Defining sharing control data sets

Sharing control is a key element in maintaining data integrity in a shared environment. Because persistent record locks are maintained in the CF, several new classes of failure might occur, such as a Parallel Sysplex®, system, or SMSVSAM address space restart, or a CF lock structure failure. The sharing control data set (SHCDS) is designed to contain the information required for DFSMS to continue processing with a minimum of unavailable data and no corruption of data when failures occur. The SCDS data can be either SMS or non-SMS managed.

The SHCDS acts as a log for sharing support. It is a logically-partitioned linear data set, with CISIZE of 4096, that must be defined with secondary extents, though all extents for each data set must be on the same volume. An SHCDS contains the following information:

The name of the CF lock structure in use
A list of subsystems and their status
A list of open data sets using the CF
A list of data sets with unbound locks
A list of data sets in permit non-RLS state

Sharing control is critical to maintaining data integrity in the event of the failures such as Parallel Sysplex, system, or SMSVSAM address space restart, or of the CF lock structure. You should always have at least two active and one spare SHCDSs. Place these data sets for maximum availability. If necessary, you can have up to five active SHCDSs and five spare SHCDSs.

Recommendation: Because the contents of these data sets are highly dynamic, do not use backup and restore functions, because these might cause the loss of VSAM RLS data set recovery information.

Consider the following as you allocate and maintain your SHCDSs:

The SHCDS must not be shared outside of the Parallel Sysplex. Refer to the restrictions in Running SMS in a parallel sysplex environment.
Allocate SHCDSs so that the number of active and spare data sets ensures the data is always duplexed. At a minimum, define and activate two SHCDSs and at least one spare SHCDS for recovery purposes. You should ensure that there are enough spare SHCDSs, because these are used when I/O errors occur on the active SHCDSs.
Place the SHCDSs on volumes with global connectivity. VSAM RLS processing is only available on those systems that currently have access to the active SHCDS. The share options for SHCDSs must be set to (3,3) so that each system in the Parallel Sysplex can properly share the data sets.
Place your SHCDSs in such a way as to maximize availability in the event of the loss of a volume. Use storage classes defined with the guaranteed space attribute. Avoid placing SHCDSs on volumes for which there might be extensive volume reserve activity.
SMSVSAM should be authorized to update SYS1.DFPSHCDS.* data sets. If you protect SYS1.* data sets, be sure SMSVSAM is able to access SYS1.DFPSHCDS.* for update.
Ensure that the space allocation for active and spare SHCDSs is the same. Ensure data sets have enough space for growth. Ensure that the SHCDS has secondary extents defined.
Use the VARY SMS,SHCDS command to activate and maintain your SHCDSs.

You must use the following naming convention when defining your SHCDSs:

SYS1.DFPSHCDS.qualifier.Vvolser

where:

qualifier: Is a 1 to 8 character qualifier.
volser: Is the volume serial number. TheV prefix allows you to specify numeric volume serial numbers.

Define the SHCDS with the following characteristics:

CISIZE equals 4096
Shareoptions are (3,3)
Contains secondary extents
Resides on a single volume

Use the following formula to calculate the size of the primary extent of your SHCDS:

 space = (number_of_recoverable_data_sets * number_of_recoverable_subsystems * 2 * 272) bytes 
        or 13MB, whichever is larger

where:

number_of_recoverable_data_sets: Is the average number of recoverable data sets open across the sysplex.
number_of_recoverable_subsystems: Is the average number of recoverable subsystems registered with RLS across the sysplex.

For example, if you have an average of 10 recoverable subsystems registered with RLS with an average of 1000 recoverable data sets open, the amount of space required for the primary extent is (10 * 1000 * 2 * 272) or 5440000 bytes (5313 KB).

You can create an SHCDS using access method services. When you use the access method services DEFINE command to create an SHCDS, specify SHAREOPTIONS(3,3) to ensure that the SHCDS can be written to and read from any system.Specify CISIZE as 4096. Select a volume that is a member of a storage class with the guaranteed space attribute.

Figure 1 shows how to create an SHCDS using IDCAMS:

Figure 1. Example for Creating Sharing Control Data Sets Using IDCAMS

//*------------------------------------------------------
//* ALLOCATE SHCDS ON XP0301 - SXPXXS04 IS GUARANTEED SPACE
//*------------------------------------------------------
//ALLOCLD1 EXEC PGM=IDCAMS
//SYSPRINT DD SYSOUT=*
//SYSIN DD *
  DEFINE CLUSTER (NAME(SYS1.DFPSHCDS.ACTIVE.VXP0301)        -
                  LINEAR                                    -
                  CISZ(4096)                                -
                  STORCLAS(SXPXXS04)                        -
                  SHAREOPTIONS(3 3)                         -
                  CYL(10 5)                                 -
                  VOLUME(XP0301) )
/*

Once the SHCDS data sets have been created, you make them available for use by using the VARY SMS, SHCDS (qualifier.Vvolser), NEW command. The (qualifier.Vvolser) is used, not the fully qualified SHCDS name. For example, VARY SMS, SHCDS (ACTIVE,XP0301). This command only needs to be entered on one system in the Parallel Sysplex. The SMSVSAM address space on the system where the command is entered communicates the name of the data set to the other SMSVSAM address spaces in the Parallel Sysplex. Each SMSVSAM address space recatalogs the data set, if it has not already been cataloged, so that you do not need to manually catalog the data set in order for it to be used. Those data sets added for use are saved, and can be accessed when an SMSVSAM address space initializes them or is restarted, or though subsequent IPLs.

The names of the SHCDS data sets are stored in the Sysplex COUPLE data sets. In the event of starting up with a new set of Sysplex couple data sets, the SHCDS data sets names are lost and you must use the VARY SMS SHCDS command to make them available.

To delete and redefine a SHCDS data set, you must first delete the SHCDS data set from SMSVSAM by entering the V SMS.SHCDS(shcdsname).DELETE command, and then you can use IDCAMS DELETE and DEFINE to modify the new SHCDS data set. You must do that even if SMSVSAM is not active. Note, that you always must have at least two active SHCDS data sets and one spare SHCDS data set. To delete a SHCDS data set when there are only the minimum number of SHCDS data sets defined, you must first define additional SHCDS data sets before you can delete the existing SHCDS data sets.

The following example shows how to delete and redefine SHCDS data sets when only the minimum number of SHCDS data sets are defined:

If the following SHCDS data sets are already defined:

16.40.09 SYSTEM           d sms,shcds
IGW612I 16:40:10   DISPLAY SMS,SHCDS
Name                      Size     %UTIL   Status   Type
TOOSMAIL.VXP0301          7200Kb     5%    GOOD     ACTIVE
TOOSMAIL.VXP0302          7200Kb     5%    GOOD     ACTIVE
WRONGVOL.VXP0201          7200Kb     5%    GOOD     SPARE

Enter these commands to define three new data sets:

V SMS,SHCDS(JUSTRITE.VXP0301),NEW
V SMS,SHCDS(JUSTRITE.VXP0302),NEW
V SMS,SHCDS(RIGHTVOL.VXP0202),NEWSPARE

These are the resulting SHCDS data sets:

SYSTEM1           d sms,shcds
IGW612I 16:45:49   DISPLAY SMS,SHCDS
Name                      Size     %UTIL   Status   Type
TOOSMAIL.VXP0301          7200Kb     5%    GOOD     ACTIVE
TOOSMAIL.VXP0302          7200Kb     5%    GOOD     ACTIVE
JUSTRITE.VXP0301         14400Kb     2%    GOOD     ACTIVE
JUSTRITE.VXP0302         14400Kb     2%    GOOD     ACTIVE
WRONGVOL.VXP0201          7200Kb     5%    GOOD     SPARE
RIGHTVOL.VXP0202          7200Kb     5%    GOOD     SPARE

Enter these commands to delete the old data sets:

V SMS,SHCDS(TOOSMAIL.VXP0301),DELETE
V SMS,SHCDS(TOOSMAIL.VXP0302),DELETE
V SMS,SHCDS(WRONGVOL.VXP0201),DELETE

The resulting SHCDS data sets are as follows:

SYSTEM1           d sms,shcds
IGW612I 16:45:49   DISPLAY SMS,SHCDS
Name                      Size     %UTIL   Status   Type
JUSTRITE.VXP0301         14400Kb     2%    GOOD     ACTIVE
JUSTRITE.VXP0302         14400Kb     2%    GOOD     ACTIVE
RIGHTVOL.VXP0202          7200Kb     5%    GOOD     SPARE

Note:

If you swap in a new set of SHCDS data sets or make changes to an existing SHCDS data set, you must communicate the changes to SMSVSAM through the VARY SMS SHCDS command.
Do not move a SHCDS data set from one volume to another. The SHCDS data set naming convention depends on the volume in which the SHCDS data set resides.

For more information see Changing the state of coupling facility cache structures and volumes.