z/OS V1R6.0 DFSMS Access Method Services for Catalogs

DEFINE CLUSTERMEGABYTES(primary[ secondary])|RECORDS(primary[ secondary])|TRACKS(primary[ secondary])|The amount of space in cylinders, kilobytes, megabytes, records, or tracksallocated to the cluster from the volume’s available space. A kilobyte ormegabyte allocation resolves to either tracks or cylinders; record allocationresolves to tracks.Exception: If allocation resolves to tracks, the space is contiguous. See“Optimizing Control Area Size” in z/OS DFSMS: Using Data Sets for moreinformation.Requests for space are directed to DADSM and result in a format-1 DSCB forall entries.If the cluster is not SMS-managed, you must use the amount of spaceallocated, either through this parameter, or through the DATACLASS, MODEL,or RECATALOG parameters. This parameter is optional if the cluster ismanaged by SMS. If it is used, it overrides the DATACLASS spacespecification. If it is not used, it can be modeled or defaulted by SMS. If itcannot be determined, the DEFINE is unsuccessful.If you select KILOBYTES or MEGABYTES, the amount of space allocated is theminimum number of tracks or cylinders required to contain the specifiednumber of kilobytes or megabytes.If you select RECORDS, the amount of space allocated is the minimum numberof tracks that are required to contain the given number of records. Themaximum number of records is 16,777,215. If RECORDS is specified for alinear data set, space is allocated with the number of control intervals equal tothe number of records.Recommendation: To maintain device independence, do not use the TRACKSor CYLINDERS parameters. If you use them for an SMS-managed data set,space is allocated on the volumes selected by SMS in units equivalent to thedevice default geometry. If there is an allocation failure due to lack of space,SMS retries allocation with a reduced space quantity. However, any retry,including reduced space quantity, is only attempted if Space Constraint Relief⇒ Y is specified. SMS also removes other limitations if the data class allowsspace constraint relief.Regardless of the allocation type, the calculation of the CA (control area) size isbased on the smaller of the two allocation quantities (primary or secondary) inthe DEFINE command. A CA is never greater than a single cylinder, it mightbe less (that is, some number of tracks), depending on the allocation amountand type used. When tracks or records are used, the space allocation unit (theCA size) can be adjusted to one cylinder. This adjustment is made if thecalculated CA size contains more tracks than exist in a single cylinder of thedevice being used. The CA area size assigned by VSAM is the smallest of:v One cylinderv The primary space quantityv The secondary space quantityIf the CA size assigned is not evenly divisible into either the primary orsecondary space quantity, VSAM increases that space to a value evenlydivisible by the CA size. If you are defining an extended format data set, youshould review ″Defining an Extended Format Key-Sequenced Data Set″ in z/OSDFSMS: Using Data Sets for information about additional space requirements.148 z/OS V1R6.0 DFSMS Access Method Services for Catalogs