5.5 DASD Similarities and Differences

5.5.1 Volume Interchangeability

DASD file label conventions, requirements, and handling techniques differ between VSE and OS/390 systems. However, OS/390 should be able to read and process DASD volumes that have been created on a VSE system. Similarly, VSE systems should be able to read volumes created by OS/390 systems. The following exceptions to this are noted below.

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OS/390 doesn¢t support FBA (Fixed Block Architecture) DASD devices; for example, 3310, 3370, 9332, 9335, 9336. Data files located on FBA devices will have to be copied to an OS/390-supported device using a VSE backup/restore program.

Concurrent sharing of a volume between the two systems is not supported and should not be attempted as data loss could result.

OS/390 does not allow more than one volume with an identical volume serial number (VOLID) to be online at any one time. VSE would allow this condition. Thus, if you are used to operating with identical serial number volumes (either tape or DASD) mounted concurrently, you will have to change to unique volume serial number identifiers for OS/390 operations.

∙It is not recommended that OS/390 volumes which contain Indexed VTOCs be accessed by a VSE system. To do so requires special procedures - See 5.5.3, ªIndexed VTOC Considerations (OS/390)º on page 109.

5.5.2DASD (VTOC) Processing

DASD volumes are managed by both VSE and OS/390 through a Volume Table of Contents (VTOC) which is a type of file. There is always one VTOC per volume. Each VTOC is composed of various records called Labels in VSE, but named Data Set Control Blocks (DSCBs) in OS/390. DSCBs are used to store information about the contents of the volume. DSCBs come in various record formats with varying field layouts, and are customized by the type of information stored. There can be up to six different DSCB types in a VTOC. They are appropriately named: ²Format-1 DSCB ,..., Format-6 DSCB². For a detailed description of a VTOC layout, index VTOC and the various DSCBs, see the DFSMSdfp Advanced Services. Also, see VSE/AF Data Management Concepts , GC33-6192. For purposes of this publication, only the Format-1, Format-4, and Format-5 DSCB need to be discussed.

Both VSE and OS/390 use the Format-1 DSCB. There is a Format-1 DSCB record for each data set allocated on a volume. OS/390 stores information about each data set in addition to the information which VSE stores. As such, VSE does not maintain all fields in the Format-1 DSCB. For example, the record format, record length, and block size are not maintained by VSE. Thus, to access VSE-created files from an OS/390 system, the user must supply (preferably through JCL) file information which otherwise would be obtained from the file¢s Format-1 label or DSCB.

Both VSE and OS/390 use a Format-4 DSCB -- there is one (and only one) per VTOC. The Format-4 DSCB contains a 1-bit flag designator (called the ²DOS bit²) which is used to indicate whether the Format-5 DSCB(s) are valid. Turning the bit ²on² or ²off² is the responsibility of the operating system. VSE when it first uses a volume turns the ²DOS bit² ²on². The use of this bit is mainly for the purpose of

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IBM OS/390 manual Dasd Similarities and Differences Volume Interchangeability, Dasd Vtoc Processing
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OS/390 specifications

IBM OS/390, a versatile operating system, was a cornerstone in enterprise environments and played a pivotal role in mainframe computing. Released in the mid-1990s, OS/390 combined the strengths of IBM's MVS (Multiple Virtual Storage) with new features and enhancements, targeting scalability, reliability, and performance in demanding business applications.

One of the key features of OS/390 was its robust support for multiple users and processes. The system allowed thousands of concurrent users to access applications and data, ensuring high availability and minimizing downtime—a critical requirement for many large organizations. This scalability was supported through various enhancements in memory management and processor scheduling, enabling optimal resource allocation across diverse workloads.

OS/390 was known for its superior workload management capabilities. The Workload Manager (WLM) component allowed administrators to define service policies, specifying how system resources would be allocated according to the priority of tasks. This ensured that critical business processes received the necessary resources while less critical tasks were managed more flexibly.

Another significant characteristic of OS/390 was its commitment to security. The operating system provided comprehensive security features, including user authentication, data encryption, and auditing capabilities. This focus on security was vital for organizations handling sensitive data, ensuring compliance with regulations and safeguarding against unauthorized access.

OS/390 also supported advanced technologies that facilitated integration and development. The system included features like the IBM CICS (Customer Information Control System) for transaction processing and IMS (Information Management System) for database management. These technologies allowed organizations to build robust, high-performance applications tailored to specific business needs.

The ease of network integration was another strength of OS/390. With the advent of the Internet and global connectivity, OS/390 systems could easily interface with various network protocols, enabling businesses to operate in a connected world. This inclusion paved the way for many organizations to expand their capabilities and offer new services, driving digital transformation.

In conclusion, IBM OS/390 represented a significant advancement in mainframe technology, combining scalability, security, and robust workload management. Its rich feature set and support for critical enterprise applications solidified its role as a vital component of many organizations' IT infrastructures, ensuring they could meet their operational challenges head-on while supporting future growth. As technology continues to evolve, the legacy of OS/390 remains influential in the realm of computing.
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