HP StorageWorks XP Remote Web Console Software - Manual (Page 23)

Snapshot XP user guide 23
Therefore, you can create a maximum of 6,826 Snapshot XP pairs.
Using shared memory capacity for the virtual volume management area
The V-VOL management area consists of the following:
• Pool association information
• Pool management block
• Other management information (shared memory is fixed at 8 MB)
To calculate the maximum number of Snapshot XP pairs you can create for an array, use the following
formula:
{ ( (X) - 8) ÷ ( ( α ) + ( β ) ) }
where:
(X) is shared memory capacity for the whole V-VOL management area (in megabytes)
(
α ) is shared memory capacity for the pool association information (in bytes)
( β ) is shared memory capacity for the pool management block (in bytes)
You should round off the calculation result to the nearest whole number.
The following is the calculation of the shared memory capacity of the pool association information (
α ) and
the shared memory capacity of the pool management block ( β ):
NOTE:
•
You can check the shared memory capacity of the pool association information and pool management
block in the Snapshot XP pane ( Figure 18 ).
• If all the V-VOL management area’s capacity is used by the pool management block and pool
association information, the capacity ratio of the pool management block and pool association
information in the V-VOL management area is fixed. For more information, see ” Creating virtual
volume management areas ” on page 16.
( α ) = ↑ ( (Y) ч 256) ↑ Ч 4 Ч 1.08
( β ) = ↑ ( (Z) ч 256) ↑ Ч 32
where:
(Y) is pool-VOL’s capacity (in kilobytes)
(Z) is pool usage (in kilobytes)
You must round up the value enclosed by two upward arrows ( ↑ ) to the nearest whole number.
NOTE: When calculating a pool’s usage:
• When copying snapshot data to the pool, Snapshot XP always uses a multiple of 256 KB of the pool’s
capacity, even when the snapshot data is smaller than 256 KB.
• When snapshot data is stored consecutively, snapshot data capacity and pool usage is almost the
same. However, pool usage might be larger than snapshot data capacity when snapshot data is
stored randomly.
When shared memory capacity for the whole V-VOL management area ((X) in the previous formula) is
2,047 MB, pool-VOL capacity ((Y) in the previous formula) is 16 GB (16,777,216 KB), and pool usage
((Z) in the previous formula) is 1 GB (1,048,576 KB), the number of Snapshot XP pairs you can create for
an array is calculated as:
(2047 - 8) ч [ { (16,777,216 ч 256) Ч 4 Ч 1.08} + { (1,048,576 ч 256) Ч 32} ] = 5162.02933...
According to these results, you can create a maximum of 5,162 Snapshot XP pairs.