IBM OS/390 User Manual

You are in a cross-system sharing environment whenever you allow more than
one copy of any operating system to access the same DASD volume
concurrently. This includes multiple OS/390 guests running under VM or PR/SM.
You must not attempt to update via DASD sharing between VSE and OS/390
systems. The methods of DASD sharing protection are totally incompatible and
you risk contamination or loss of:

•

data

•

data sets

•

catalogs

•

VTOCs

Should you be planning to share ICF catalogs or VSAM data sets between 

more OS/390 systems, you should read the chapter entitled 

″

Sharing a VSAM

Data Set

″ 

in the 

Using Data Sets manual. How catalogs are shared is

documented in

 Managing Catalogs. The volume the catalogs are allocated on

must be defined as SHARED to all images and the shareoptions for the catalog
must be 3,4.

OS/390 Global Resource Serialization (GRS) and the new Record Level Sharing
feature (part of SYSPLEX support) can provide additional cross-system and
DASD sharing capabilities. For more information on GRS, refer to 

OS/390 V1R1.0

MVS Planning - Global Resource Serialization, GC28-1818, or OS/390 V1R3.0 MVS
Planning - Global Resource Serialization, GC28-1759.

In order to provide any protection in a DASD sharing environment you must let
OS/390 know that the device may be shared. This is done by specifying the
parameter 

″

SHARED

″ 

in the Hardware Configuration Definition data set. Without

this parameter, OS/390 will provide no DASD sharing protection.

Unlike VSE/VSAM, the cross-region shareoption has no meaning for
cross-system sharing with OS/390 VSAM 

unless OS/390 GRS is used to control

cross-region sharing of data sets. What is important is the 

second field of the

SHR parameter. This field can only be 3 or 4. In other words, only SHR(x 3) or
SHR(x 4) is valid for an AMS DEFINE or ALTER.

Record Level Sharing (RLS), available to users of OS/390 systems with Parallel
Sysplex capabilities (Coupling Facility, integrated or stand-alone), together with
DFSMS support included in DFSMS 1.3 or later provides a much higher level of
sharing with protection for OS/390 users. It is an alternative to using
shareoptions for accessing VSAM data sets.

RLS can be used in batch environments but there are restrictions. RLS is
designed primarily for VSAM data sets used by CICS applications. With VSAM
RLS, multiple CICS systems can directly access a shared VSAM data set,
eliminating the need for function shipping between AORs and file owning regions
(FORs).

Information on Record Level Sharing can be found in several manuals:

Chapter 5. Disk and Tape Storage Considerations