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Considerations for setting fractional reserve

Fractional reserve, also called LUN overwrite reserve, enables you to control the size of the overwrite reserve for reserved LUNs and files in a volume. By using this volume attribute correctly you can maximize your storage utilization, but you should understand how it interacts with other technologies.

The fractional reserve setting is expressed as a percentage; the only valid values are 0 and 100 percent. You use the vol options command to set fractional reserve.

Setting fractional reserve to 0 increases your storage utilization. However, an application accessing data residing in the volume could experience a data outage if the volume is out of free space, even with the volume guarantee set to volume, when any of the following technologies and Data ONTAP features are in use:

If you are using one or more of these technologies with no fractional reserve and you need to prevent errors due to running out of space, you must use all of the following configuration settings for the volume:

In addition, you can optionally use the volume autogrow capability to decrease the likelihood of volume Snapshot copies needing to be deleted automatically. If you enable the autogrow capability, you must monitor the free space in the associated aggregate. If the aggregate becomes full enough that the volume is prevented from growing, more Snapshot copies will probably be deleted as the free space in the volume is depleted.

If you do not want to monitor aggregate free space or have volume Snapshot copies automatically deleted, you can set the volume's fractional reserve setting to 100. This requires more free space up front, but guarantees that data modification operations will succeed even when the technologies listed above are in use.

The default value and allowed values for the fractional reserve setting depend on the guarantee of the volume:

Volume guarantee Default fractional reserve Allowed values
Volume 100 0, 100
None 0 0, 100
File 100 100
For more information about using fractional reserve, see the following Technical Reports:
  • TR-3965: Thin Provisioning Deployment and Implementation Guide
  • TR-3483: Thin Provisioning in a NetApp SAN or IP SA Enterprise Environment