Metro Mirror Concepts

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 a fibre, either ESCON or Fibre Channel. The boxes can be from 2km to 103km apart using ESCON. However, if you plan to mirror over distances exceeding 9km, then you are better off using Fibre Channel, as performance will not suffer so much.

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.

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 take an hour or more if a lot of disks have to be synchronised

The following table explains some Metro Mirror terms and disk states

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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