User guide

1. Calculate the number using the number of differential tables (see (page 31)).
2. Compare the results of the three calculations. The smallest of the three is the maximum number
of FS pairs that you can create in the storage system.
When you create FS pairs again, you do not need to use the snapshot estimated manageable
capacity and the number of cache management devices. Use only the number of pair tables and
calculate the number of FS pairs that you can create.
For more information, see “Calculating the number of Fast Snap pairs based on pair tables”
(page 31).
If you have multiple P-VOLs, calculate the number of FS pairs that you can create for each P-VOL
in the storage system.
Calculating the maximum number of pairs based on differential tables
Differential tables track updates that take place in pairs. A base that does not have shared memory
has 26,176 differential tables. The remaining number of differential tables on a system is determined
by the amount of shared memory that you have installed on the system.
The following table shows various combinations and the number of differential tables that are
associated with them.
Differential tables requiredCombination
26,176Base that does not have shared memory
104,768Extension 1
209,600Extension 2
Differential tables are also used by the following software applications:
• BC/SS
• BC Z
• SS
• FC Z
• Compatible FlashCopy SE
• Auto LUN
Calculating the number of Fast Snap pairs based on pair tables
The following relationships and pairs require a pair table, which contains the information required
to manage them:
• FC Z relationship
• Compatible FlashCopy SE relationship
• SS pair
• FS pair
You can use up to 1,048,575 pair tables in a storage system, you can share the pair tables among
the following relationships and pairs.
To calculate the number of FS pairs, use the following formula:
1,048,575 - (the number of SS pairs) - (the number of FC Z or Compatible
FlashCopy SE relationships) - (the number of existing FS pairs)
For example, the number of FS pairs that you can create in a storage system is calculated using
the following formula, if the number of pair tables used by SS is 15,000 and the number of pair
tables used by FC Z and Compatible FlashCopy SE is 30,000:
Requirements for Fast Snap tasks 31