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Volume Table of Contents

Another important mechanism that manages data on a computer system is the volume table of contents (VTOC). The VTOC keeps track of the locations of all data sets on a volume. You cannot find data sets without the VTOC, even if they exist on a volume. It is similar to looking for information in a huge library without using the card catalog.

VTOCs can become damaged in a number of ways. Hardware errors are the most common cause of VTOC errors. Because the VTOC is needed to find any data set on a volume, the system spends a lot of time reading the VTOC. If a disk controller malfunctions, and data is damaged on the disk pack, most likely the VTOC is damaged.

Programmer error is another cause of VTOC damage. Sometimes, you must alter a VTOC with an APF‑authorized version of the superzap program. For instance, you might have to alter a VTOC when trying to recover inadvertently deleted data sets or to recover from a hardware error. If the VTOC is altered in error, it can be damaged or unusable. There are very few times when it is necessary to alter a VTOC in this way. These updates must be tightly controlled because it is possible to bypass data security this way. A password‑protect bit can be zapped off, or a new data set name can be zapped in so that access control software thinks that it is a different data set.

The Disk Volume Analysis option (6.1.3) scans the VTOCs and displays any errors that it finds. VTOC errors must be described in somewhat esoteric terms. The analysis can tell you that the DOS/DIRF bit is on or you have a VTOC error in Format 4 High Water Mark. These errors are tied in with the internal structure of the VTOC and are hard to describe in any other way.

With the information from this display, you can approach the data center’s technical support staff. You can ask why the VTOCs are this way, and what they intend to do to correct the situation.