IBM manual 104 DS6000 Series Concepts and Architecture

Note: If you want to use PPRC for mirroring, you need to compare its function with OS mirroring.

Generally speaking, you will have some disruption to recover your system with PPRC secondary volumes in an open systems environment because PPRC secondary volumes are not online to the application servers during the PPRC relationship.

You may need some operations before assigning PPRC secondary volumes. For example, in an AIX environment, AIX assigns specific IDs to each volume (PVID). PPRC secondary volumes have the same PVID as PPRC primary volumes. AIX cannot manage the volumes with the same PVID as different volumes. Therefore, before using the PPRC secondary volumes, you need to clear the definition of the PPRC primary volumes or reassign PVIDs to the PPRC secondary volumes.

Some operating systems (OS) or file systems (for example, AIX LVM) have a function for disk mirroring. OS mirroring needs some server resources, but usually can keep operating with the failure of one volume of the pair and recover from the failure non-disruptively. If you use PPRC for the mirroring in the local site only, you need to consider which solution (PPRC or OS mirroring) is better for your system.

Global Copy (PPRC-XD)

￿Description

Global Copy is a function for continuous copy without data consistency.

￿Advantages

It can copy your data at nearly an unlimited distance, even if you are limited by the network and channel extender capabilities. It is suitable for data migration and daily backup to the remote site.

￿Considerations

The copy is normally fuzzy but can be made consistent through synchronization.

Note: When you operate to create a consistent copy for Global Copy, you need the go-to-sync (synchronize the secondary volumes to the primary volumes) operation. During the go-to-sync operation, PPRC changes from a non-synchronous copy to a synchronous copy. Therefore, the go-to-sync operation may cause performance impact to your application system. If the data is heavily updated and the network bandwidth for PPRC is limited, the time for the go-to-sync operation becomes longer.

Global Mirror (Asynchronous PPRC)

￿Description

Global Mirror is an asynchronous copy; you can create a consistent copy in the secondary site with an adaptable Recovery Point Objective (RPO).

Note: Recovery Point Objective (RPO) specifies how much data you can afford to recreate should the system need to be recovered.

￿Advantages

Global Mirror can copy over nearly an unlimited distance. It is scalable across the server enclosures. It can realize low RPO with enough link bandwidth. Global Mirror causes little or no impact to your application system.

104DS6000 Series: Concepts and Architecture