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:
A flashcopy repository logical drive must be no smaller than 8 MB.
The amount of write activity to the after the flashcopy logical drive has been created dictates how large the flashcopy repository logical drive must be. As
the amount of write activity to the base logical drive increases, the number of original data blocks to copy from the base
logical drive to the flashcopy repository logical drive also increases.
The estimated life expectancy of the flashcopy logical drive helps determine the capacity of the flashcopy repository logical
drive. If the flashcopy logical drive is created and stays enabled for a long time, the flashcopy repository logical drive
risks reaching its maximum capacity. For more information, refer to the Learn About Estimating the FlashCopy Repository Logical
Drive Life Expectancy online help topic.
The management overhead that the flashcopy repository logical drive needs to store flashcopy logical drive data adds to determining
the capacity of the flashcopy repository logical drive. The required management overhead is small. You can use the formula
that follows to calculate overhead.
There might not be a one-to-one correlation between the number of data blocks that change on the base logical drive and the
amount of copy-on-write data stored on the flashcopy repository logical drive. For performance reasons, and depending on where
the data blocks are that must be copied, the might copy a full set of 32 blocks, even if only one set of blocks has changed. Keep this guideline in mind when determining
the percentage of the capacity of the base logical drive to copy to 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:
Convert the base logical drive's capacity to bytes.
The result is the management overhead required, 2.75 MB (or 0.002686 GB).
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).
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
The final estimated flashcopy repository logical drive capacity for this example is as follows:
1.5 GB + 0.002686 GB = 1.502686 GB
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.