The FASTREPLICATION(REQUIRED | PREFERRED | NONE) keyword tells DFSMSdss how you want to use SnapShot. The default is FASTREPLICATION(PREFERRED).

FASTREPLICATION(REQUIRED) specifies that DFSMSdss must use SnapShot for the DEFRAG operation. If SnapShot cannot be used to move an extent DFSMSdss issues error message ADR938E, which indicates that the DEFRAG operation failed.

FASTREPLICATION(PREFERRED) specifies that DFSMSdss attempt to use SnapShot before any other I/O method. If SnapShot cannot be used, DFSMSdss uses traditional I/O methods to move the current extent and all the subsequent extents on the volume.

FASTREPLICATION(NONE) specifies that DFSMSdss not attempt to use SnapShot during the DEFRAG operation.

For more information about the FASTREPLICATION(REQUIRED | PREFERRED | NONE) keyword, refer to z/OS DFSMSdfp Storage Administration.