HP StorageWorks 2000 Modular Smart Array Reference Guide (481599-003, August 2008)

106 HP StorageWorks 2000 Family Modular Smart Array reference guide • August 2008
■ How much modified (write) data will the snapshots have? Of the snapshots
that will be mounted as read-write and actually written to, factor in the
average amount of data that will be modified.
■ Add 750 Mbyte of reserve space for internal use.
■ Add a recommended 25% safety margin to the snap pool in case actual capacity
use exceeds the estimate.
You can estimate snap-pool size as shown in following example. Assume you want
to create a snap-pool for two 10-Gbyte master volumes, and will use these values:
■ Master volume size = 10,000 Mbyte
■ Average percent of change = 5% (0.05)
■ Number of snapshots retained = 4
■ Number of modified snapshots = 4
■ Average write data = 1,000 Mbyte
■ Snap-pool reserve space = 750 Mbyte
■ Safety margin = 25%
The snap-pool size calculation is:
Therefore, the snap pool should be approximately 16,000 Mbyte (16 Gbyte).
Note – If you cannot estimate the size values, you can specify a reasonable initial
size and use the Auto Expand policy to expand the snap pool when it reaches a
capacity threshold. See “Setting Snap Pool Policies and Thresholds” on page 110.
Note – For an explanation of sizes represented by various units, see “Size
Representations in SMU” on page 27 for more information.
MV1 space (10,000 x 0.05 x 4) + (4 x 1000): 6000.00 Mbyte
+ MV2 space (10,000 x 0.05 x 4) + (4 x 1000): 6000.00 Mbyte
+ Snap-pool reserve space : 750.00 Mbyte
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= Master volume space required : 12750.00 Mbyte
x Safety margin : 1.25
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= Total snap-pool space required : 15937.50 Mbyte