Prior to MVS/ESA 4.3 the BLKSIZE parameter had to be coded as a sub-parameter of DCB, as in DCB=(BLKSIZE=nnnnn. BLKSIZE has subsequently been propogated to full parameter status, and does not need to be part of the DCB sub-parameter list.
It is the BLKSIZE which establishes the physical limit of data blocks written to output devices. The maximum block size available for disk devices is 32760 bytes. For a commentary on Disk BLKSIZE Optimisation click here. On OS/390 V2R10 systems and higher, i.e. z/OS V1.R1 upwards, cartridge data sets BLKSIZEs can be bigger at around 64K for 3490 and 256K for 3590.
Beware of specifying this parameter for input files, as if the chosen size is too small S001 abends will occur.
If the largest block size supported by a device is used, there are two benefits:
Be aware that the larger the block size the larger the REGION needed. As Sequential files use 5 buffers, there will be a 5 fold increase per data set.
Coding BLKSIZE=0 will indicate that a System Determined Block Size is requested, which will be the optimum for the device in use. If a data set is being read it is best to omit BLKSIZE from the DD statement, letting the system detect the size from the program or file label.
Copyright © KMS-IT Limited 2002