HP StorageWorks XP Remote Web Console Software - Manual (Page 30)

Note that you must round up the value enclosed by the two upward arrows (↑) to the nearest whole
number.
To calculate P (total capacity of all pools), use the formula below:
P = Y – (4,116,000 + 84,000 × Number of pool-VOLs)
Notes on calculating the usage of a pool:
• When copying snapshot data to the pool, XP Snapshot always consumes a multiple of 256
kB of the capacity of the pool. 256 kB will be consumed even if the snapshot data is smaller
than 256 kB.
• When snapshot data is stored consecutively, the capacity of the snapshot data and the usage of
the pool will be almost the same. However, the usage of the pool will be larger than the capacity
of the snapshot data in some cases, when snapshot data is stored randomly.
When the shared memory capacity for the whole V-VOL management area (S in the formula above) is
1,024 MB, the capacity of the pool-VOL (Y in the formula above) is 4 TB (4,294,967,296 kB), the number
of the pool-VOLs is 100, and the capacity of one P-VOL is 10 GB, the number of XP Snapshot pairs that
can be created for a subsystem can be calculated as follows.
Example
P = 4,294,967,296 － (4,116,000 + 84,000 × 100)
= 4,282,451,296
S2 = ↑[{( 4,282,451,296 ч 256) Ч 32} ч 8,736↑ Ч 8,736 +
↑[{( 4,282,451,296 ч 256) ч 272} ч 2,048 ]↑ Ч 8,736
=↑61,275.917...↑ Ч 8,736 + ↑30.029...↑ Ч 8,736
= 61,276 Ч 8,736 + 31 Ч 8,736
= 535,577,952
S1 = 1,073,741,824 – 535,577,952 – (10 Ч 1,024 Ч 1,024)
=527,678,112
Total capacity of P-VOLs (kB) = ↑( 527,678,112 ч 8,736)↑ Ч 2,048 Ч 256
= ↑60,402.714...↑ Ч 2,048 Ч 256
= 60,403 Ч 2,048 Ч 256
= 31,668,568,064
Since the total capacity of P-VOLs is 30,926,336 in megabytes, maximum number of XP Snapshot
pairs will be as follows:
Maximum number of XP Snapshot pairs =
30,926,336 ч [↑{ (10 Ч 1,024) ч 512 }↑ Ч 512]
= 30,926,336 ч 10,240
= 3,020
According to the result above, the maximum number of XP Snapshot pairs that can be created is 3,020.
Calculating the Number of XP Snapshot Pairs by Using the Number of Pair Tables
Pair tables are used by Compatible FlashCopy Version 2 and XP Snapshot pairs in order to store control
information which is needed to manage the pairs. To create a Compatible FlashCopy Version 2 pair or
an XP Snapshot pair, you need one pair table for each pair.
Up to 1,048,575 pair tables are available in one subsystem.
Among storage system program products, only Compatible FlashCopy Version 2 and XP Snapshot use
pair tables. Therefore, if you use Compatible FlashCopy Version 2, the number of available pair tables
for XP Snapshot pairs can be calculated by subtracting the number of differential tables used by the
Compatible FlashCopy Version 2 pairs from the number of pair tables for the whole subsystem.
For information about how to check the number of pair tables used by Compatible FlashCopy Version
2 pairs (that is, the number of already created Compatible FlashCopy Version 2 pairs), see the HP
StorageWorks XP24000 for FlashCopy Mirroring Software User’s Guide.
For example, when the relationship expansion function is effective and the number of pair tables used by
Compatible FlashCopy Version 2 is 30,000, the number of XP Snapshot pairs that can be created for a
subsystem can be calculated as follows.
Example
1,048,575 – 30,000 = 1,018,575
30 Preparing for XP Snapshot Operations