For volume FlashCopy®, all FlashCopy withdraws are considered full-volume withdraws. All source tracks on the source device are removed from their FlashCopy relationship to all target tracks on the target device. Host software ensures that the volumes that are specified as source and target are the source and target devices, respectively, in an active FlashCopy relationship.
Anytime that a FlashCopy relationship is withdrawn, the data on the target volume might be left in an unpredictable state. Figure 1 shows how data on the target volume might be inconsistent with the source volume after a FCWITHDR command has been processed. It shows a typical backup cycle that starts with a complete backup on a Saturday night, updates to the source volume during the first three days of the week, and a point-in-time backup on a Wednesday night.
- 1
- On Saturday, at the end of a business week, the customer wants
to do a full backup. A FlashCopy MODE(COPY)
relationship is established between volume A and volume B. A background
copy process starts and all tracks are copied from volume A to volume
B (a T0 copy). After the FlashCopy relationship
is established, read access from the target copy is immediately possible.
As part of the backup cycle the customer uses DFSMSdss to create a
backup copy from volume B to tape. By creating the tape backup copy
from volume B, the customer gets the volume A data from the point-in-time
of the establish and eliminates I/O contention with application programs
continuing to run on volume A. When the background copy finishes the FlashCopy relationship is
removed.
NOTE: The data on volume B is unpredictable in the case where a FlashCopy MODE(COPY) relationship is withdrawn before the background copy to volume B completes.
- 2
- On Monday and Tuesday, application programs continue to make updates to volume A. The backup copy that was created on tape on Saturday from volume B is a virtual copy of volume A at the FlashCopy establish time on Saturday. Updates that continue on volume A after the FlashCopy relationship was established are not included on volume B. Consequently, the data on volume B is no longer consistent with the data on volume A.
- 3
- Application updates continue on volume A. A new FlashCopy establish with MODE(NOCOPY) is done Wednesday night in preparation for another tape backup cycle. The new point-in-time data is now accessible from B, and the tape backup copy can be made from volume B. Since this establish was done using MODE(NOCOPY), the state of the physical data on volume B at this point is unpredictable. Reading from volume B while the relationship is established will return the point-in-time data from volume A, while some tracks may also have been physically copied from volume A to volume B during this time. After the tape backup copy is complete, a FlashCopy withdraw command removes the FlashCopy relationship between volume A and volume B. At this time the contents of volume B are unreliable and should not be used.