5.1 Using Basic Sequential Access Method I/O Starting with z/OS V1R12, DFSMSdss now uses Basic Sequential Access Method (BSAM) instead of EXCP to read from and write to DFSMSdss dump data sets during DUMP, COPYDUMP, and RESTORE operations. DFSMSdss is designed to use larger blocks when possible for DUMP, COPYDUMP, and RESTORE operations, and to support Extended Format Sequential dump data sets on DASD for DUMP, RESTORE, and COPYDUMP. The use of larger block sizes improves performance for these operations, and using Extended Format dump data sets supports striping and compression. This allows DFSMSdss to support 256 K blocks when writing to and reading from a tape. Before z/OS V1R12, the maximum was 65,520 bytes. 5.1.1 Enhanced BLKSIZE DD The DUMP and COPYDUMP commands require a DD to be passed using the OUTDDNAME keyword that identifies a basic or large format sequential data set. DFSMSdss uses this data set to write the output blocks using EXCP during the DUMP and COPYDUMP. Users are not required to specify the BLKSIZE parameter on this output DD, but when the BLKSIZE is specified it currently only supports a limited set of the overall valid values for a tape device. Currently, users can specify a block size using the BLKSIZE parameter of the DD with either a value of zero or a value in the range of 7892 - 32760 bytes for both DASD and tape. DFSMSdss would ignore any block size over 32760 bytes and behaves as though no block size was specified. OUTDDNAME specifying a DASD data set The maximum supported block size for DASD is 32,760 bytes. DFSMSdss DUMP and