The volume table of contents (VTOC) is a data set that describes the contents of the DASD volume on which it resides. The VTOC locates data sets on that volume. The VTOC is composed of data set control blocks (DSCBs). The DSCBs describe the type and location of data sets on that volume, unused records in the VTOC, and the unallocated space on the volume.

A VTOC index decreases search time for DSCBs. If the system detects a logical or physical error in a VTOC index, the system disables further access to the index from all systems that might be sharing the volume. If a VTOC index becomes disabled, the VTOC remains usable but with possibly degraded performance.

If a VTOC index becomes disabled, you can rebuild the index without taking the volume offline to any system. All systems can continue to use that volume without interruption to other applications, except for a brief pause during the index rebuild. After the system rebuilds the VTOC index, it automatically reenables the index on each system that has access to it.

Related reading: For more information, see z/OS DFSMSdfp Advanced Services and Device Support Facilities (ICKDSF) User's Guide and Reference.