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

Control interval (CI)
VSAM stores records in control intervals. A control interval is a continuous area of direct
access storage that VSAM uses to store data records and control information that describes
the records. Whenever a record is retrieved from direct access storage, the entire control
interval containing the record is read into a VSAM I/O buffer in virtual storage. The desired
record is transferred from the VSAM buffer to a user-defined buffer or work area.
Control area (CA)
The control intervals in a VSAM data set are grouped together into fixed-length contiguous
areas of direct access storage called control areas. A VSAM data set is actually composed of
one or more control areas. The number of control intervals in a control area is fixed by VSAM.
The maximum size of a control area is one cylinder, and the minimum size is one track of
DASD storage. When you specify the amount of space to be allocated to a data set, you
implicitly define the control area size. Refer to 4.32, “VSAM: Control interval (CI)” on
page 162, for more information.
Component
A component in systems with VSAM is a named, cataloged collection of stored records, such
as the data component or index component of a key-sequenced file or alternate index. A
component is a set of CAs. It is the VSAM terminology for an MVS data set. A component has
an entry in the VTOC. An example of a component can be the data set containing only data
for a KSDS VSAM organization.
Cluster
A cluster is a named structure consisting of a group of related components. VSAM data sets
can be defined with either the DEFINE CLUSTER command or the ALLOCATE command. The
cluster is a set of components that have a logical binding between them. For example, a
KSDS cluster is composed of the data component and the index component. The concept of
cluster was introduced to make the JCL to access VSAM more flexible. If you want to access
a KSDS normally, just use the cluster’s name on a DD card. Otherwise, if you want special
processing with just the data, use the data component name on the DD card.
Sphere
A sphere is a VSAM cluster and its associated data sets. The cluster is originally defined with
the access method services ALLOCATE command, the DEFINE CLUSTER command, or through
JCL. The most common use of the sphere is to open a single cluster. The base of the sphere
is the cluster itself.
Chapter 4. Storage management software 161