ABCs of z/OS System Programming Volume 3 - IBM Redbooks

4.5 IEBCOPY: Compress operation
DATA.SET1
Directory
ABCDF
Member F
Figure 4-7 IEBCOPY compress operation
IEBCOPY compress operation
A partitioned data set will contain unused areas (sometimes called gas) where a deleted
member or the old version of an updated member once resided. This unused space is only
reclaimed when a partitioned data set is copied to a new data set, or after a
compress-in-place operation successfully completes. It has no meaning for a PDSE and is
ignored if requested.
The simplest way to request a compress-in-place operation is to specify the same ddname for
both the OUTDD and INDD parameters of a COPY statement.
Example
In our example in 4.4, “IEBCOPY: Copy operation” on page 114, the pointers in the
DATA.SET1 directory are changed to point to the new members B and C. Thus, the space
occupied by the old members B and C is unused. The members currently on DATA.SET1 are
compressed in place as a result of the copy operation, thereby eliminating embedded unused
space. However, be aware that a compress-in-place operation may bring risk to your data if
something abnormally disrupts the process.
116 ABCs of z/OS System Programming Volume 3
A
Unused
B
D
C
After Copy,and
Before Compress
Compress
DATA.SET1
Directory
ABCDF
Member F
A
B
D
C
Available
After Copy,and
After Compress