IBM System storage DS6000 Series, Redbooks

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Chapter 6. Copy Services
105
򐂰 Considerations
When the link bandwidth capability is exceeded with a heavy workload, the RPO might
grow.
6.2.5 What is 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 the data contains any of these combinations, then the data is
inconsistent
(the
order of dependent writes was
not
preserved):
򐂰 Operation 2 and 3
򐂰 Operation 1 and 3
򐂰
Operation 2
򐂰 Operation 3
In the Consistency Group operations, data consistency means this sequence is always kept in
the backup data.
And, the order of non-dependent writes does not necessarily need to be preserved. For
example, consider the following two sequences:
1. Deposit paycheck in checking account A
2. Withdraw cash from checking account A
3. Deposit paycheck in checking account B
4. Withdraw cash from checking account B
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.