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HP P9000 Snapshot User Guide (AV400-96394, October 2011)

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Table 2 Shared memory and number of differential tables
number of differential tablesShared Memory
26,176Base (without shared memory)
104,768Extension 1
209,600Extension 2
Besides Snapshot, differential tables are used by these other program products:
• Business Copy
• Business Copy Z
• Compatible FlashCopy
• Auto LUN
Calculate the number of differential tables these products use and subtract the amount from the
total number of differential tables. The remainder is available for Snapshot pairs. See the products'
documentation for instructions.
Using differential tables, you will find the maximum number of Snapshot pairs allowed on your
system using two calculations:
1. Number of differential tables required for one Snapshot pair
2. Maximum number of Snapshot pairs
NOTE: "ceil" indicates that the result within the parenthesis () that follows must be rounded up.
1. Calculate the number of differential tables required for one Snapshot pair (O) using the following
formula:
ceil((X ÷ 256) ÷ Z)
where:
X = capacity of the volume in kilobytes
Z = number of slots that a differential table can manage (639 × 32)
Round up to the nearest whole number.
For example:
(16,777,216 kB ÷ 256) ÷ (639 x 32)
= 3.19523, which rounds up to 4.
Therefore, four differential tables are required to create one Snapshot pair.
2. Calculate the maximum number of Snapshot pairs using the following formula:
T ÷ D = M
where:
T = total number of differential tables
D = number of differential tables required for one pair
M = maximum number of Snapshot pairs
For example: (In this example, it is assumed that no other program product is using differential
tables, and that the number of differential tables is 57,600.)
576,00 ÷ 4 = 14,400
In this example, the maximum number allowed for Snapshot based on differential tables is 14,400
pairs.
One Snapshot pair consists of a P-VOL and up to 64 V-VOLs. Generations of Snapshots do not
apply when performing differential table calculations.
14 Planning and design