Volume Table of Contents

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 is the functional equivalent of either the Microsoft DOS File Allocation Table (FAT) or GUID Partition Table (GPT) on a desktop PC, and the root directory of a mass storage device (floppy, jump drive, hard disk, etc.) on a PC or minicomputer, e.g. / on Unix or Linux, C: on DOS or Windows. The VTOC is not used to contain any IPLTEXT 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, 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 volume of DASD (i.e. disk pack) 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 disk volume when it is initialized using the Device Support Facilities utility program, ICKDSF. VTOC was originally designed for removable disk packs.For example, if the dataset is created with the DD card specification SPACE=(CYL,(4,2)) the dataset is initially allocated four cylinders, and an additional two cylinders, not necessary contiguous, will be allocated each time more space is needed, up to fifteen additional extents.extent ending cylinder and track (four bytes, binary). Pointer to Format-3 (extension) DSCB if this dataset has more than three extents. 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 is the functional equivalent of either the Microsoft DOS File Allocation Table (FAT) or GUID Partition Table (GPT) on a desktop PC, and the root directory of a mass storage device (floppy, jump drive, hard disk, etc.) on a PC or minicomputer, e.g. / on Unix or Linux, C: on DOS or Windows. The VTOC is not used to contain any IPLTEXT 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, 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 volume of DASD (i.e. disk pack) 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 disk volume 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 consists of a sequence of 140-byte records known as Data Set Control Blocks, or 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 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 the 'extents' of data sets, giving their start address and end address 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.