Metro Mirror Concepts

Metro Mirror can be run between two storage subsystems like DS8000s, or it can be run between two SVC SAN controllers. When run from SVCs, Metro Mirror is always Open Systems, but subsystem based Metro Mirror can be either Mainframe or Open Systems. These pages concentrate on Mainframe mirroring.

Metro Mirror, or Peer to Peer Remote Copy as it used to be called, is used to provide synchronous data mirroring between two different storage subsystems. PPRC is a real time hardware function, the data is always kept exactly synchronised between the two boxes, and is independent of any operating system. The storage systems have to be connected by Fibre Channel and the boxes can be up to 300km apart without needing any extra hardware.

Reads are not affected by PPRC, they always come from the primary system. For every WRITE I/O, data is written to cache on the primary system, then replicated to the cache on secondary system. Without PPRC, the data write is complete once the data is stored in cache on the primary subsystem. With PPRC, a write is not complete until the secondary reports that the data is safely stored in cache. This can significantly add to the time taken for a write operation.
mirroring a disk

PPRC works at logical disk level, i.e. disks as they are defined to the operating system. The whole disk is mirrored. In a disaster, it will be possible to switch to the other set of disks and boot a system from them, without having to restore any data.
When PPRC is first initiated, all the data is copied. This can several hours if a lot of disks have to be synchronised

The following table explains some Metro Mirror terms and disk states

Primary disk The disk which is addressed directly by the operating system. It will usually be in your main computer centre
Secondary disk The mirror of your primary disk
Simplex This disk is not part of any PPRC pair
Duplex This Disk is part of an established PPRC Pair, and all is working ok.
Pending A PPRC pair command has been issued, but all the data has not all been copied over yet
Suspend This disk is part of a PPRC pair, but mirroring is suspended. The Primary disk will keep a record of updated tracks in a log, the data on the secondary disk will probably be out of step with the primary

If an error condition occurs and a mirrored volume cannot be updated, the primary disk will either go into an "extended long busy" or "queue full" condition and will pass an error message back to the host. This message can be trapped by automation to freeze the all the disk subsystems.

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