ABCs of z/OS System Programming Volume 3 - IBM Redbooks

2.17 VTOC and DSCBs
VTOC
DSCBs
Figure 2-17 Data set control block (DSCB)
Data set control block (DSCB)
The VTOC is composed of 140-byte data set control blocks (DSCBs) that point to data sets
currently residing on the volume, or to contiguous, unassigned (free) tracks on the volume
(depending on the DSCB type).
DSCBs also describe the VTOC itself. CVAF routines automatically construct a DSCB when
space is requested for a data set on the volume. Each data set on a DASD volume has one or
more DSCBs (depending on its number of extents) describing space allocation and other
control information such as operating system data, device-dependent information, and data
set characteristics. There are seven kinds of DSCBs, each with a different purpose and a
different format number.
The first record in every VTOC is the VTOC DSCB (format-4). The record describes the
device, the volume the data set resides on, the volume attributes, and the size and contents of
the VTOC data set itself. The next DSCB in the VTOC data set is a free-space DSCB
(format-5) that describes the unassigned (free) space in the full volume. The function of
various DSCBs depends on whether an optional Index VTOC is allocated in the volume. Index
VTOC is a sort of B-tree, to make the search in VTOC faster.
Table 2-1 on page 47 describes the various types of DSCBs, taking into consideration
whether the Index VTOC is in place or not.
In z/OS V1R7 there is a new address space (DEVMAN) containing trace information about
CVAF events.
46 ABCs of z/OS System Programming Volume 3
F4 F0 F1 F1 F1
F4
DATA SET C
DATA SET A
DATA SET B