IBM DS6000 Series ユーザーズマニュアル

Chapter 6. Copy Services 

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Considerations

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

With Copy Services, you can create 

 for FlashCopy and PPRC. 

Consistency Group is a function to keep 

 in the backup copy. Data 

consistency means that the order of dependent writes is kept in the copy. 

In this section we define 

 and 

, and then we explain how 

Consistency Group operations keep 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:

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Operation 1, 2, and 3

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Operation 1 and 2

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Operation 1

If the copy of the data contains any of these combinations, then the data is 

 (the 

order of dependent writes was 

 preserved):

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Operation 2 and 3

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Operation 1 and 3

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Operation 2

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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

 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.