How guarantees on FlexVol volumes affect fractional reserve

What guarantees are

Guarantees on a FlexVol volume ensure that write operations to a specified FlexVol volume or write operations to LUNs with space reservation on that file do not fail because of lack of available space in the containing aggregate. Guarantees determine how the aggregate preallocates space to the FlexVol volume. Guarantees are set at the volume level. There are three types of guarantees:

volume
A guarantee of volume ensures that the amount of space required by the FlexVol volume is always available from its aggregate. This is the default setting for FlexVol volumes. Fractional reserve is adjustable from the default of 100 percent only when a FlexVol volume has guarantees.
file
The aggregate guarantees that space is always available for overwrites to space-reserved LUNs. Fractional reserve is set to 100 percent and is not adjustable.
none
A FlexVol volume with a guarantee of none reserves no space, regardless of the space reservation settings for LUNs in that volume. Write operations to space-reserved LUNs in that volume might fail if its containing aggregate does not have enough available space.

Command for setting guarantees

You can specify guarantees when you create a FlexVol volume by using the -s option of the vol create command:

vol create f_vol_name [-l language_code] [-s {volume|file|none}] 
aggr_name size{k|m|g|t}

You can change the guarantee setting of the volume by using the vol options command:

vol options f_vol_name guarantee guarantee_value

f_vol_name is the name of the FlexVol volume whose space guarantee you want to change.

guarantee_value is the space guarantee you want to assign to this volume. The possible values are volume, file, and none.

For detailed information about setting guarantees, see the Data ONTAP Storage Management Guide.

Overcommitting an aggregate

You might want to overcommit an aggregate to enable flexible provisioning. For example, you might need to assign large volumes to specific users, but you know they will not use all their available space initially. When your users require additional space, you can increase the size of the aggregate on demand by assigning additional disks to the aggregate.

To overcommit an aggregate, you create FlexVol volumes with a guarantee of none or file, so that the volume size is not limited by the aggregate size. The total size of the FlexVol volumes you create might be larger than the containing aggregate.

The following example shows a 1-TB aggregate with two FlexVol volumes. The guarantee is set to file for each FlexVol volume. Each FlexVol volume contains a 200-GB LUN. The file guarantee ensures that there are 200 GB of intended reserve space in each FlexVol volume so that write operations to the space-reserved LUNs do not fail, regardless of the size of the FlexVol volumes that contain the LUNs.

Each FlexVol volume has space for other data. For example, you can create non-space-reserved LUNs in a FlexVol volume, but write operations to these LUNs or LUNs might fail when the aggregate runs out of free space.

For detailed information

For detailed information about using guarantees, see the Data ONTAP Storage Management Guide.