What is SMS in mainframe ? SMS stands for Storage Management Subsystem

Introduction:
The Storage Management Subsystem (SMS) automates the use of storage for data sets.  The z/OS ,  system is not aware of how much space a dataset will need, where it will be stored(Disk or Tape) ,what will be its record format and other details. Z/oS storage team need to decide whether to backup the large files and also need to recall it when necessary. All these data management activities can be done either manually or through the use of automated processes which is nothing but SMS.  With SMS activated , the z/OS  programmer or storage admin team  may, for example, create model data definitions for typical data sets, so that SMS automatically assigns those  attributes to data sets when they are created.
The data sets allocated through SMS are called system-managed data sets or SMS-managed data sets. For example, suppose you want to create a new data set named DATA.LIST. If SMS is active, you could use JCL like this:

//NEWDS DD DSN=myid.test.dataset1,
// DISP=(NEW,CATLG),
// DATACLAS=DCSL002,
// STORCLAS=SRTCL00

In this case, z/OS can use characteristics from predefined data and storage classes when it creates the DATA.LIST data set.

If SMS is not active or not in use, you need to manually specify the space requirements and storage location for the new data set, and your JCL would look like this :

//NEWDS DD DSN=myid.test.dataset1,
// DISP=(NEW,KEEP),
 // SPACE=(CYL,(1,1)),
 // UNIT=SYSDA,
// VOL=SER=SHARED  

The advantages associated with an SMS environment.
 • The user is relieved of making decisions about resource allocation of data sets, since it is done by SMS.
 • SMS provides the capability of concatenating data sets of unlike devices. For example, a tape data set can be concatenated with a disk data set. This capability is not available in a non-SMS environment.
 • SMS managed data sets cannot be deleted unless they are first uncataloged. Due to this extra step, erroneous deletion of data sets is minimized.
 • Additional features are available in the use of IDCAMS in an SMS environment. For example, the ALTER command can be used to increase the limit of the number of generation data sets in a GDG. This feature is not available in a non-SMS system.
 • VSAM data sets created in an SMS environment offer more flexibility that those created through JCL in a non-SMS environment.

Read about the  SMS Parameters in the next post