Creating a couple data set (CDS)

The TYPE(BPXMCDS) couple data set (CDS) contains the sysplex-wide mount table and information about all participating systems, and all mounted file systems in the sysplex. To allocate and format a TYPE(BPXMCDS) CDS, customize and invoke the BPXISCDS sample job in SYS1.SAMPLIB. The job will create two couple data sets: one is the primary and the other is a backup that is referred to as the alternate. In BPXISCDS, you also specify the number of mount records that are supported by the CDS.

Use of the CDS functions in the following manner:

The first system that enters the sysplex with SYSPLEX(YES) initializes the CDS for z/OS® UNIX System Services. The z/OS UNIX CDS controls shared file system mounts and will eventually contain information about all systems participating in the shared file system configuration.
This system processes its BPXPRMxx parmlib member, including all its ROOT and MOUNT statement information. The MOUNT and ROOT information are logged in the CDS so that other systems that eventually join the participating group can read data about systems that are already using shared file system.
Subsequent systems joining the participating group will read what is already logged in the CDS and will perform all mounts. Any new BPXPRMxx mounts are processed and logged into the CDS. Systems already in the participating group will then process the new mounts added to the CDS.

Following is the sample JCL with comments. The statements in bold contain the values that you specify based on your environment. Place the primary and alternate couple data sets on separate volumes.

//*
//STEP10    EXEC PGM=IXCL1DSU
//STEPLIB   DD   DSN=SYS1.MIGLIB,DISP=SHR
//SYSPRINT  DD   SYSOUT=A
//SYSIN     DD   *
 /* Begin definition for OMVS couple data set(1)            */
  DEFINEDS SYSPLEX(PLEX1) 
   /* Name of the sysplex in which the OMVS couple data set is to be used.*/
   DSN(SYS1.OMVS.CDS01) VOLSER(3390x1)
    /* The name and volume for the OMVS couple data set.
    The utility will allocate a new data set by the name specified on the 
    volume specified.*/
    MAXSYSTEMS(8)
    /* Specifies the number of systems to be supported by the z/OS UNIX CDS.
        Default =     8          */
        NOCATALOG       
    /* Default is not to CATALOG */          
    DATA TYPE(BPXMCDS)      
    /* The type of data in the data set being created for OMVS.
    
     BPXMCDS is the TYPE for OMVS. */ITEM NAME(MOUNTS) NUMBER(100)
  /* Specifies the number of MOUNTS that can be supported by OMVS.*/
     Default =   100
     Suggested minimum =    10
     Suggested maximum = 35000  */
  ITEM NAME(AMTRULES) NUMBER(50)
   /* Specifies the number of automount rules that can be supported by OMVS.*/
      Default =    50
      Minimum =    50
      Maximum =  5000           */

    /* Begin definition for OMVS couple data set(2)               */
   DEFINEDS SYSPLEX(PLEX1)         
   /* Name of the sysplex in which the OMVS couple data set is to be used.     */
   DSN(SYS1.OMVS.CDS02) VOLSER(3390x2) 
   /* The name and volume for the OMVS couple data set.  The utility will
      allocate a new data set by the namespecified on the volume specified.     */
    MAXSYSTEMS(8)
    /* Specifies the number of systems to be supported by the z/OS UNIX CDS.
       Default =     8           */
       NOCATALOG                 
    /* Default is not to CATALOG */
       DATA TYPE(BPXMCDS)         
    /* The type of data in the data set being created is for OMVS.  
       BPXMCDS is the TYPE for OMVS.            */
 ITEM NAME(MOUNTS) NUMBER(100)
    /* Specifies the number of MOUNTS that can be supported by OMVS.
       Default =   100
       Suggested minimum =    10
       Suggested maximum = 35000 */
     ITEM NAME(AMTRULES) NUMBER(50)
    /* Specifies the number of automount rules that can be supported by OMVS.
       Default =     50
       Minimum =     50
       Maximum =   5000     */

AMTRULES specifies the number of automount rules that can be supported by z/OS UNIX. It must be large enough to describe each automount-managed directory in your automount policy. The maximum value is 5000. Generally, each managed directory requires one AMTRULE, and each generic or specific rule for that managed directory requires one-half of an AMTRULE. For example, an automount policy consisting of two managed directories where the first managed directory contains a single generic rule and the second managed directory contains three specific rules would require a total of four AMTRULES. The first managed directory requires 1.5 AMTRULES and the second managed directory requires 2.5 AMTRULES.

Rule: Automount mounts must be included in the MOUNTS value. The number of automount mounts is the expected number of concurrently mounted file systems using the automount facility. For example, you might have specified 1000 file systems to be automounted, but if you expect only 50 to be used concurrently, then factor these 50 into your MOUNTS value.

For more information about setting up a z/OS system to run in a sysplex, see z/OS MVS Setting Up a Sysplex.

The NUMBER(nnnn) specified for mounts and automount rules (a generic or specific entry in an automount map file) is directly linked to function performance and the size of the CDS. If maximum values are specified, the size of the CDS will increase accordingly and the performance level for reading and updating it will decline.

Conversely, if the NUMBER values are too small, the function (for example, the number of mounts supported) would fail after the limit is reached. However, a new CDS can be formatted and switched in with larger values specified in NUMBER. To make the switch, issue the SETXCF COUPLE,PSWITCH command. The number of file systems required (factoring in an additional number to account for extra mounts), determines your minimum and maximum NUMBER value.

After the CDS is created, it must be identified to XCF for use by z/OS UNIX.