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Determining the volume size and fractional reserve setting when you need Snapshot copies

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.

About this task

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.

Steps

  1. Add up all of the space-reserved LUNs.

    Example

    If you know your database needs 40 GB of space, you must create a 40 GB space-reserved LUN.

  2. 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.

  3. Calculate the amount of space you need for Snapshot copies by multiplying your ROC by the number of days of Snapshot copies you intend to keep.

    Space required for Snapshot copies = ROC x number of days of Snapshot copies.

    Example

    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.

  4. Determine how much space you need for overwrites by multiplying your ROC by number of days you want to keep Snapshot copies before deleting.

    Space required for overwrites = ROC × number of days you want to keep Snapshot copies before deleting

    Example

    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.

  5. Calculate the required volume size by adding together the total data size and the space required for Snapshot copies.

    Volume size = Total data size + space required for Snapshot copies

    Example

    You have a 40 GB LUN and 12 GB of Snapshot copies. The volume size needs to be 52 GB.

  6. Calculate the minimum fractional reserve value for this volume by dividing the size of space required for Snapshots by the total size of the space-reserved LUNs in the volume. Setting this value will enable Data ONTAP to create Snapshots only when the minimum amount of space is available in the volume.

    Fractional reserve = space required for overwrites ÷ total data size.

    Example

    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.

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 10 days of Snapshot copies. You need 100 GB of space for Snapshot copies (10 GB ROC × 10 Snapshot copies).

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).