HP StorageWorks XP Remote Web Console Software - Manual (Page 36)

Calculating the Maximum Number of Pairs
The number of XP Snapshot pairs that can be created for a storage system depends on the number
of differential tables, the capacity of shared memory for the V-VOL management area, or the
number of pair tables. Calculate the number of XP Snapshot pairs that can be created, first by
using the number of differential tables, then by using the capacity of shared memory for the V-VOL
management area, and finally by using the number of pair tables. Compare the three calculation
results. The smallest value is the maximum number of XP Snapshot pairs that can be created for
the storage system.
The following are the descriptions of the calculation of the number of XP Snapshot pairs that can
be created based on the number of differential tables, the capacity of shared memory for the V-VOL
management area, or the number of pair tables.
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 storage system. Other than XP
Snapshot, the following program products use differential tables in the storage system:
•
XP Business Copy
•
ShadowImage for Mainframe
•
Compatible FlashCopy Version 1
•
Compatible FlashCopy Version 2
•
XP Auto LUN
If any of the mentioned program products other than XP Snapshot are used in the same storage
system, 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 XP Auto LUN) of
the previously mentioned program products from the number of differential tables of the whole
storage system.
See the manual of each program product for the calculation of the number of required differential
tables for XP Business Copy, XP Business Copy for Mainframe, Compatible FlashCopy Version 1,
Compatible FlashCopy Version 2, and XP Auto LUN.
For information on using XP Auto LUN, contact HP technical support (see “Contacting HP”
(page 116) ).
To calculate the number of differential tables for each XP Snapshot pair, use the following formula:
(X ч 256) ч (Z)
(X): Capacity of the volume (in kilobytes)
(Z): The number of slots that a differential table can manage (639 × 32)
You must round up the result of the calculation to the nearest whole number.
When the capacity of the volume (X in the formula) 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 (639 Ч 32) = 3.19523
3.19523 will be 4 when the value after the decimal point is rounded up. This indicates that four
differential tables are required to create an XP Snapshot pair.
Assume that no other program products use differential tables in a storage system where the number
of differential tables is 26,176. The number of differential tables that XP Snapshot can use is
26,176. To create XP Snapshot pairs that require four differential tables for each pair, the calculation
will be as follows.
36 Preparing for XP Snapshot Operations