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Chapter 7. Copy Services 131
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.
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).
Advantages:
Global Mirror can copy with nearly an unlimited distance. It is scalable across the storage
units. It can realize a low RPO with enough link bandwidth. Global Mirror causes only a
slight impact to your application system.
Note: If you want to use a PPRC copy from the application server which has the PPRC
primary volume, you need to compare its function with OS mirroring.
You will have some disruption to recover your system with PPRC secondary volumes in an
open system environment because PPRC secondary volumes are not online to the
application servers during the PPRC relationship.
You may also 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 a PPRC copy from the application server for recovery, you need to consider which
solution (PPRC or OS mirroring) is better for your system.
Note: When you 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.
Note: Recovery Point Objective (RPO) specifies how much data you can afford to
re-create should the system need to be recovered.