IBM DS8000 User Manual page 139

Multiple relationship FlashCopy
The multiple relationship FlashCopy function allows a source volume to have
multiple targets simultaneously.
When a FlashCopy operation copies data from a source volume to a target volume,
that source volume can be involved in more than one FlashCopy relationship at
the same time (known as a multiple relationship FlashCopy). If a track on a
volume is not a target track of an existing FlashCopy relationship, it can become
the target in a new FlashCopy relationship. Up to twelve different target
FlashCopy relationships can exist for a single source volume.
If you require multiple copies of the same data, this function allows you to copy a
single source volume multiple (up to twelve) times to different target volumes as
soon as a FlashCopy volume is established. For example, suppose you use
FlashCopy to copy volume A to volume B. As soon as that FlashCopy relationship
is established, you can copy volume A to volume C. Once that relationship is
established, you can copy volume A to volume D, and so on. Multiple sessions
within a single volume are also possible.
Persistent FlashCopy
A persistent FlashCopy relationship is one that does not end when the background
physical copy ends. The relationship persists until it is explicitly withdrawn.
Persistent FlashCopy overrides normal FlashCopy processing and allows a
FlashCopy relationship to stay active until it is explicitly withdrawn. Normally, a
FlashCopy relationship is automatically withdrawn when the background physical
copy ends.
Designating that a FlashCopy relationship be persistent can help protect against
inadvertent updates of recently created target volumes. For example, if a source
volume is regularly copied to alternating target volumes (thereby ensuring that a
complete copy of the source volume is always available), the persistent relationship
identifies the target volume for the most recently completed FlashCopy.
Consider the following situation:
v A company wants to maintain more than one point-in-time version of a set of its
production volumes.
v The company creates a FlashCopy version of its production volumes every eight
hours, each time using a different set of target volumes.
v The production volume is Volume A.
v The copies are Volume B, Volume C, and Volume D.
v The volumes that are copied contain a different point-in-time version.
The following represents the procedure that FlashCopy operations follow in this
scenario:
1. FlashCopy processing from Volume A to Volume B
2. FlashCopy from processing Volume A to Volume C
3. FlashCopy from processing Volume A to Volume D
This FlashCopy procedure presents a possible exposure for mistakes. For instance,
if the FlashCopy operation (step 2 of the FlashCopy procedure) is mistakenly
performed from Volume A to Volume B rather than from Volume A to Volume C,
the result could be data that overwrites the original point-in-time copy in volume
B.
Chapter 8. Optional feature codes for the DS8000
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