Use the fractional reserve method to estimate the size of volumes on which you need to create Snapshot copies. Fractional reserve is not necessary for volumes when you do not need Snapshot copies.
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.
If you know your database needs 40 GB of space, you must create a 40 GB space-reserved LUN.
This value depends on how often you overwrite data. It is expressed as GB per day.
Space required for Snapshot copies = ROC x number of days of Snapshot copies.
You need a 40 GB LUN, and you estimate that your data changes at a rate of about 10 percent each day, which in this case is 4 GB per day. You want to take one Snapshot copy each day and want to keep three weeks of Snapshot copies, for a total of 21 days of Snapshot copies. The amount of space you need for Snapshot copies is 21 × 4 GB, or 84 GB.
Space required for overwrites = ROC × number of days you want to keep Snapshot copies before deleting
You have a 40 GB LUN and your data changes at a rate of 4 GB each day. You want to retain daily snapshots for 3 days. You need 4 GB × 3, or 12 GB of additional space in the volume reserved for overwrites to the LUN.
Volume size = Total data size + space required for Snapshot copies
You have a 40 GB LUN and 12 GB of Snapshot copies. The volume size needs to be 52 GB.
Fractional reserve = space required for overwrites ÷ total data size.
You have a 40 GB LUN. You require 12 GB of changes held in Snapshot copies. 12 GB is 30 percent of the total LUN size. Therefore the smallest volume size is 52 GB and you must set the Fractional Reserve value to 30 to enable Snapshot creation to succeed.
You would calculate the size of your volume as follows:
Volume size = Total data size + Space required for Snapshot copies.
The size of the volume in this example is 200 GB (100 GB + 100 GB).