IBM DS8000 manual How does Consistency Group keep data consistency?

￿Operation 3

In the Consistency Group operation, 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

In order for the data to be consistent, the deposit of the paycheck must be applied before the withdraw of cash for each of the checking accounts. However, it does not matter whether the deposit to checking account A or checking account B occurred first, as long as the associated withdrawals are in the correct order. So for example, the data copy would be consistent if the following sequence occurred at the copy. In other words, the order of updates is not the same as it was for the source data, but the order of dependent writes is still preserved.

1.Deposit paycheck in checking account B

2.Deposit paycheck in checking account A

3.Withdraw cash from checking account B

4.WIthdraw cash from checking account A

How does Consistency Group keep data consistency?

Consistency Group operations cause the storage units to hold I/O activity to a volume for a time period by putting the source volume into an extended long busy state. This operation can be done across multiple LUNs or volumes, and even across multiple storage units.

In the storage subsystem itself, each command is managed with each logical subsystem (LSS). This means that there are slight time lags until each volume in the different LSS is changed to the extended long busy state. Some people are concerned that the time lag causes you to lose data consistency, but, it is not true. We explain how to keep data consistency in the Consistency Group environments in the following section.

See Figure 7-13 on page 134. In this case, three write operations (1st, 2nd, and 3rd) are dependent writes. It means that these operations must be completed sequentially.

Chapter 7. Copy Services 133