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Determining the volume size when using Snapshot autodelete

Before you create a volume for use with Snapshot autodelete, you should estimate how large it needs to be.

About this task

Snapshot autodelete is a volume-level option that allows you to automatically delete Snapshot copies when a pre-defined threshold called a "trigger" is met. You can set the trigger for autodelete when the volume is nearly full, when the snap reserve space is nearly full, or when the overwrite reserved space is consumed. Using Snapshot autodelete is recommended in most SAN configurations, but is particularly useful when:
  • You do not want your volumes to automatically grow, because automatic growth consumes space in the aggregate.
  • Ensuring availability of your LUNs is more important to you than maintaining old Snapshot copies.

Steps

  1. 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.
  2. 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 200 GB LUN, and you estimate that your data changes at a rate of about 10 percent each day, which in this case is 20 GB per 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 days of Snapshot copies. The amount of space you need for Snapshot copies is 21 ?? 20 GB, or 420 GB.
  3. Calculate the required volume size by adding together the total LUN size and the space required for Snapshot copies.
    The total LUN size = the size of all the LUNs in the volume.
    Example
    The following example shows how to calculate the size of a volume based on the following information:
    • You need to create two 200 GB LUNs. The total LUN size is 400 GB.
    • You take one Snapshot copy each day and you want to keep 10 days of Snapshot copies. This means you need 400 GB of space for Snapshot copies (40 GB ROC ?? 10 Snapshot copies).
    • You rate of change varies due to periodic increases. You do not want to add additional space to your volume to accommodate the variable rate. In this case, you can configure Snapshot autodelete with a volume space trigger to delete snapshots, so that space remains available for additional overwrites even when your rate of change increases more than usual.

    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 800 GB (400 GB + 400 GB).

    For more information about the Snapshot autodelete function, see the Data ONTAP Storage Management Guide for 7-Mode, and for more information about working with traditional and FlexVol volumes, see the Data ONTAP Storage Management Guide for 7-Mode.