HP StorageWorks XP Remote Web Console Software - Manual (Page 28)

Calculating the Number of XP Snapshot Pairs by Using the Number of Differential Tables
If additional shared memory is installed, the maximum number of differential tables will be 26,176 or
57,600 or 104,768 or 146,688 or 209,600. To calculate the number of XP Snapshot pairs that can be
created, calculate the number of differential tables required for the XP Snapshot pair and compare it with
the number of differential tables of the whole subsystem. Other than XP Snapshot, the following program
products use differential tables in the storage system:
• Business Copy
• ShadowImage for Mainframe
• Compatible FlashCopy Version 1
• Compatible FlashCopy Version 2
• Auto LUN
If any of the above mentioned program products other than XP Snapshot are used in the same subsystem,
the number of differential tables that can be used by XP Snapshot pairs is calculated by subtracting the
number of differential tables used by the pairs (migration plan for Auto LUN) of the above mentioned
program products from the number of differential tables of the whole subsystem.
Please see the manual of each program product for the calculation of the number of required differential
tables for Business Copy, ShadowImage for Mainframe, Compatible FlashCopy Version 1, Compatible
FlashCopy Version 2, and Auto LUN.
To calculate the number of differential tables for each XP Snapshot pair, use the formula below:
((X) ч 256) ч (Z)
(X): Capacity of the volume (in kilobytes)
(Z): The number of slots that a differential table can manage (1,916 × 32)
Note that you must round up the result of the calculation to the nearest whole number.
When the capacity of the volume ((X) in the formula above) is 16 GB (16,777,216 kB), the number of
differential tables for each XP Snapshot pair can be calculated as follows.
Example
(16,777,216 ч 256) ч (1,916 Ч 32) = 1.06889
1.06889 will be 2 when the value after the decimal point is rounded up. This indicates that two
differential tables are required to create an XP Snapshot pair.
Assume that no other program products use differential tables in a subsystem where the number of
differential tables is 26,176. The number of differential tables that XP Snapshot can use is 26,176. To
create Snapshot pairs that require two differential tables for each pair, the calculation will be as follows.
26,176 ÷ 2 = 13,088
Thus, the maximum number of XP Snapshot pairs that can be created is 13,088.
Calculating the Number of XP Snapshot Pairs by Using the Shared Memory Capacity for the
Virtual Volume Management Area
The V-VOL management area consists of the following three elements.
• Pool association information
• Pool management block
• Management information other than those above (shared memory is fixed to 10 MB)
The shared memory of the V-VOL management area differs according to the additional shared memory.
The following table shows the detailed information.
28 Preparing for XP Snapshot Operations