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130 IBM System Storage DS6000 Series: Copy Services with IBM System z
12.1 Metro Mirror overview
Metro Mirror (previously known as synchronous Peer-to-Peer Remote Copy, or PPRC)
provides real-time mirroring of logical volumes between two DS6000s that can be located up
to 300 km from each other. It is a synchronous copy solution where write operations are
completed on both copies (local and remote site) before they are considered to be complete.
It is typically used for applications that cannot suffer any data loss in the event of a failure.
As data is synchronously transferred, the distance between primary and secondary disk
subsystems will determine the effect on application response time. Figure 12-1 illustrates the
sequence of a write update with Metro Mirror.
Figure 12-1 Metro Mirror
When the application performs a write update operation to a primary volume, this is what
happens:
1. Write to primary volume (DS6000 cache and NVS).
2. Write to secondary volume (DS6000 cache and NVS).
3. Signal write complete from the secondary DS6000.
4. Post I/O complete to host server.
The Fibre Channel connection between primary and secondary subsystems can be direct,
through a switch, or through other supported distance solutions (for example, Dense Wave
Division Multiplexor, DWDM).
2
3
1
4
Server
write
Write to secondary
Write acknowledge
Write complete
acknowledgement
LUN or
volume
Primary
(source)
LUN or
volume
Primary
(source)
LUN or
volume
Secondary
(target)