IBM manual Shared Dasd vs. Cloned Dasd, Shared Dasd between OS/390 Test Systems vs. Cloned Dasd

26.6 Shared DASD vs. Cloned DASD

The issue of whether to share DASD volumes and data sets between systems is decided on the basis of DASD space availability, need for multiple versions of a file, and the ability to manage updates between the two systems.

26.6.1 Shared DASD between OS/390 Test Systems (vs. Cloned DASD)

The decision to share data sets and volumes or to make copies of them for each OS/390 system should be thought out carefully. Many OS/390 data sets and some volumes can be shared between multiple systems as long as updates are serialized and good change control procedures are followed. The recommended approach is to put multiple OS/390 systems in the same sysplex and use GRS to guarantee serialization of these resources. The alternative is to ²clone² or make copies of the volumes or data sets, but this obviously takes more DASD space.

Referring to Table 45 on page 403, there are some volumes that can be shared between active OS/390 systems, and others that should never be shared:

System Libraries Separate SYSRES volumes should be maintained for each logical OS/390 system.

Distribution Libraries Share, but only update from the maintenance system.

CatalogsThe master catalog should be fairly static, contain only the necessary entries, with the rest in user catalogs. The master catalog can be shared but highly controlled as to who can update it. User catalogs can be shared between systems, use GRS or manual procedures to serialize updates.

Paging Data Sets Use by only one system at a time (re-use by different test system as long as they are not both active at once.)

Spool & Checkpoint JES2 can share the spool and checkpoint between multiple members if you are comfortable with

multi-access spool. Otherwise, they should be dedicated to each OS/390 system. The backup OS/390 system should have its own spool and checkpoint.

Softcopy Library Share these between all OS/390 systems.

DFSMShsm ML1 Share if using HSM on both systems.

Storage/WorkShare or isolate depending on how much space you want to reserve for the ²production² system.

User LibrariesYou can share the user program and data libraries, but you must keep track of separate version levels with different data set names or different user catalogs and volumes.

A lot of these decisions whether to share volumes, data sets and work space depends on how much you want to isolate the systems and manage change control.

432VSE to OS/390 Migration Workbook