Extended-Format VSAM Data Sets

VSAM extended format data sets might have any combination of the following optional attributes:

Data striping. This is called a striped data set.
Data compression. This is called a compressed format data set.
Extended addressability.

For example, a data set might be a striped compressed format data set with extended addressability. Striping can reduce sequential access time. Compression can reduce the disk space. Extended addressability increases the maximum size of the data set.

VSAM data sets must also be in extended-format to be eligible for the following advanced functions:

Partial space release (PARTREL)
Candidate volume space
System-managed buffering (SMB)

An extended-format data set for VSAM can be allocated for key-sequenced data sets, entry-sequenced data sets, variable-length or fixed-length relative-record data sets, and linear data sets.

Certain types of key-sequenced data set types are excluded. The following data sets cannot have an extended format:

Other system data sets
Temporary data sets

Note: The maximum number of paging slots for each page space is 16M. Page spaces regardless of their size have the extended format and extended addressable attributes assigned to them whether they are on an SMS managed volume or a non-SMS managed volume.

When a data set is allocated as an extended format data set, the data and index are extended format. Any alternate indexes related to an extended format cluster are also extended format.

If a data set is allocated as an extended format data set, 32 bytes (X'20') are added to each physical block. Consequently, when the control interval size is calculated or explicitly specified, this physical block overhead may increase the amount of space actually needed for the data set. Figure 1 shows the percentage increase in space as indicated. Other control intervals do not result in an increase in needed space.

Figure 1. Control interval size

  - 3390 Direct Access Device
 
     Control Interval Size    Additional Space Required
               512                        2.1%
              1536                        4.5%
             18432                       12.5%

  - 3380 Direct Access Device
 
     Control Interval Size    Additional Space Required
               512                        2.2%
              1024                        3.2%