How is space allocated on DASD volumes?

To allocate a data set using JCL, you specify the amount of space required in blocks, records, tracks, or cylinders.

When creating a DASD data set, you specify the amount of space needed explicitly through the SPACE parameter, or implicitly by using the information available in a data class. The system can use a data class if SMS is active even if the data set is not SMS-managed. For system-managed data sets, the system selects the volumes, saving you from having to specify a volume when you allocate a data set.

If you specify your space request by average record length, space allocation is independent of device type. Device independence is especially important to system-managed storage. A logical record length (LRECL) is a unit of information about a unit of processing (for example, a customer, an account, a payroll employee, and so on). It is the smallest amount of data to be processed, and it is comprised of fields that contain information recognized by the processing application. The maximum length of a logical record (LRECL) is limited by the physical size of the used media.

Logical records, when located on DASD, tape, or optical devices, are grouped within physical records named blocks. BLKSIZE indicates the length of those blocks. Each block of data on a DASD volume has a distinct location and a unique address, thus making it possible to find any block without extensive searching. Logical records can be stored and retrieved either directly or sequentially.

When the amount of space required is expressed in blocks, you must specify the number and average length of the blocks within the data set.

Let us take an example of a request for disk storage as follows:

Average block length in bytes = 300
Primary quantity (number) of blocks = 5000
Secondary quantity of blocks, to be allocated if the primary quantity gets filled with data = 100

From this information, the operating system estimates and allocates the amount of disk space required.

Space for a disk data set is assigned in extents. An extent is a contiguous number of disk drive tracks, cylinders, or blocks. Data sets can increase in extents as they grow. Older types of data sets can have up to 16 extents per volume. Newer types of data sets can have up to 128 extents per volume or 255 extents total on multiple volumes.

Extents are relevant when you are not using PDSEs and have to manage the space yourself, rather than through DFSMS™. Here, you want the data set to fit into a single extent to maximize disk performance. Reading or writing contiguous tracks is faster than reading or writing tracks scattered over the disk, as might be the case if tracks were allocated dynamically. But if there is not sufficient contiguous space, a data set goes into extents.