Learn About Estimating the FlashCopy Repository Logical Drive Capacity

During the creation of a , a physical called the is created to store data and data.

The default setting for the flashcopy repository logical drive capacity is 20 percent of the capacity of the base, and usually this capacity is sufficient. However, use this information to help determine the appropriate capacity of the flashcopy repository logical drive:

Calculating Expected Overhead


Important:

Using a utility to perform a defragment operation on a base logical drive with an associated flashcopy repository logical drive causes a copy-on-write of every data block in the base logical drive. This situation can cause the flashcopy repository logical drive to fill before the defragment operation is completed. The flashcopy logical drive fails or writes fails to the base logical drive, depending on the flashcopy logical drive full condition that was originally set for the flashcopy logical drive. To prevent any failures during a base logical drive defragment operation, make sure that the capacity of the flashcopy repository logical drives is set to 105 percent of the size of the base logical drive before starting the defragment utility. This minimum size is needed to support a copy-on-write of every data block in the base logical drive.

Use this formula to calculate the amount of management overhead that is required to store flashcopy data on the flashcopy repository logical drive. This formula is only a guide. You should periodically re-estimate the capacity of flashcopy repository logical drives.


Note:

To use this formula, first convert bytes to kilobytes, and convert kilobytes to megabytes.

192 KB + (X/2000)

In this formula, X is the capacity of the base logical drive in bytes.

Example

For a 5-GB base logical drive, where 30 percent 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.
    The 5 GB equals 5,368,709,120 bytes.
  2. Divide the base logical drive's capacity by 2000.
    The result is 2,684,354.56 bytes
  3. Convert the result from step 2 to kilobytes.
    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 into megabytes.
    The result is the management overhead required, 2.75 MB (or 0.002686 GB).
  6. In this example, 30 percent of the data blocks on the base logical drive is expected to change. To calculate the capacity of the flashcopy repository logical drive, allow enough space for the copy-on-write data as well as the management overhead (calculated in step 5).
    1. To calculate the copy-on-write space, calculate the percentage of the base logical drive expected change:
      30 percent of 5 GB = 1.5 GB
    2. The final estimated flashcopy repository logical drive capacity for this example is as follows:
      1.5 GB + 0.002686 GB = 1.502686 GB
  7. In the Create FlashCopy Logical Drive Wizard - Specify Repository Capacity dialog, use the percentage (percent) 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. Use the spinner box to increase or decrease the percentage until the FlashCopy Repository Logical Drive Capacity value matches the estimated capacity calculated in step 6. You might need to round up some of the values.

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