HP XP RAID Manager Software - Manual (Page 13)

Figure 5 Basic Factors for Data Pool Sizing
In addition, when finalizing a data pool’s capacity, it is advisable to factor in the following:
•
A safety factor of 20-percent. Having a safety factor calculated in minimizes the chances of
over-filling the data pool. To factor in a safety factor of 20-percent, multiply the base data
pool size times 1.2.
•
Annual increase in data transactions. Your business data is likely to grow. Multiply the base
pool size times the percentage of expected annual growth.
•
Data is copied to a pool in units of 256kB. This is true even when the actual write-data is
smaller than 256kB. In such a case, 256kB of the pool capacity is consumed by the write.
Therefore, when planning pool size, you must factor in the unit size of 256kB.
Number of pairs
The number of Snapshot pairs you can create in your storage system is limited by available system
resources for copy operations. Confirm with your service provider that additional shared memory
for differential tables and the V-VOL management area are installed on your system.
:
If you need more Snapshots, an extension area is available. Contact your HP account representative
for more information.
The maximum number of pairs supported on a system is the lowest value of the following three
calculations:
•
Maximum number of pairs based on the number of differential tables
•
Maximum number of pairs based on the capacity of shared memory for the V-VOL management
area
•
Maximum number of pairs based on the number of pair tables
Calculating maximum pairs based on differential tables
Differential tables keep track of the updates that take place in pairs. The number of these tables a
pair requires is determined by the size the pair volumes.
This section helps you calculate the number of differential tables required per pair, then calculate
the maximum number of pairs your system supports, based on the number of differential tables on
the system.
The number of differential tables on a system is determined by the amount of shared memory
installed on the system. When Snapshot is used, the shared memory level is COW1 with Extension1
and Extension2. At these levels, the number of differential tables is 26,176, 104,768, or 209,600.
The following table shows the shared memory and the number of differential tables.
Number of pairs 13