
extension, secondary capability) properly sized and implemented to support
the workload requirement, solution success is virtually assured. Workload
requirements, however, are not always fixed or predictable. Workloads
change, coincidental application demands occur, and workload additions to
satisfy growth requirements are usually inevitable.
Workload demand profiles consist of a series of “peaks and valleys” which are
unpredictable in both magnitude and duration. It becomes very important that
measures and methodologies be put in place to minimize the impact of these
anomalies.
Caching usage is the prime method of buffering peaks when demand exceeds
network capacity and the prime method of unloading when valleys occur.
Cache availability is mandatory for these sequences to occur and the quantity
of available cache dictates the size and duration of the peaks which can be
accommodated. Cache is a shared resource in that it is the prime host access
facility. It must also serve additional functions such as DFW, CFW, Concurrent
Copy Sidefile, and in this case, Remote Copy Sidefile buffering.
Rules of Thumb (ROT) based on past experiences are used to provide a best
effort recommendation as to the amount of cache required to sustain the
environment with minimal impact to host applications. With undersized cache
to accommodate these demands, probable negative effects are decreased
cache hit ratios, inability to start CC sessions, busy states to Write Pending
operations, and so on.
Hitachi’s offerings for Asynchronous Remote Copy include TrueCopy Async
(TCA Open and TCzA). Each is unique in both operational structure and
capability, and different ROTs are used for cache sizing recommendations. For
TCA, one and a half times the current, none DR, amount of cache is
recommended. An additional methodology called device blocking is used to
sustain the environment and is implemented for TCA.
This methodology is interchangeably referred to as device blocking be it at
channel or device level, flow control, or device throttling. All are methods of
controlling application demand thus allowing the DR solution to sustain and
each is tuned to minimize impact on host operations.