IBM System storage DS6000 Series, Redbooks

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DS6000 Series: Concepts and Architecture
Figure 6-7 Metro Mirror
Global Copy (PPRC-XD)
Global Copy copies data non-synchronously and over longer distances than is possible with
Metro Mirror. When operating in Global Copy mode, the source volume sends a periodic,
incremental copy of updated tracks to the target volume, instead of sending a constant
stream of updates. This causes less impact to application writes for source volumes and less
demand for bandwidth resources, while allowing a more flexible use of the available
bandwidth.
Global Copy does not keep the sequence of write operations. Therefore, the copy is normally
fuzzy, but you can make a consistent copy through synchronization (called a go-to-sync
operation). After the synchronization, you can issue FlashCopy at the secondary site to make
the backup copy with data consistency. After you establish the FlashCopy, you can change
the PPRC mode back to the non-synchronous mode.
If you want to make a consistent copy with FlashCopy, you must purchase a Point-in-Time
Copy function authorization (2244 Model PTC) for the secondary storage unit.
Note: When you change PPRC mode from synchronous to non-synchronous mode, you
change the PPRC mode from synchronous to suspend mode at first, and then you change
PPRC mode from suspend to non-synchronous mode.
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1
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Server write
Write to secondary
Write acknowledge
Write hit
to secondary