maintaining the accuracy of the Format-5 DSCB. The Format-5 DSCB, sometimes called the ²free space DSCB², is used only by OS/390. OS/390 keeps track of unallocated space on a volume by creating one or more Format-5 DSCBs; that is, each Format-5 contains up to 26 extents of free space information.

VSE, although it doesn¢t keep track of free space on a volume, will still be able to process a volume created by OS/390 (and on which a Format-5 DSCB is located). However, VSE does not update the Format-5 DSCB when new space is allocated, or allocated space is freed. Although VSE ignores Format-5 DSCBs, it ²tells² OS/390 that the Format-5 DSCBs may no longer be valid (for example, a new file being allocated). This ²telling² is accomplished through the ²DOS bit² (previously discussed). OS/390, when it first allocates to a volume, always checks the Format-4 DSCB and the ²DOS bit.² If it finds this bit ²on², OS/390 creates (by invoking a VTOC, conversion routine) new Format-5 DSCBs representing all unallocated space it finds on the volume. Should there already be Format-5 DSCBs on a volume from prior OS/390 uses, OS/390 invalidates these and creates new ones. (An appropriate OS/390 message is displayed on the operator console whenever new Format-5 DSCBs are created as a result of VSE¢ s previous use of the volume.)

You may obtain OS/390 dump, formatted, or abridged listings of a VTOC by using the LISTVTOC command of the OS/390 IEHLIST utility program. The DFSMSdss PRINT command can also be used to print all or part of the VTOC. A VTOC listing can be obtained under VSE by executing the LVTOC utility program. In both environments, DITTO/ESA¢s DVT function may also be used.

5.5.3 Indexed VTOC Considerations (OS/390)

OS/390 has a facility for improved DASD VTOC performance called the Indexed VTOC. Indexed VTOCs are optional but are strongly recommended in OS/390. Their major benefit is improved VTOC performance by avoiding lengthy hardware keyed searches of the VTOC, which tie up the channel and device, and by managing free space information in such a way that the number of I/O operations required to obtain or release space on the volume is reduced. We recommend that you use indexed VTOCs. Details can be found in DFSMSdfp Advanced Services, SC26-4921.

The VTOC index itself is a specialized data set which resides on the same volume as the VTOC to which it refers. As such, it has a Format-1 DSCB in the VTOC which contains the index¢s data set name and extent information. The index data set name must adhere to the naming convention

²SYS1.VTOCIX.xxxxxxxx² where xxxxxxxx is user defined: It is recommended that you include the volume¢s serial number.

It is not recommended that OS/390 volumes with Indexed VTOCs be used on VSE

systems. If the need exists, care must be exercised - each volume has to be converted from an Indexed VTOC to a non-indexed VTOC (that is, a VTOC with no index) before transporting the volume to the VSE system. Otherwise, serious errors may result when the volume is returned to the OS/390 system; that is, VTOC changes made on the VSE system not causing reconstruction of the VTOC are not recorded in the index and, in effect, invalidate the index.

For more information on the Indexed VTOC facility, see DFSMSdfp Advanced

Services, SC26-4921. For more information on creating Index VTOCs, see the

Device Support Facilities User¢s Guide and Reference.

Chapter 5. Disk and Tape Storage Considerations 109

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IBM manual Indexed Vtoc Considerations OS/390

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.