If the total space occupied by the prime area and index area does not exceed one volume, you can request that the separate index area be imbedded in the prime area (to reduce access arm movement) by indicating an index size in the SPACE parameter of the DD statement defining the prime area.
If you request space for prime and index areas only, the system automatically uses any space remaining on the last cylinder used for master and cylinder indexes for overflow, provided the index area is on a device of the same type as the prime area.
Overflow tracks = total tracks
- (track index tracks + prime data tracks)
When you allocate a 1-cylinder data set, ISAM reserves 1 track on the cylinder for the end-of-file mark. You cannot request an independent index for an indexed sequential data set that has only 1 cylinder of prime data.
When you request space for an indexed sequential data set, the DD statement must follow several rules, shown as follows and summarized in Table 1.
As your data set is allocated, the operating system builds the track indexes in the prime data area. Unless you request a separate index area or an imbedded index area, the cylinder and master indexes are built in the independent overflow area. If you did not request an independent overflow area, the cylinder and master indexes are built in the prime area.
If an error is found during creation of a multivolume data set, the IEHPROGM utility program should be used to scratch the DSCBs on the volumes where the data set was successfully allocated. You can use the IEHLIST utility program to determine whether part of the data set has been allocated. The IEHLIST utility program also determines whether space is available or whether identically named data sets exist before space allocation is attempted for indexed sequential data sets. These utility programs are described in z/OS DFSMSdfp Utilities. Table 1 lists the criteria for requesting indexed sequential data sets.
Index Size Coded? | Restrictions on Unit Types and Number of Units | Resulting Arrangement of Areas |
---|---|---|
n/a | None | Separate index, prime, and overflow areas. See Specifying an Independent Overflow Area. |
n/a | None | Separate index and prime areas. Any partially used index cylinder is used for independent overflow if the index and prime areas are on the same type of device. See Specifying a Separate Index Area. |
No | None | Prime area and overflow area with an index at its end. See Specifying a Prime Area and Overflow Area. |
Yes | Prime area cannot have more than one unit. | Prime area, imbedded index, and overflow area. See Specifying a Prime Area and Overflow Area. |
No | None | Prime area with index at its end. Any partially used index cylinder is used for independent overflow. See Prime Data Area. |
Yes | Prime area cannot have more than one unit. | Prime area with imbedded index area; independent overflow in remainder of partially used index cylinder. See Prime Data Area. |