ABCs of z/OS System Programming Volume 3 - IBM Redbooks

5.27 Device-independence space allocation
//ALLOC EXEC PGM=IDCAMS
//SYSPRINT DD SYSOUT=*
//SYSIN DD *
ALLOCATE
DSNAME('FILE.PDSE') -
NEW -
DSNTYPE(LIBRARY)
SUB
Figure 5-27 Device independence
SMS
FILE.PDSE
VOLSMS
Ensuring device independence
The base configuration contains a default unit that corresponds to a DASD esoteric (such as
SYSDA). Default geometry for this unit is specified in bytes/track and tracks/cylinder for the
predominant device type in the esoteric. If users specify the esoteric, or do not supply the
UNIT parameter for new allocations, the default geometry converts space allocation requests
into device-independent units, such as KBs and MBs. This quantity is then converted back
into device-dependent tracks based on the default geometry.
System-determined block size
During allocation, DFSMSdfp assists you to assign a block size that is optimal for the device.
When you allow DFSMSdfp to calculate the block size for the data set, you are using a
system-determined block size. System-determined block sizes can be calculated for
system-managed and non-system-managed primary storage, VIO, and tape data sets.
The use of system-determined block size provides:
► Device independence, because you do not need to know the track capacity to allocate
efficiently
► Space usage optimization
► I/O performance improvement
► Simplifies JCL, because you do not need to code BLKSIZE
You take full advantage of system-managed storage when you allow the system to place data
on the most appropriate device in the most efficient way, when you use system-managed data
Chapter 5. System-managed storage 287