In the

IBM System/360 The IBM System/360 (S/360) is a family of mainframe computer systems that was announced by IBM on April 7, 1964, and delivered between 1965 and 1978. It was the first family of computers designed to cover both commercial and scientific applica ...

Including the successors

S/370 The IBM System/370 (S/370) is a model range of IBM mainframe computers announced on June 30, 1970, as the successors to the System/360 family. The series mostly maintains backward compatibility with the S/360, allowing an easy migration path f ...

through

z/Architecture z/Architecture, initially and briefly called ESA Modal Extensions (ESAME), is IBM's 64-bit complex instruction set computer (CISC) instruction set architecture, implemented by its mainframe computers. IBM introduced its first z/Architecture- ...

storage architecture, the Volume Table of Contents (VTOC), is a data structure that provides a way of locating the data sets that reside on a particular DASD

volume Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). The def ...

. With the exception of the ''IBM Z compatible disk layout''The VTOC for an IBM Z compatible minidisk has a VTOC with up to three datasets, each containing a Linux File system. in Linux on Z, it is the functional equivalent of the MS/PC

DOS DOS is shorthand for the MS-DOS and IBM PC DOS family of operating systems. DOS may also refer to: Computing * Data over signalling (DoS), multiplexing data onto a signalling channel * Denial-of-service attack (DoS), an attack on a communicatio ...

File Allocation Table File Allocation Table (FAT) is a file system developed for personal computers. Originally developed in 1977 for use on floppy disks, it was adapted for use on hard disks and other devices. It is often supported for compatibility reasons by c ...

(FAT), the Windows NT Master File Table (MFT), and the equivalent structure in, e.g., a

Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which in ...

file system In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one lar ...

.While the VTOC only needs to associate names directly with storage allocations, a Linux file system has an indirect association via

inode The inode (index node) is a data structure in a Unix-style file system that describes a file-system object such as a file or a directory. Each inode stores the attributes and disk block locations of the object's data. File-system object attribu ...

s. The VTOC is not used to contain any IPLTEXTHowever, the IPL text on cylinder 0 track 0 does read and use the VTOC. and does not have any role in the IPL process, therefore does not have any data used by or functionally equivalent to the MBR. It lists the names of each data set on the volume as well as

size Size in general is the magnitude or dimensions of a thing. More specifically, ''geometrical size'' (or ''spatial size'') can refer to linear dimensions (length, width, height, diameter, perimeter), area, or volume. Size can also be measur ...

, location, and permissions. Additionally, it contains an entry for every area of contiguous free space on the volume. The third record on the first track of the first cylinder of any DASD (e.g., disk) volume is known as the volume label and must contain a pointer to the location of the VTOC. The location of the VTOC may be specified when the volume is initialized. For performance reasons it may be located as close to the center of the volume as possible, since it is referenced frequently. A VTOC is added to a DASD volume when it is initialized using the Device Support FacilitiesDSF replaces the original IBCDASD and IEHDASDR, which IBM no longer distributes and which can not run on current hardware. program,

ICKDSF This article discusses support programs included in or available for OS/360 and successors. IBM categorizes some of these programs as utilities and others as service aids; the boundaries are not always consistent or obvious. Many, but not all, of ...

, in current systems. When in

OS/360 and successors OS/360, officially known as IBM System/360 Operating System, is a discontinued batch processing operating system developed by IBM for their then-new System/360 mainframe computer, announced in 1964; it was influenced by the earlier IBSYS/IBJO ...

allocates a data set, it generally searches the catalog to determine the volumes on which it resides.It is possible, although unusual, for a job to bypass the catalog and specify the volume via dynamic allocation or JCL parameters. When a program opens a

Direct Access Storage Device A direct-access storage device (DASD) (pronounced ) is a secondary storage device in which "each physical record has a discrete location and a unique address". The term was coined by IBM to describe devices that allowed random access to data, ...

(DASD) dataset, the OPEN routine searches the VTOC index (VTOCIX) if there is one, or directly searches the VTOC if there is no VTOCIX.

Data Set Control Block types

The VTOC consists of a sequence of 140-byte records known as Data Set Control Blocks (DSCBs). There are ten types of DSCB. The VTOC must reside within the first 64K tracks on the volume, and The first DSCB in the VTOC is always a format 4 DSCB which describes the VTOC itselfThe VTOC has a dataset name as the VTOC is, indeed, a dataset; the VTOC's dataset name is (44) X'04' characters, which, in later instances of the OS, has become a protected name; a program may not write into it without authorization. and attributes of the DASD volume on which this VTOC resides. The second DSCB is always a format 5 DSCB which describes free space within the VTOC. Normally, the rest of the VTOC will contain format 0 DSCBs, which are empty entries, and format 1 or format 3 DSCBs, which describe the extents of data sets, giving their start address and end addressEach extent is described by a starting CCHH and an ending CCHH, relative to the beginning of the disk, which is CCHH=x'00000000'. of up to 16 such ''extents'' on disk. The initial part of a data set is described by a format 1 DSCB. If necessary, format 3 DSCBs are used to describe further extents of the data set. When a data set is deleted, its format 1 DSCB is overwritten to become a format 0 DSCB, and the format 3 DSCB, if one exists, is similarly deleted. Originally, a VTOC search was a sequential scan of the DSCBs, stopping when the correct format 1 DSCB was found or the end of the VTOC was reached. As DASD volumes became larger, VTOC search became a bottleneck and so a VTOC index was added.

Format 1 DSCB

This VTOC entry describes a dataset and defines its first three extents. This is the format of the DSCB from OS/360 Release 21.7 in 1973, prior to changes for

Y2K The year 2000 problem, also known as the Y2K problem, Y2K scare, millennium bug, Y2K bug, Y2K glitch, Y2K error, or simply Y2K refers to potential computer errors related to the formatting and storage of calendar data for dates in and after ...

Notes

References

External links

z/OS DFSMS: Using Data Sets

z/OS: DFSMSdfp Advanced Services
* {{cite manual , url = https://www.ibm.com/support/knowledgecenter/zosbasics/com.ibm.zos.zsysprog/zsysprogc_systemIPL.htm , title = z/OS System IPL: Sequence and key controls , publisher = IBM IBM mainframe operating systems IBM storage software File system management IBM file systems

Data Set Control Block types

Format 1 DSCB

See also

Notes

References

External links