Volume Table of Contents

In the storage architecture of <a href="/facts/OS%2f360_and_successors/nQbdNbx6">OS/360 and successors</a>, <a href="/facts/Conversational_Monitor_System/qmWXRkQT">CMS</a>, and <a href="/facts/DOS%2f360_and_successors/bT9PzVHd">DOS/360 and successors</a>, the Volume Table of Contents (VTOC) is a data structure that provides a way of locating the <a href="/facts/Data_set_(IBM_mainframe)/ker4c5te">data sets</a> that reside on a particular <a href="/facts/Direct_access_storage_device/dr0tg5Hl">DASD</a> <a href="/facts/Volume_(computing)/g94wpmdC">volume</a>. With the exception of the IBM Z compatible disk layout in Linux on Z, it is the functional equivalent of the MS/PC <a href="/facts/DOS/sWwDXNmX">DOS</a> <a href="/facts/File_Allocation_Table/uFvpmWxa">File Allocation Table</a> (FAT), the <a href="/facts/NTFS/dryNM3BK">NTFS</a> <a href="/facts/Master_File_Table/dryNM3BK">Master File Table</a> (MFT), and an <a href="/facts/Inode/Ej7JIzyP">inode</a> table in a <a href="/facts/File_system/FdPm90uc">file system</a> for a <a href="/facts/Unix-like/YAKrAxzd">Unix-like</a> system. 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 <a href="/facts/File_size/nd3d84MN">size</a>, location, and <a href="/facts/File_system_permissions/DHq5SwI4">permissions</a>. Additionally, it contains an entry for every area of contiguous free space on the volume. The third record on the first <a href="/facts/Cylinder-head-sector/rUMqWXob">track</a> of the first <a href="/facts/Cylinder_(disk_drive)/rUMqWXob">cylinder</a> 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 Facilities program, <a href="/facts/ICKDSF/d0kWFFAf">ICKDSF</a>, in current systems.
When in OS/360 and successors allocates a data set, it generally searches the catalog to determine the volumes on which it resides. When a program opens a <a href="/facts/Direct_Access_Storage_Device/dr0tg5Hl">Direct Access Storage Device</a> (DASD) dataset, the OPEN routine searches the VTOC index (VTOCIX) if there is one, or directly searches the VTOC if there is no VTOCIX.

Volume Table of Contents open-in-new

Volume Table of Contents