IBM DS8000 manual What is a Consistency Group?, What is data consistency?

￿Considerations:

When the link bandwidth capability is exceeded with a heavy workload, the RPO might grow.

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.

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.

7.2.5What 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

132DS8000 Series: Concepts and Architecture