The SPACE parameter in a DD statement that defines a library must always be of the form:
SPACE=(units,(quantity,increment,directory))
Although you can omit the third term (increment), indicating its absence by a comma, the last term, specifying the number of directory blocks to be allocated, must always be present.
The amount of auxiliary storage required for a library depends on the number and sizes of the members to be stored in it and on how often members will be added or replaced. (Space occupied by deleted members is not released.) The number of directory blocks required depends on the number of members and the number of aliases. You can specify an incremental quantity in the SPACE parameter that allows the operating system to obtain more space for the data set, if such is necessary at the time of creation or at the time a new member is added; the number of directory blocks, however, is fixed at the time of creation and cannot be increased.
For example, the DD statement:
// PDS DD UNIT=SYSDA,VOL=SER=3412, // DSNAME=ALIB, // SPACE=(CYL,(5,,10)), // DISP=(,CATLG)
requests the job scheduler to allocate 5 cylinders of the DASD with a volume serial number 3412 for a new library name ALIB, and to enter this name in the system catalog. The last term of the SPACE parameter requests that part of the space allocated to the data set be reserved for ten directory blocks.