VTOC
Encyclopedia
In the IBM mainframe
storage architecture, Volume Table Of Contents, or VTOC, is a data structure, that provides a way of locating the data sets
that reside on a particular disk volume
. It can reside within the first 64K tracks on the volume, and lists the names of each data set on the volume as well as size
, location, and permission
s. 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 volume of DASD
(i.e. disk pack) is known as the volume label and must contain a pointer to the location of the VTOC. A VTOC is added to a disk when it is initialized using the Device Support Facilities utility program, ICKDSF. VTOC was originally designed for removable disk packs.
To locate a data set, a program will generally interrogate a z/OS
catalog to find the volume where the data set resides. Having found the correct volume, the VTOC is searched to find out where on the disk the data set is stored.
The VTOC comprises records known as Data Set Control Blocks, or DSCBs. There are seven types of DSCB.
The first DSCB in the VTOC is always a format 4 DSCB which describes the VTOC itself and attributes of the disk 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 data sets, giving their start address 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 parts of the data set. When a data set is deleted, its format 1 DSCB is overwritten to become a format 0 DSCB.
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 disk volumes became larger, VTOC search became a bottleneck and so a VTOC index was added.
IBM mainframe
IBM mainframes are large computer systems produced by IBM from 1952 to the present. During the 1960s and 1970s, the term mainframe computer was almost synonymous with IBM products due to their marketshare...
storage architecture, Volume Table Of Contents, or VTOC, is a data structure, that provides a way of locating the data sets
Data set (IBM mainframe)
data set , dataset , is a computer file having a record organization. The term pertains to the IBM mainframe operating system line, starting with OS/360, and is still used by its successors, including the current z/OS. Those systems historically preferred this term over a file...
that reside on a particular disk volume
Volume (computing)
In the context of computer operating systems, volume is the term used to describe a single accessible storage area with a single file system, typically resident on a single partition of a hard disk. Similarly, it refers to the logical interface used by an operating system to access data stored on...
. It can reside within the first 64K tracks on the volume, and lists the names of each data set on the volume as well as size
File size
File size measures the size of a computer file. Typically it is measured in bytes with a prefix. The actual amount of disk space consumed by the file depends on the file system....
, location, and permission
Permission
Permission, in philosophy, is the attribute of a person whose performance of a specific action, otherwise ethically wrong or dubious, would thereby involve no ethical fault. The term "permission" is more commonly used to refer to consent...
s. Additionally, it contains an entry for every area of contiguous free space on the volume. The third record on the first track
Cylinder-head-sector
Cylinder-head-sector, also known as CHS, was an early method for giving addresses to each physical block of data on a hard disk drive. In the case of floppy drives, for which the same exact diskette medium can be truly low-level formatted to different capacities, this is still true.Though CHS...
of the first cylinder
Cylinder (disk drive)
A disk drive cylinder is a division of data in a disk drive, as used in the CHS addressing mode of a Fixed Block Architecture disk or the cylinder–head–record addressing mode of a CKD disk. The concept is concentric, hollow, cylindrical slices through the physical disks , collecting the respective...
of any volume of DASD
Direct access storage device
In mainframe computers and some minicomputers, a direct access storage device, or DASD , is any secondary storage device which has relatively low access time relative to its capacity....
(i.e. disk pack) is known as the volume label and must contain a pointer to the location of the VTOC. A VTOC is added to a disk when it is initialized using the Device Support Facilities utility program, ICKDSF. VTOC was originally designed for removable disk packs.
To locate a data set, a program will generally interrogate a z/OS
Z/OS
z/OS is a 64-bit operating system for mainframe computers, produced by IBM. It derives from and is the successor to OS/390, which in turn followed a string of MVS versions.Starting with earliest:*OS/VS2 Release 2 through Release 3.8...
catalog to find the volume where the data set resides. Having found the correct volume, the VTOC is searched to find out where on the disk the data set is stored.
The VTOC comprises records known as Data Set Control Blocks, or DSCBs. There are seven types of DSCB.
| DSCB format type | Purpose | Notes |
|---|---|---|
| 0 | Empty entry | |
| 1 | Data set primary | Describes first three extents of a data set |
| 2 | ISAM data set | Describes ISAM data sets |
| 3 | Data set extension | Describes data set extents after the third |
| 4 | VTOC | Describes volume attributes |
| 5 | Free space | Redundant if the VTOC is indexed |
| 7 | Free space | Only used for a limited set of devices |
The first DSCB in the VTOC is always a format 4 DSCB which describes the VTOC itself and attributes of the disk 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 data sets, giving their start address 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 parts of the data set. When a data set is deleted, its format 1 DSCB is overwritten to become a format 0 DSCB.
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 disk volumes became larger, VTOC search became a bottleneck and so a VTOC index was added.
See also
- Data Control BlockData Control BlockIn IBM mainframe operating systems, such as OS/360, MVS, z/OS, a Data Control Block is a description of a dataset in a program. A DCB is coded in Assembler programs using the DCB macro instruction...
(DCB) - Data set (IBM mainframe)Data set (IBM mainframe)data set , dataset , is a computer file having a record organization. The term pertains to the IBM mainframe operating system line, starting with OS/360, and is still used by its successors, including the current z/OS. Those systems historically preferred this term over a file...
- Direct access storage deviceDirect access storage deviceIn mainframe computers and some minicomputers, a direct access storage device, or DASD , is any secondary storage device which has relatively low access time relative to its capacity....
(DASD)