Defining Data Classes to Simplify Data Set Allocations

A data class defines what the data looks like and contains attributes that correspond to parameters that can be coded on JCL DD statements, TSO ALLOCATE commands, or requests for dynamic allocation. It is a collection of allocation and space attributes used to create a data set. You define data classes for data sets that have similar attributes. When end users allocate a data set and refer to a data class, SMS allocates the data set using the attribute values of its associated data class, with the exception of override Data Class attribute. In that case, only the Data Class space attributes will be used.

Figure 1, Figure 2, Figure 3 and Figure 4 show examples of the ISMF Data Class Define panel, with the attributes used to allocate an extended format, VSAM key-sequenced data set (KSDS).

Related Reading: For detailed descriptions of the data class attributes listed on the Data Class Define panel, see z/OS DFSMSdfp Storage Administration.

You can use Figure 1 to define record and space attributes.

Figure 1. Data Class Define Panel, Page 1

   Panel  Utilities  Scroll  Help                                              
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                              DATA CLASS DEFINE                 Page 1 of 6   
Command ===>                                                                  
                                                                              
SCDS Name . . . : USER6.MYCDS                                                 
Data Class Name : DC1                                                         
                                                                              
To DEFINE Data Class, Specify:                                                
  Description ==>                                                             
              ==>                                                             
  Recfm . . . . . . . . . .             (any valid RECFM combination or blank)
  Lrecl . . . . . . . . . .             (1 to 32761 or blank)                 
  Override Space  . . . . . N           (Y or N)                              
  Space Avgrec  . . . . . .             (U, K, M or blank)                    
        Avg Value . . . . .             (0 to 65535 or blank)                 
        Primary . . . . . .             (0 to 999999 or blank)                
        Secondary . . . . .             (0 to 999999 or blank)                
        Directory . . . . .             (0 to 999999 or blank)                
  Retpd or Expdt  . . . . .             (0 to 9999, YYYY/MM/DD or blank)      
  Volume Count  . . . . . . 1           (1 to 255 or blank)                   
  Add'l Volume Amount . . .             (P=Primary, S=Secondary or blank)     
Use ENTER to Perform Verification; Use DOWN Command to View next Panel;       
Use HELP Command for Help; Use END Command to Save and Exit; CANCEL to Exit.

You can specify additional volume and VSAM attributes on the second panel, shown in Figure 2.

Figure 2. Data Class Define Panel, Page 2

  Panel  Utilities  Scroll  Help                                              
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                                DATA CLASS DEFINE                 Page 2 of 6 
Command ===>                                                                
                                                                            
SCDS Name . . . : USER6.MYSCDS                                            
Data Class Name : DC1                                                     
                                                                            
To DEFINE Data Class, Specify:                                              
                                                                            
  Data Set Name Type  . . . . .        (EXT, HFS, LIB, PDS, Large or blank) 
    If Ext  . . . . . . . . . .        (P, R or blank)                      
    Extended Addressability . . N      (Y or N)                             
    Record Access Bias  . . . .        (S, U, DO, DW, SO, SW or blank)      
    RMODE31 . . . . . . . . . .        (ALL, BUFF, CB, NONE or blank)       
  Space Constraint Relief . . . N      (Y or N)                             
    Reduce Space Up To (%)  . .        (0 to 99 or blank)                   
    Dynamic Volume Count  . . .        (1 to 59 or blank)                   
  System Managed Buffering  . .            (1K to 2048M or blank)           
                                                                            
                                                                            
                                                                            
Use ENTER to Perform Verification; Use UP/DOWN Command to View other Panels;
Use HELP Command for Help; Use END Command to Save and Exit; CANCEL to Exit.

On this panel, you can specify the following attributes for a data class:

Whether to allocate VSAM data sets in extended format.
Whether to allocate a VSAM data set to use extended addressability, so that it can grow beyond the four gigabyte (GB) size (the data set must be allocated in extended format to be eligible for extended addressability)
Whether to let VSAM determine how many and which type of buffers to use when allocating VSAM data sets in extended format
Whether to retry new volume allocations or extends on new volumes that have failed due to space constraints
Whether to dynamically add volumes to a data set when a new volume is needed, and how many to add (up to 59; valid only when space constraint relief is Y). See z/OS DFSMSdfp Storage Administration for more information about dynamic volume count.
Whether to support VSAM data sets (both system-managed and non-system-managed data sets) with spanned record formats. Spanned record formats are those in which a data record can cross control interval boundaries
Whether extended format KSDSs are able to contain compressed data. You can request that physical sequential data sets be compressed using either tailored or generic compression dictionaries. You can use the access method services DCOLLECT command, the ISMF display function, and SMF type 14, 15, and 64 records to assess the overall space savings due to compression
Whether the data set could support extended attributes (format 8 and 9 DSCBs) and optionaly reside in EAS. (EATTR option) .

Use page 4 of the ISMF Data Class Define panel, shown in Figure 3, to further modify data classes.

Figure 3. Data Class Define Panel, Page 4

  Panel  Utilities  Scroll  Help                                              
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                              DATA CLASS DEFINE                 Page 4 of 6   
Command ===>                                                                  
                                                                              
SCDS Name . . . : Y421252.MYSCDS                                             
Data Class Name : DCDC1                                                        
                                                                              
To DEFINE Data Class, Specify:                                                
                                                                              
  Media Interchange                                                           
    Media Type . . . . . . .          (1 to 13 or blank)                         
    Recording Technology . .          (18,36,128,256,384,E1,E2-E4,EE2-EE4 or ' ')
    Performance Scaling  . . .        (Y, N or blank)                         
    Performance Segmentation          (Y, N or blank)                         
  Block Size Limit . . . . .             (32760 to 2GB or blank)              
  Recorg . . . . . . . . . .             (KS, ES, RR, LS or blank)            
  Keylen . . . . . . . . . .             (0 to 255 or blank)                  
  Keyoff . . . . . . . . . .             (0 to 32760 or blank)                
  CIsize Data  . . . . . . .             (1 to 32768 or blank)                
  % Freespace CI . . . . . .             (0 to 100 or blank)                  
              CA . . . . . .             (0 to 100 or blank)                  
Use ENTER to Perform Verification; Use UP/DOWN Command to View other Panels;  
Use HELP Command for Help; Use END Command to Save and Exit; CANCEL to Exit.

Use page 6 of the ISMF Data Class Define panel, shown in Figure 4, to specify whether to assign attributes for VSAM record-level sharing (RLS) to system-managed data sets.

You can specify whether the data set is eligible for backup-while-open processing and whether the data set is recoverable. You can specify the name of the forward recovery log stream. You can also specify the size of VSAM RLS data that is cached in the CF cache structure that is defined to DFSMS. You can also specify whether SMSVSAM is allowed to use 64-bit addressable virtual storage for its data buffers, moving them above the 2 gigabyte bar.

Figure 4. Data Class Define Panel, Page 6

  Panel  Utilities  Scroll  Help                                           
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                              DATA CLASS DEFINE                 Page 6 of 6
Command ===>                                                               
                                                                           
SCDS Name . . . : USER6.MYSCDS                                             
Data Class Name : DC2                                                     
                                                                           
To DEFINE Data Class, Specify:                                             
  Shareoptions Xregion  . . .      (1 to 4 or blank)                       
               Xsystem  . . .      (3, 4 or blank)                         
  Reuse . . . . . . . . . . . N    (Y or N)                                
  Initial Load  . . . . . . . R    (S, R or blank)                         
  BWO . . . . . . . . . . . .      (TC, TI, NO or blank)                   
  Log . . . . . . . . . . . .      (N, U, A or blank)                      
  Logstream Id  . . . . . . .                                              
  FRlog . . . . . . . . . . .      (A, N, R, U or blank)                   
  RLS CF Cache Value  . . . . A    (A, N, U or D)                          
  RLS Above the 2-GB Bar  . . N    (Y or N)                                
  Extent Constraint Removal   N    (Y or N)                                
  CA Reclaim  . . . . . . . . Y    (Y or N)                                
  Log Replicate . . . . . . . N    (Y or N)

You can develop data classes as you migrate permanent data to system-managed storage, or you can defer this until after most permanent data has been migrated. First, have users externally specify the keyword DATACLAS to select these data classes. Later, you can further automate by assigning them in your ACS routines. This latter level of automation requires a plan to identify specific data set types by interrogating the values of ACS variables. A simple example is the ACS variable, &LLQ, which represents the data set's low-level qualifier.

Data classes can assist you in enforcing standards for data set allocation. The need to maintain MVS™ or DFSMSdfp installation exits that enforce allocation standards might be eliminated by using the data class ACS routine facilities. All keywords on the JCL DD statement, TSO ALLOCATE command, or dynamic allocation are passed to your data class ACS routine to help you determine how to allocate the data set. You can issue a warning message to your users if their allocation violates standards, such as specifying a specific volume or not using a secondary allocation request. Or, you can fail an allocation by setting a non-zero return code in your data class ACS routine. DFSMSdfp enables you to include any ACS variable in the informational messages you create.

You can override a data class attribute using JCL or dynamic allocation parameters. However, overriding a subparameter of a parameter can override ALL of the subparameters for that parameter. For example, SPACE=(TRK,(1)) in JCL can cause primary, secondary, and directory quantities, as well as AVGREC and AVGUNIT, in the data class to be overridden. However, if you also specify DSNTYPE=PDS, the directory quantity is taken from the data class.

If you want the data class to supply the default value of a parameter, do not specify a value for that parameter in the JCL or dynamic allocation. Users cannot override the data class attributes of dynamically allocated data sets if you use the IEFDB401 user exit. By default, SMS cannot change values that are explicitly specified because doing so would alter the original meaning and intent of the allocation. The default for the 'override space' attribute in Data Class is NO, but you can use this attribute to allow SMS to change explicitly specified values.