IBM 990 - Manual (Page 182)

170 IBM
^
zSeries 990 Technical Guide
Cost benefits
Cost benefits are realized by enabling the use of non-standalone Coupling Facilities (for
example, ICFs) for all resource sharing and data sharing environments.
7.3.2 CF structure duplexing
System-managed Coupling Facility structure duplexing creates a duplexed copy of the
structure in advance of any failure, providing a robust failure recovery capability through
failover to the unaffected structure instance. This results in:
An easily-exploited common framework for duplexing the structure data contained in any
type of CF structure, with installation control over which structures are duplexed
Minimized overhead of duplexing during mainline operation via hardware-assisted
serialization and synchronization between the primary and secondary structure updates
Maximized availability in failure scenarios by providing a rapid failover to the unaffected
structure instance of the duplexed pair, with very little disruption to the ongoing execution
of work by the exploiter and applications
System-managed duplexing rebuild provides robust failure recovery capability via the
redundancy of duplexing, and low exploitation cost via system-managed, internalized
processing. Structure failures, CF failures, or losses of CF connectivity can be handled by:
1. Hiding the observed failure condition from the active connectors to the structure, so that
they do not perform unnecessary recovery actions
2. Switching over to the structure instance that did not experience the failure
3. Re-establishing a new duplex copy of the structure if appropriate as the Coupling Facility
becomes available again, or on a third CF in the Parallel Sysplex
System messages are generated as the structure falls back to simplex mode for monitoring
and automation purposes. The structure operates in simplex mode until a new duplexed
structure can be established, and can be recovered using whatever existing recovery
techniques are supported by the exploiter.
System-managed duplexing's main focus is providing this robust recovery capability for
structures whose users do not support user-managed duplexing rebuild processes, or do not
even support user-managed rebuild at all.
7.3.3 Configuration planning
A new connectivity requirement for system-managed CF structure duplexing is that there
must be bi-directional CF-to-CF connectivity between each pair of CFs in which duplexed
structure instances reside. With peer links, this connectivity can be provided by a single
bi-directional link (two with redundancy).
CF-to-CF links can either be dedicated or shared via MIF. They can be shared with
z/OS-to-CF links between z/OS and Coupling Facility images in the pair of servers they
connect. When planning sharing links, remember that receiver links cannot be shared, and
peer links can only be shared by one Coupling Facility partition.
Figure 7-7 on page 171 gives an overview of system-managed CF structure duplexing.