Defining sharing control data sets

VSAM RLS requires sharing control data sets. These are used to maintain data integrity in the sharing environment. The sharing control data set is used sysplex-wide by all the SMSVSAM servers, and is always duplexed.

Two active (and at least one spare) sharing control data sets must be available at all times.

The size required depends on the number of MVS™ images in the sysplex, and on the number of files that are expected to be open concurrently. The z/OS DFSMSdfp Storage Administration Reference manual gives information about calculating the amount of space that is needed for the sharing control data sets.

Sharing control data sets are VSAM linear data sets that must reside on volumes which have global connectivity. The data sets may have multiple extents, but only on the same volume. You define them using standard techniques for defining data sets. The names must have SYS1.DFPSHCDS as the first and second qualifiers. See the z/OS DFSMSdfp Storage Administration Reference manual for other rules relating to the definition of sharing control data sets.

You must not issue RESERVEs on any volumes on which sharing control data sets reside. Convert any such RESERVEs to enqueues.

You can check that the data sets are available to the sysplex with the MVS DISPLAY SMS command, on any MVS image, as follows:

D SMS,SHCDS

This command shows the names of the two active, and the spare data set as in the following example:

Name Size %UTIL Status Type
ACTIVE1.VP2SS03 7920KB 7920KB 74% Good ACTIVE
ACTIVE2.VP2SS03 7920KB 7920KB 74% Good ACTIVE
SPARE.VP2SS03 7920KB 7920KB 74% Good SPARE
Note:
The DISPLAY command shows only the third and fourth qualifiers of the sharing control data set names; the first and second qualifiers are always SYS1.DFPSHCDS.

The first time an SMSVSAM server is started in the sysplex, the sharing control data sets need to be varied online using

V SMS,SHCDS,NEW

for the active data sets, and

V SMS,SHCDS,NEWSPARE

for the spare data set (or data sets). The server cannot come up properly if this is not done.

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