IBM DS8000 - Manual (Page 154)

132 DS8000 Series: Concepts and Architecture
Considerations:
When the link bandwidth capability is exceeded with a heavy workload, the RPO might
grow.
z/OS Global Mirror (XRC)
Description
z/OS Global Mirror is an asynchronous copy controlled by z/OS host software, called
System Data Mover .
Advantages
It can copy with nearly unlimited distance. It is highly scalable, and it has very low RPO.
Considerations
Additional host server hardware and software is required. The RPO might grow if
bandwidth capability is exceeded, or host performance might be impacted.
7.2.5 What is a Consistency Group?
With Copy Services, you can create Consistency Groups for FlashCopy and PPRC.
Consistency Group is a function to keep data consistency in the backup copy. Data
consistency means that the order of dependent writes is kept in the copy.
In this section we define data consistency and dependent writes , and then we explain how
Consistency Group operations keep data consistency.
What is data consistency?
Many applications, such as databases, process a repository of data that has been generated
over a period of time. Many of these applications require that the repository is in a consistent
state in order to begin or continue processing. In general, consistency implies that the order
of dependent writes is preserved in the data copy. For example, the following sequence might
occur for a database operation involving a log volume and a data volume:
1. Write to log volume: Data Record #2 is being updated.
2. Update Data Record #2 on data volume.
3. Write to log volume: Data Record #2 update complete.
If the copy of the data contains any of these combinations then the data is consistent:
Operation 1, 2, and 3
Operation 1 and 2
Operation 1
If the copy of data contains any of those combinations then the data is inconsistent (the order
of dependent writes was not preserved):
Operation 2 and 3
Operation 1 and 3
Operation 2
Note: To manage Global Mirror, you need many complicated operations. Therefore, we
recommend management utilities (for example, Global Mirror Utilities) or management
software (for example, IBM Multiple Device Manager) for Global Mirror.