CA Dynam for VSE User Guide

Managing the VTOC
VTOC Indexes
A VTOC index is a separate data set on a volume. Each index record identifies
a data set residing on the volume, with a pointer to the DSCB for that file. This
allows direct access by data set name, without resource-consuming scans. To
locate the file, the VTOC index is read into storage, and the search for the data
set name is made there.
To locate a data set by data set name in a nonindexed VTOC, it is necessary to
sequentially scan DSCBs on the device. This scan involves use of the channel,
the DASD control unit, and the device itself.
The VTOC is open for shorter periods of time, allowing more
multiprogramming and multi-CPU activity to progress in a given period of
time, because the processing of an indexed VTOC is streamlined. In short,
system throughput is significantly enhanced, whether DASD files are controlled
by CA Dynam/D.
Processing the VTOC
Depending on whether a volume has a VTOC index, the VTOC itself is
maintained in native IBM format, and can be processed by any IBM operating
system. If a volume has a VTOC index, however, all systems updating the
VTOC must also be running CA Dynam VTOC management.
A VTOC is normally initialized using the IBM utility, ICKDSF. This program also
can build a VTOC index for the OS user. CA Dynam, while able to process a
VTOC built with ICKDSF, cannot process a VTOC index built with this utility. If
a VTOC index is built this way, it will be ignored. (For more information about
the program ICKDSF, see the IBM publication Device Support Facilities User's
Guide and Reference.)
Building the VTOC Index
A CA Dynam VTOC index is built using the CA Dynam utility, DYNCAT.
Thereafter, each call to the VSE VTOC management routines is processed
through the VTOC index.
Chapter 4: Disk File Management 4–11