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202 IBM System Storage DS6000 Series: Copy Services with IBM System z
17.1 Global Copy overview
Global Copy (formerly known as PPRC Extended Distance, or PPRC-XD) is a
non-synchronous remote copy function for System z and open systems for longer distances
than are possible with Metro Mirror. It is appropriate for remote data migration, offsite
backups, and transmission of inactive database logs at virtually unlimited distances.
Figure 17-1 Global Copy
With Global Copy, write operations complete on the primary disk subsystem before they are
received by the secondary disk subsystem. This capability is designed to prevent the primary
system’s performance from being affected by wait time from writes on the secondary system.
Therefore, the primary and secondary copies can be separated by any distance. Figure 17-1
illustrates how Global Copy operates, and the flow is described here:
1. The host server makes a write I/O to the primary DS6000. The write is staged through
cache and non-volatile storage (NVS).
2. The write returns as completed to the host server’s application.
3. At a later time (that is, in a non-synchronous manner), the primary DS6000 sends the
necessary data so that the updates are reflected on the secondary volumes. The updates
are grouped in batches for efficient transmission.
4. The secondary DS6000 returns write complete to the primary DS6000 when the updates
are secured in the secondary DS6000 cache and NVS. The primary DS6000 then resets
its Global Copy change recording information.
Note: The efficient extended distance mirroring technique of Global Copy is achieved with
sophisticated algorithms. For example, if changed data is in the cache, then Global Copy
sends only the changed sectors. There are also sophisticated queuing algorithms to
schedule the processing of updating the tracks for each volume and set the batches of
updates to be transmitted.
2
1
Server
write
Write to secondary
(non-synchronously)
Write acknowledge
1
3
1
2
4
LUN or
volume
Primary (source)
LUN or
volume
Secondary (target)