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The following considerations apply to sysplex couple data sets
as well as
to couple data sets that hold policy data. - A Couple Data Set Can Be Formatted with the Couple Data Set
Format Utility
IBM® provides
a couple data set format utility, IXCL1DSU, to format the couple data
sets. Sample JCL to invoke the format utility, along with explanations
of the information you need to specify when formatting each type of
couple data set is provided in Format utility for couple data sets.
(The
WLM couple data set can also be formatted interactively using the
WLM administrative application. See z/OS MVS Planning: Workload Management.)
- Format the couple data set to support the number
of systems in the sysplex
All systems in a sysplex that use  function-specific data must be able to access both the
primary and alternate couple data sets that were created to contain
that data. The value specified on the MAXSYSTEM parameter for function
couple data sets should match the value specified for MAXSYSTEM when
formatting the sysplex couple data set. 
- An Alternate Couple Data Set Is Highly Recommended
To
avoid a single point of failure in the sysplex, IBM recommends that, for all couple data sets,
you create an alternate couple data set on a different device, control
unit, and channel from the primary. Note: For storage
subsystem devices like the 2105 ESS, which emulates multiple logical
control units, you should attempt to place the alternate couple data
set on a different physical control unit if possible.
Information
about the sysplex and the services that use couple data sets is maintained
in both the primary and alternate couple data sets concurrently. If
the primary data set fails, the sysplex automatically makes the alternate
the primary.
Besides preventing a single point of failure,
having an alternate couple data set has other advantages. You can
issue the SETXCF COUPLE,PSWITCH command to dynamically make the alternate
couple data set the primary. Thus, in a non-disruptive way, you can:
- Expand the size of the primary couple data set by switching to
an alternate that is formatted to accommodate additional ITEMS appropriate
to that type of couple data set.
- Change the couple data set to a different device, by defining
the alternate couple data set on a different device from the primary.
Thus, you can circumvent device errors or take advantage of the characteristics
of a different device.
Then,
take a formatted spare couple data set (or format a new alternate
couple data set) and define it to XCF: SETXCF COUPLE,ACOUPLE=(alt-dsname[,alt-volume]),TYPE=dstype
The system issues message IXC267E when the primary or
alternate couple data set of any type is removed, to indicate that
the sysplex is operating without an alternate. Since the remaining
couple data set constitutes a single point of failure, consider defining
automation rules to respond to message IXC267E and automatically activate
a pre-formatted spare couple data set using the SETXCF COUPLE,ACOUPLE
command.
- An Alternate Couple Data Set Must Be At Least As Large As the
Primary
After a primary couple data set has become active
in the sysplex, the couple data set can be removed from service by
adding an alternate couple data set (if one does not currently exist)
to the sysplex and by issuing a “PSWITCH” to make the alternate couple
data set the primary. When an alternate couple data set is added to
the sysplex, it must be at least as large as the current primary data
set. That is, the alternate couple data set must be formatted with
ITEM values equal to or greater than the ITEM values that were used
to format the primary couple data set. For example, if the primary
sysplex couple data set was formatted for 50 XCF groups and 200 XCF
group members, the new sysplex couple data set must be formatted for
50 or more XCF groups and 200 or more XCF group members. Likewise,
the values specified for the MAXSYSTEM parameter and if applicable,
the MAXGROUP/MAXMEMBER parameters, must be equal to or greater that
the values that were used to format the primary couple data set.
When
formatting couple data sets consider specifying parameter values commensurate
with your hardware and software configuration. You might wish to add
some contingency (for example, 20%) to individual parameter values
to accomodate growth.
- Avoid Overspecifying Parameter Values
Do not excessively
overspecify parameter values when formatting couple data sets. Formatting
couple data sets with parameters that are overspecified results in
wasted space and degraded couple data set performance. Furthermore,
if the current primary couple data set is formatted with parameter
values that are overspecified and it becomes necessary to format and
switch to a couple data set with smaller parameter values, the sysplex
might need to be reinitialized in order to switch to the smaller format
couple data set.
It is legitimate to add some contingency to
parameter values to allow for growth. However, do not add excessive
contingency. Remember that you can always increase couple data set
capacity by issuing “PSWITCH” to switch to an alternate couple data
set that is formatted with parameter values greater than the parameter
values that were used to format the primary couple data set.
- A Spare Couple Data Set Is Recommended
When an alternate
couple data set replaces a primary, the original primary data set
is deallocated, and there is no longer an alternate couple data set.
Because
it is advisable to have an alternate couple data set always available
to be switched, for each couple data set you plan to use, consider
formatting three data sets before the IPL. For example, for the sysplex
couple data set, you might format the following: SYS1.XCF.CDS01 - specified as the primary couple data set
SYS1.XCF.CDS02 - specified as the alternate couple data set
SYS1.XCF.CDS03 - a spare
Then, if the alternate
(CDS02) becomes the primary, you can issue the SETXCF COUPLE,ACOUPLE
command to make the spare data set (CDS03) the alternate.
- The Couple Data Set Must Not Exist Prior to Formatting
The
format utility cannot use an existing data set. This prevents the
accidental re-formatting of an active couple data set. You must delete
an existing couple data set before reformatting it.
- The Couple Data Set Must Be Formatted, Not Copied
The contents of an existing, in-use couple data set should
not be copied to another data set for use by another sysplex, or another
instance of a sysplex. If a couple data set must be moved to another
volume, the preferred way to do this would be to use the IXCL1DSU
couple data set format utility to format a new, empty couple data
set of the required type on the required volume (making sure that
the formatting parameters for the data set provide enough capacity
to bring the couple data set into use with the currently-active primary
couple data set of that type), and then bring the data set into use
as the alternate couple data set using the SETXCF COUPLE,ACOUPLE command.
Then, if you want, the new data set might be made the active primary
couple data set using the SETXCF COUPLE,PSWITCH command.
Note that couple data sets contain the following: - The sysplex name of the sysplex to which they pertain and in which
they will be used.
- Sysplex ownership records that contain information about the instance
of the sysplex that last used the couple data set.
- Various real-time status and policy information that pertains
to the function for which the couple data set is formatted, for example,
SYSPLEX, CFRM, SFM, and so forth.
Simply making a copy of a couple data set creates a snapshot
of all of this information at a point in time; as time then passes,
this snapshot of the function's usage and policy information can
become more and more out-of-date. Subsequently bringing this couple
data set copy into use as a primary couple data set in a sysplex (for
example, by initializing a sysplex specifying the couple data set
copy as a PCOUPLE) can cause serious problems, as the status information
in the couple data set copy is an obsolete snapshot rather than the
most up-to-date sysplex information, and might experience inconsistencies
with more current information stored in databases or other types of
data sets that the sysplex is using. When re-initializing
a sysplex, the most current couple data sets previously in use in
the sysplex should always be used. Regressing to use older copies
of couple data sets, or couple data sets from another sysplex instance,
is not recommended.
For additional information and
recommendations regarding the use of remote copy technologies (for
disaster recovery) to replicate couple data sets, see Planning disaster recovery actions.
- A Multiple Extent Couple Data Set Is Not Supported
For
the sysplex couple data set, the format utility determines the size
of the data set based on the number of groups, members, and systems
specified or defaulted, and allocates space on the volume specified
for the data set. There must be enough contiguous space available
on the volume for the sysplex couple data set.
For the couple
data sets that hold policy information, the format utility determines
the size of the data set based on parameter values within the policy.
- A Couple Data Set Cannot Span Volumes
XCF does not
support multiple-volume data sets.
- A Couple Data Set Can Be Used by Only One Sysplex
The
name of the sysplex for which a data set is intended must be specified
when the data set is formatted. The data set can be used only by systems
running in the sysplex whose name matches that in the couple data
set.
- A Couple Data Set Can Share a Volume with Other Data Sets
However,
if you decide to format a couple data set on a volume with other data
sets, - Avoid a volume that has the RESERVE macro issued against it.
- Avoid a volume with high use data sets, such as page or spool
data sets.
- Performance and Availability Considerations
The
placement of couple data sets can improve performance, as well as
availability. For maximum performance and availability, each couple
data set should be on its own volume. While this is an expensive approach,
placing some data sets on different volumes from others makes sense.
If a system cannot access a couple data set for an extended
period of time (for example, if the volume on which it resides is
reserved by another system), MVS™ switches
to its alternate couple data set. To minimize sysplex disruption,
when you use DFDSS (or another data mover) to back up a volume with
a couple data set, it is recommended that you: The system drives periodic I/O to all couple
data sets, which acts to verify their continued accessibility. In
the case of the sysplex couple data sets, the normal system status
updates serve this purpose. For other couple data sets, which are
not otherwise frequently accessed, the system "probes" about once
per minute to determine whether the data set can still be accessed.
Any data set found to be inaccessible due to I/O or device error is
removed from service in accordance with XCF's normal recovery protocols.
This ensures that problems are surfaced in a timely manner, preventing
an undetected single point of failure and allowing the installation
to provide an accessible replacement.
- Security Considerations
It
is the responsibility of the installation to provide the security
environment for the couple data sets. Consider protecting the couple
data sets with with the same level of security as the XCF address
space (XCFAS).
z/OS Security Server RACF System Programmer's Guide explains
how to add a started procedure to the started procedures table.
- The ADRDSSU Utility Can Dump the Contents of a Couple Data
Set
If you experience problems with a couple data set or with
the usage of a couple data set, you can use ADRDSSU to dump the data
set. For example, if you are having problems with coupling facility
structures, use ADRDSSU to dump the CFRM data set.
The following
JCL could be used to dump a couple data set named SYS1.PRIMARY, which
resides on the 3380 volume, SHR001. //DUMP JOB MSGLEVEL=(1,1)
//STEP1 EXEC PGM=ADRDSSU,REGION=4M
//SYSPRINT DD SYSOUT=*
//DD1 DD DISP=SHR,VOL=SER=SHR001,UNIT=3380
//SYSIN DD *
PRINT DATASET(SYS1.PRIMARY) INDDNAME(DD1)
/*
- A Couple Data Set Can Be Defined on an SMS-Managed Volume
You
can define a couple data set on a DASD volume that the Storage Management
Subsystem (SMS) manages. When
placing a couple data set on an SMS-managed volume, consider the following:
- Ensure that all systems that need to can access the
couple data set. Catalog both the primary and alternate couple data
sets. If the catalog is not shared by all systems, ensure that the
couple data set is cataloged on each system. IBM recommends that you catalog the couple data
set in the master catalog, rather than a user catalog. If you catalog
the couple data set in a user catalog, the volsers must be specified
in COUPLExx in order for the data sets to be found during IPL, so
there is no benefit in using a user catalog.
- To prevent a single point of failure, define the primary and alternate
couple data sets to different control units and different volumes.
The couple data sets can reside on SMS-managed and non-SMS-managed
volumes. For example, the primary couple data set can reside on an
SMS-managed volume while the alternate can reside on a non-SMS-managed
volume.
- Using the Interactive Storage Management Facility (ISMF) panels,
define a storage class for the SMS-managed volume with GUARANTEED
SPACE=YES. When you use the format utility to format the primary and
alternate couple data sets, you can specify the appropriate storage
classes (STORCLAS).
You must ensure that the volumes you choose
for the couple data set are available to all systems that require
it. Specifying the guaranteed space attribute for the storage class
ensures that SMS can select from these eligible volumes.
Protect the couple data set from being migrated or deleted
by DFHSM. To prevent accidental expiration or migration of the volume
by DFHSM, define the following management class attributes through
the ISMF panels: - EXPIRE AFTER NON-USAGE=NOLIMIT
- EXPIRE AFTER DATE/DAYS=NOLIMIT
- COMMAND or AUTO MIGRATE=NONE
You can use the format utility to format the couple data
set on an SMS-managed volume by specifying the STORCLAS, MGMTCLAS, or VOLSER keywords. For a description of these
keywords, see Format utility for couple data sets.
For
information about SMS storage classes and management classes, see Storage
Management Subsystem Migration Planning Guide.
- Use of Remote Copy Technology with Couple Data
Sets
See Planning disaster recovery actions for considerations
on the use of remote copy technology with couple data sets.
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z/OS MVS Setting Up a Sysplex
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