Allocation of work data sets

Dynamic allocation has the following advantages over JCL allocation:

Installation options can be used to automatically activate dynamic allocation for all sort applications.
To use JCL allocation, appropriate DD statements must be specified for each individual application.
As the characteristics (file size, virtual storage, and so on) of an application change over time, DFSORT can automatically optimize the amount of dynamically allocated work space for the application. This eliminates unneeded allocation of disk space.
JCL allocation is fixed; DFSORT cannot adjust it. Disk space might be wasted.
Dynamically allocated work data sets are automatically allocated as large format so they can use more than 65535 tracks on a single volume.
When disk work space requirements are larger than expected, DFSORT can automatically recover by increasing allocation sizes or using additional work data sets.
As the amount of Hiperspace available to the application varies from run to run, DFSORT can automatically adjust the amount of space it dynamically allocates to complement the amount of Hiperspace. This eliminates unneeded allocation of disk space.
JCL allocation is fixed; DFSORT cannot adjust it, even if all sorting can be done in Hiperspace. Disk space might be wasted.

Dynamic allocation has one drawback: for certain applications, as described in File size and dynamic allocation, you might need to give DFSORT a reasonable estimate of the input file size. Later, if the input file size for the application increases significantly, you must update the file size estimate accordingly.

However, JCL allocation has a similar drawback, except that it applies to all applications. Unless you overallocate the work data sets initially and waste space, you have to update the JCL allocation when the input file size increases significantly for any application to avoid out-of-space abends.

If you can allocate enough work data set space with JCL to guarantee your applications will never exceed the space allocated, you do not need dynamic allocation. However, since efficient use of disk space is usually desirable, dynamic allocation is recommended over JCL allocation.

For both dynamic allocation and JCL allocation:

The amount of work space actually used will often be less than the amount allocated. DFSORT tries to minimize dynamic over-allocation while making certain that the application will not fail due to lack of space. With JCL allocation, you could minimize the amount of allocated space manually, but this might require changes to JCL allocation as the characteristics of the application change over time.
Limiting the virtual storage available to DFSORT can increase the amount of work space required. With a reasonable amount of storage, 4MB for example, DFSORT can sort using a reasonable amount of work space. If storage is limited, more work space might be required. If storage is drastically limited (for example, to 200KB), significantly more work space might be required.