Using SIZE and MAXSIZE variables

The values for SIZE and MAXSIZE (and the space units the values represent) depend on the type of allocation of the data set. For all VSAM data sets, and non-VSAM data sets allocated in device dependent units (tracks or cylinders), the value represents the number of kilobytes using the maximum block size of the device. For example, the maximum block size for a 3390 is 56664. For all other non-VSAM data sets allocated in device independent units (blocks, average block, AVGREC=U, AVGREC=K, AVGREC=M), the value represents the number of data kilobytes (based on the average block size, specified block size, or 4096 if no block size is available).

The values DFSMSdss computes for SIZE and MAXSIZE may not match that of the original allocation. The values may be different if the device type on which the data set now resides is not the same as either of the following:

The device type used at allocation, or
The default device type in the CDS when the original allocation was done

DFSMSdss calculates the value based on the device type where the data set currently resides. DFSMSdss has no way of "knowing" what device type was specified or used for the original allocation.

DFSMSdss calculates SIZE and MAXSIZE variables as follows:

For PDS and PDSE data sets — The values DFSMSdss computes may be different from those computed by SMS because SMS adds space for the directory. Also, DFSMSdss computes MAXSIZE for PDSE data sets based on 122 secondary extents.
For VSAM data sets — DFSMSdss computes the SIZE and MAXSIZE for key-sequenced data sets (KSDS) from the current size and space values of the data component. The index component size is not included. Also, DFSMSdss computes MAXSIZE for VSAM data sets based on 122 secondary extents.
For extended-format sequential data sets — DFSMSdss computes MAXSIZE for extended-format sequential data sets based on 122 secondary extents.
For data sets allocated with AVGREC=U, K, or M — The size values computed during the initial allocation used the specified average block value. Since the average block size value is not stored anywhere and was only available during the initial allocation, DFSMSdss uses the DCB BLKSIZE value. If the DCB BLKSIZE value is not the same as the average block size value, the values computed for SIZE and MAXSIZE may be different from those computed at initial allocation.