The required volume size for a volume when you need Snapshot copies depends on several factors, including how much your data changes, how long you need to keep Snapshot copies, and how much data the volume is required to hold.
- Calculate the Rate of Change (ROC) of your data per day.
This value depends on how often you overwrite data. It is expressed as GB per day.
- Calculate the amount of space you need for Snapshot copies by multiplying your ROC by the number of Snapshot copies you intend to keep.
Space required for Snapshot copies = ROC × number of Snapshot copies
Example
You need a 20-GB LUN, and you estimate that your data changes at a rate of about 10 percent, or 2 GB each day. You want to take one Snapshot copy each day and want to keep three weeks’ worth of Snapshot copies, for a total of 21 Snapshot copies. The amount of space you need for Snapshot copies is 21 × 2 GB, or 42 GB.
- Determine how much space you need for overwrites by multiplying your ROC by the amount of time, in days, you need to update your system when space is scarce.
Space required for overwrites = ROC × time needed to update system
Example
You have a 20-GB LUN and your data changes at a rate of 2 GB each day. You want to ensure that write operations to the LUNs do not fail for three days after you take the last Snapshot copy. You need 2 GB × 3, or 6 GB of space reserved for overwrites to the LUNs.
- Calculate the required volume size by adding together the total data size, the space required for Snapshot copies, and the space required for overwrites.
Volume size = Total data size + space required for Snapshot copies + space required for overwrites
- Calculate the fractional reserve value you must use for this volume by dividing the size of the space required for overwrites by the total size of the space reserved files or LUNs in the volume.
Fractional reserve = space required for overwrites ÷ total data size
Example
You have a 20-GB LUN. You require 6 GB for overwrites. Thirty percent of the total LUN size is 6 GB, so you must set your fractional reserve to 30.
Volume size calculation example
The following example shows how to calculate the size of a volume based on the following information:
- You need to create two 50-GB LUNs.
The total LUN size is 100 GB.
- Your data changes at a rate of 10 percent of the total LUN size each day.
Your ROC is 10 GB per day (10 percent of 100 GB).
- You take one Snapshot copy each day and you want to keep the Snapshot copies for 10 days.
You need 100 GB of space for Snapshot copies (10 GB ROC × 10 Snapshot copies).
- You want to ensure that you can continue to write to the LUNs through the weekend, even after you take the last Snapshot copy and you have no more free space.
You need 20 GB of space reserved for overwrites (10 GB per day ROC × 2 days). This means you must set fractional reserve to 20 percent (20 GB = 20 percent of 100 GB).
You would calculate the size of your volume as follows:
Volume size = Total data size + Space required for Snapshot copies + Space for overwrites
The size of the volume in this example is 220 GB (100 GB + 100 GB + 20 GB).
Note: This volume size requires that you set the fractional reserve setting for the new volume to 20. If you leave fractional reserve at 100 to ensure that writes could never fail, then you need to increase the volume size by 80 GB to accommodate the extra space needed for overwrites (100 GB rather than 20 GB).
