In days of old, when 64KB was a lot of real storage, to reference a data set you had to specify the data set name and the DASD volume the data set was on. DSN=MY.JCL,VOL=SER=USER00

After this, the idea of a catalog was developed. Just like the catalog in a library, it told you where things were stored. When you created a data set, and specified DISP=(NEW,CATLG), the data set name and volume were stored in the catalog. When you wanted to use a data set, and did not specify the volume, then the catalog was used to find the volume for the data set.

As systems grew and the number of data sets grew, the catalog grew and quickly became difficult to manage. For example if you deleted data sets, the entry was logically removed from the catalog, resulting in gaps in the catalog.

After this a multi level catalog was developed. You have one master catalog. You can have many user catalogs. You define an alias in the master catalog saying for data sets starting with a specific high level qualifier, use that user catalog.

When a userid was created, most system programmers would also create an alias pointing to a user catalog. They may define a user catalog for each user, or a user catalog could be shared by many aliases.

The catalogs are managed by the VSAM component of z/OS.

Entity naming

PDS and sequential files are referred to as datasets. VSAM provides simple database objects,

• Relative Record ( where you say get me the n’th record)
• Key sequence. You define a primary index, and you can an index on different columns using an ALTERNATIVE INDEX. 

VSAM uses the term cluster to what you use in you JCL or application. A cluster has a data component, and zero or more index components.

Moving systems.

I have been running on a self contained ADCD system at z/OS level 2.4. I have recently installed a self contained system at z/OS 2.5. How do I get my data sets into the new system?
You can import connect a user catalog into a new (master) catalog, and define an alias in the new master catalog pointing to the user catalog.

When I did this I could then see my COLIN.* data sets. To be able to use the data sets, I need the volumes to be attached to the z/OS system.

Useful IDCAMS commands

In batch you use the IDCAMS program (IDC = prefix for VSAM, AMS is for Access Management Services!)

If you do not specify a catalog, it defaults to the master catalog.

Create a user catalog

//IBMDF  JOB 1,MSGCLASS=H                                   
//S1  EXEC PGM=IDCAMS                                       
//SYSPRINT DD SYSOUT=*                                      
//SYSIN DD *                                                
 DEFINE USERCATALOG -                                       
      ( NAME('A4USR1.ICFCAT') -                             
        MEGABYTES(15 15) -                                  
        VOLUME(A4USR1) -                                    
        ICFCATALOG  -                                       
        FREESPACE(10 10) -                                  
        STRNO(3  ) ) -                                      
        DATA( CONTROLINTERVALSIZE(4096) -                   
              BUFND(4) ) -                                  
        INDEX(BUFNI(4) )                                    
/*                                                          

List an entry

You can list information about an entry, such as a data set, or a catalog using the LISTCAT command

• Listing Data Sets With Your Prefix
• Listing Information About Specific Data Sets
• Listing Information From a Specific Catalog
• Listing Information by Data Set Qualifier
• Listing Alias Entries in a Catalog
• Listing Data Set Information By Creation and Expiration Dates
• Listing Specific Data Set Information in a Catalog

LISTCAT ENT(COLIN.USERS) ALL

Listing aliases

You can use the IDCAMS command LISTCAT with alias

LISTCAT ALIAS

which gives a one line per entry list of all of the aliases

ALIAS --------- ADCDA       
ALIAS --------- ADCDB       
ALIAS --------- ADCDC       
ALIAS --------- ADCDD       
...

LISTCAT ALIAS ALL

gives

ALIAS --------- ADCDA                                                       
     ...          
     ENCRYPTIONDATA                                                         
       DATA SET ENCRYPTION-----(NO)                                         
     ASSOCIATIONS                                                           
       USERCAT--USERCAT.Z24C.USER                                           
ALIAS --------- ADCDB                                                       
    ...      
     ENCRYPTIONDATA                                                         
       DATA SET ENCRYPTION-----(NO)                                         
     ASSOCIATIONS
       USERCAT--USERCAT.Z24C.USER                                                             

So we can see that the the alias ADCDA maps to user catalog USERCAT.Z24C.USER

Listing a catalog

The command

LISTCAT CATALOG(USERCAT.Z24C.USER)

gives

CLUSTER ------- 00000000000000000000000000000000000000000000       
   DATA ------- USERCAT.Z24C.USER                                  
   INDEX ------ USERCAT.Z24C.USER.CATINDEX                         
NONVSAM ------- ADCDA.S0W1.ISPF.ISPPROF                            
NONVSAM ------- ADCDA.S0W1.SPFLOG1.LIST                            
NONVSAM ------- ADCDB.S0W1.ISPF.ISPPROF                            
...                           
CLUSTER ------- SYS1.VVDS.VC4CFG1                                  
   DATA ------- SYS1.VVDS.VC4CFG1                                  

Which shows there is a data component of the catalog called USERCAT.Z24C.USER, and there is index component called USERCAT.Z24C.USER.CATINDEX.

The catalog has a data component (USERCAT.Z24C.USER) and an index component USERCAT.Z24C.USER.CATINDEX.

Within the catalog are entries for data sets such as ADCDA.S0W1.ISPF.ISPPROF, and system (DFDSS) dataset SYS1.VVDS.VC4CFG1 - which contains information what is on the SMS DASD volume C4CFG1.