HP StorageWorks XP24000 Continuous Access Software User and Reference Guide, v01 (T5278-96002, June 2007)

A Con tinuous Access L oad
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 a t the home site be disrupted. As with any i mplem ented application or solution, imp a ct
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 add itions to satisfy growth requirements are usually inevitable.
Workload d emand profiles consist of a series of peaks and valleys which are unpredictable in b oth
magnitudeand duration. Itbecomes very importantthatmeasuresand methodologiesbeput in placeto
minimize the impact of these anomalies.
Caching usage is the prime metho d of buffering peaks when demand exceeds network c apacity and the
prime method of unload ing 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 a lso
serve additional functions such as D FW, 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 impa ct 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.
HP’s offerings for Asynchronous Remote Copy include Continuous Access Asynchronous (Continuous
Access Asynchronous Open and TrueCopy Asynchronous for Mainframe). Each is unique in both
operational structure and capability, and different ROTs are used for cache sizing recommendations.
For Continuous Access Asynchronous, 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 a nd
is
implemented for Continuous Access Asynchronous.
This m etho dology is interchangeably referred to as device blocking be it at the channel or device level,
flow control, or device throttling. All are m ethods of controlling application demand, thus allowing the DR
solution to sustain, and ea ch is tuned to minimize impact o n host operations.
With Continuous Access Asynchronous, a graduated level of retry d elay to application host operations is
induced dependent upon the amount of residual DR data not yet ser viced by the secondary. This is a
device-level operation, and devices deemed critical may be excluded from this algorithm via host control.
It is essential that all components of a DR solution be sized correctly, implemented accordingly, and
expectations be realized.
This design concept is considered a best practice for a Disaster Recovery/Business Continuation (DR/BC)
environment. This concept d epends upon the capability of a network implemented to support the demand
of host write update activity to devices associated with the DR/BC environment without induced delay
impact to host performance.
Operational requirements must dictate the direction and design of the delivered solution. The following
options are recommended to lessen the host impact of the best practice solution.
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