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

A GDS has sequentially ordered absolute and relative names that represent its age. The
catalog management routines use the absolute generation name in the catalog. Older data
sets have smaller absolute numbers. The relative name is a signed integer used to refer to the
most current (0), the next to most current (-1), and so forth, generation. See also 4.20,
“Absolute generation and version numbers” on page 141 and 4.21, “Relative generation
numbers” on page 142 for more information about this topic.
A generation data group (GDG) base is allocated in a catalog before the GDS’s are
cataloged. Each GDG is represented by a GDG base entry. Use the access method services
DEFINE command to allocate the GDG base (see also 4.19, “Defining a generation data
group” on page 139).
The GDG base is a construct that exists only in a user catalog, it does not exist as a data set
on any volume. The GDG base is used to maintain the generation data sets (GDS), which are
the real data sets.
The number of GDS’s in a GDG depends on the limit you specify when you create a new
GDG in the catalog.
GDG example
In our example in Figure 4-26 on page 137, the limit is 5. That means, the GDG can hold a
maximum of five GDSs. Our data set name is ABC.GDG. Then, you can access the GDSs by
their relative names; for example, ABC.GDG(0) corresponds to the absolute name
ABC.GDG.G0005V00. ABC.GDG(-1) corresponds to generation ABC.GDG.G0004V00, and
so on. The relative number can also be used to catalog a new generation (+1), which will be
generation number 6 with an absolute name of ABC.GDG.G0006V00. Because the limit is 5,
the oldest generation (G0001V00) is rolled-off if you define a new one.
Rolled in and rolled off
When a GDG contains its maximum number of active generation data sets in the catalog,
defined in the LIMIT parameter, and a new GDS is rolled in at the end-of-job step, the oldest
generation data set is rolled off from the catalog. If a GDG is defined using DEFINE
GENERATIONDATAGROUP EMPTY and is at its limit, then when a new GDS is rolled in, all the
currently active GDSs are rolled off.
The parameters you specify on the DEFINE GENERATIONDATAGROUP IDCAMS command determine
what happens to rolled-off GDSs. For example, if you specify the SCRATCH parameter, the GDS
is scratched from VTOC when it is rolled off. If you specify the NOSCRATCH parameter, the
rolled-off generation data set is re-cataloged as rolled off and is disassociated with its
generation data group.
GDSs can be in a deferred roll-in state if the job never reached end-of-step or if they were
allocated as DISP=(NEW,KEEP) and the data set is not system-managed. However, GDSs in
a deferred roll-in state can be referred to by their absolute generation numbers. You can use
the IDCAMS command ALTER ROLLIN to roll in these GDSs.
For further information about generation data groups, see z/OS DFSMS: Using Data Sets,
SC26-7410.
138 ABCs of z/OS System Programming Volume 3