z/OS MVS Setting Up a Sysplex
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What is a single point of failure?

z/OS MVS Setting Up a Sysplex
SA23-1399-00

A single point of failure is an environment where one failure can result in the simultaneous loss of both the coupling facility list structure for a log stream and the local storage buffer copy of the data on the system making the connection. If you were using local storage buffers to duplex data, this can result in loss of log data. When your configuration contains a single point of failure, you can safeguard your data by duplexing data to staging data sets.

A single point of failure can also exist for a duplex-mode structure where one failure can result in the simultaneous loss of both structure instances. The two structure instances in this environment are failure-dependent, meaning a single failure will cause the structure data to be lost, even though it is in two copies of the structure. Since a single point of failure can cause the loss of all the log data in the failure-dependent duplex-mode structure, Logger will continue to safeguard the data by using one of its own duplexing methods.

When a configuration is not vulnerable to a single point of failure, it is failure independent.

System logger evaluates a connection for a single point of failure based on the location of the coupling facility and its status of volatile or non-volatile. The coupling facility can reside in one of the following configurations:
  • The coupling facility executes in a logical partition or LPAR (virtual server), sharing a central processing complex (CPC) with a system that is connected to a log stream associated with the coupling facility.
  • The coupling facility is in a stand-alone CPC and does not share the CPC with a system with a connection to the log stream.
  • The coupling facility executes as an Integrated Coupling Migration Facility (ICMF) partition on a CPC. Only connections from a system on the same CPC can communicate with this coupling facility.
System logger uses the following rules for determining whether a log stream connection contains a single point of failure:
  • When a coupling facility shares a CPC with a system that connects to log streams mapping to the same coupling facility, each active connection between the system and the coupling facility contains a single point of failure, regardless of the volatility state of the coupling facility. The connections have a single point of failure because if the CPC should fail, coupling facility resident log data will be lost when both the system, with local storage buffers, and the coupling facility fail with the CPC.
  • When a coupling facility (or the composite view of a duplex-mode structure) is separate from the system connected to the associated log stream, the volatility status of the coupling facility determines whether the connection is vulnerable to a single point of failure:
    • If the coupling facility (or the composite view of a duplex-mode structure) is non-volatile, the connection is failure independent.
    • If the coupling facility (or the composite view of a duplex-mode structure) is volatile, the connection is vulnerable to a single point of failure. A power failure could knock out both the system, with its local storage buffer copy of coupling facility data, and the coupling facility.

    A connection is failure independent when it is from a system on a separate CPC from the non-volatile coupling facility to which it is connected.

If a connection contains a single point of failure and STG_DUPLEX(YES) DUPLEXMODE(COND) has been specified in the log stream definition, system logger will duplex coupling facility log data to staging data sets for that connection. If a coupling facility or CPC failure occurs, system logger can retrieve lost coupling facility log data from the staging data sets for a connection with a single point of failure.

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