Learn About Estimating FlashCopy Repository Logical Drive Capacity

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During the creation of a FlashCopy logical drive , a physical logical drive called the FlashCopy repository logical drive is created to store FlashCopy data and copy-on-write data.

The default setting for the FlashCopy repository logical drive capacity is 20% of the base logical drive's capacity and, in general, this capacity should be sufficient. However, use the following information to help determine the appropriate capacity of the FlashCopy repository logical drive:

Calculating Expected Overhead

Use the following formula to calculate the amount of management overhead required to store FlashCopy data on the FlashCopy repository logical drive. This formula should be used merely as a guide, and FlashCopy repository logical drive capacity should be re-estimated periodically.

Note: Conversion from bytes, to kilobytes, and then to megabytes is required for this formula.

The formula to calculate the amount of management overhead required is:

192 KB + (X/2000)

where X is the capacity of the base logical drive in bytes.

Example:

For a 5 GB base logical drive, where 30% of the data blocks are expected to change on the base logical drive, the estimated FlashCopy repository logical drive capacity can be calculated as follows:

1

Convert the base logical drive's capacity to bytes.

Result: When converted, 5 GB equals 5,368,709,120 bytes.

2

Divide the base logical drive's capacity (in bytes) by 2000.

Result: When divided, the result is 2,684,354.56 bytes

3

Convert the result from step 2 (in bytes) to kilobytes (KB).

Result: When converted, the result is 2621.44 KB.

4

Add 192 KB to the results from step 3.

192 KB + 2621.44 KB = 2813.44 KB

5

Convert the result from step 4 to megabytes (MB).

Result: When converted, the amount of management overhead required is calculated to be 2.75 MB (or 0.002686 GB).

6

In this example, 30% of the data blocks on the base logical drive are expected to change. To accurately calculate the FlashCopy repository logical drive capacity, sufficient space would need to be allowed for the copy-on-write data as well as the management overhead (calculated in step 5).

To calculate the copy-on-write space required, calculate the percentage of the base logical drive expected change:

30% of 5 GB = 1.5 GB

The final estimated FlashCopy repository logical drive capacity for this example is:

1.5 GB + 0.002686 GB = 1.502686 GB

7

In the Create FlashCopy Logical Drive Wizard: Specify Repository Capacity screen, use the percentage (%) of base logical drive spinner box to set the estimated FlashCopy repository logical drive capacity.

Note: The spinner box sets the FlashCopy repository logical drive capacity as a percentage of the base logical drive. Using the spinner box, increase or decrease the percentage until the FlashCopy Repository Logical Drive Capacity value matches the estimated capacity calculated in step 6. (Some rounding up may be required.)

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Learn About Estimating FlashCopy Repository Logical Drive Life Expectancy

Increasing the Capacity of a FlashCopy Repository Logical Drive

Creating a FlashCopy Logical Drive Using the FlashCopy Logical Drive Wizard