Chapter 9. IBM Flex System V7000 Storage Node Copy Services 367
Write I/O request to target: Writes to the target cause the grain to be copied from the
source to the target unless the entire grain is being written, and then the write completes
to the target.
FlashCopy mappings
FlashCopy bit mappings define the relationship between a source volume and a target
volume. FlashCopy mappings can be either a stand-alone mapping or a member of a
consistency group, as described in “FlashCopy consistency groups” on page 370.
Incremental FlashCopy mappings
In an incremental FlashCopy, the initial mapping copies all of the data from the source volume
to the target volume. Subsequent FlashCopy mappings only copy data that has been modified
since the initial FlashCopy mapping. This action reduces the amount of time that it takes to
re-create an independent FlashCopy image. You can define a FlashCopy mapping as
incremental only when you create the FlashCopy mapping.
Multiple target FlashCopy mappings
You can copy up to 256 target volumes from a single source volume. Each relationship
between a source and target volume is managed by a unique mapping such that a single
volume can be the source volume for up to 256 mappings.
Each of the mappings from a single source can be started and stopped independently. If
multiple mappings from the same source are active (in the copying or stopping states), a
dependency exists between these mappings.
If a single source volume has multiple target FlashCopy volumes, the write to the source
volume does not cause its data to be copied to all of the targets. Instead, it is copied to the
newest target volume only. The older targets refer to new targets first before referring to the
source. A dependency relationship exists between a particular target and all newer targets
that share a source until all data has been copied to this target and all older targets.
Cascaded FlashCopy mappings
The cascaded FlashCopy function allows a FlashCopy target volume to be the source volume
of another FlashCopy mapping. Up to 256 mappings can exist in a cascade. If cascaded
mappings and multiple target mappings are used, a tree of up to 256 mappings can be
created.
Cascaded mappings differ from multiple target FlashCopy mappings in depth. Cascaded
mappings have an association in the manner of A > B > C, while multiple target FlashCopy
has an association in the manner A > B1 and A > B2.
Background copy rate
The background copy rate is a property of a FlashCopy mapping defined as a value of 0 -
100. The background copy rate can be defined and dynamically changed for individual
FlashCopy mappings. A value of 0 disables background copy. This option is also called the
no-copy option, which provides pointer-based images for limited lifetime uses.
With FlashCopy background copy, the source volume data is copied to the corresponding
target volume in the FlashCopy mapping. If the background copy rate is set to 0, which means
disable the FlashCopy background copy, only data that changed on the source volume is
copied to the target volume.
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