Choosing space efficient FlashCopy

A space efficient volume does not have all of its physical space allocated when it is created. Instead, its physical space is allocated in a way that is dependent on the type of space efficient volume in use.
Track space efficient (TSE)
Physical space is allocated on a track basis. When data is written to a TSE volume, a track of physical space is taken from the segments that are assigned to a repository volume, and is used to hold the data for the TSE volume. A repository volume can provide the physical space for multiple space efficient volumes. These types of volumes are intended for limited use.
Extent space efficient (ESE)
Physical space is allocated on an extent basis. When data is written to an ESE volume, a physical space extent is taken directly from an extent pool as writes occur. Extent sizes vary.

DFSMSdss can use extent space efficient volumes as the source or target of a FlashCopy relationship, or track space efficient volumes as the target of a FlashCopy relationship. These types of FlashCopy relationship are called space efficient FlashCopies. Track space efficient volumes have limitations that are described below.

Allowing track space efficient volume use with the FCSETGTOK keyword

During full volume copy operations, DFSMSdss can use a track space efficient volume as the target of a FlashCopy relationship, when you specify the FCSETGTOK keyword on your COPY command.

You can use track space efficient FlashCopy® for a full-volume copy operation only; that is, a COPY FULL operation or a COPY TRACKS command that specifies a full volume. Observe the following considerations:
  • If you specify FCSETGTOK with COPY FULL, and the target is a track space efficient volume, DFSMSdss attempts to establish a full-volume FlashCopy relationship without excluding free space, which results in one FlashCopy relationship for the entire volume. If FlashCopy cannot be used, DFSMSdss issues an error message and the copy operation fails. DFSMSdss does not try any other methods of data movement.
  • To use FCSETGTOK with COPY TRACKS, your command must specify one track range (an extent) that includes the entire volume (tracks 0 through n). Otherwise, the FCSETGTOK keyword has no effect on the copy operation.
DFSMSdss ignores the FCSETGTOK keyword for COPY operations in which:
  • FlashCopy is not used to perform the copy operation
  • The target volume is not a track space efficient volume
  • Less than a full volume is to be copied, for example, a COPY DATASET operation.

Along with the FCSETGTOK keyword, you must also specify the FAILRELATION sub-keyword to indicate the action that the storage facility is to take if the space on the repository volume is exhausted while the space efficient FlashCopy relationship still exists.

Using FCSETGTOK might require RACF® authorization. If your installation has defined the RACF FACILITY class profile, STGADMIN.ADR.COPY.FCSETGT, your user ID requires READ access to the profile. For more information, see Protecting DFSMSdss functions with RACF FACILITY class profiles.

Note: Space efficient FlashCopy to a track space efficient volume is intended for full volume copies that are short term in nature, such as those that are to be backed up to tape. Space efficient FlashCopy to a track space efficient volume might also be appropriate for longer term copies, if the source and target volumes are not frequently updated. The physical background copy option is not permitted for space efficient FlashCopy to a track space efficient volume. That is, you must also specify FCNOCOPY with the FCSETGTOK keyword.

For an overview of FlashCopy, refer to z/OS DFSMS Advanced Copy Services.

For more information about the FCSETGTOK keyword and the FAILRELATION sub-keyword, refer to FCSETGTOK.