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.