How aggregate overcommitment works

Using aggregate overcommitment, the storage administrator can appear to provide more storage than is actually available from a given aggregate, as long as not all of that storage is currently being used. Aggregate commitment is also called thin provisioning.

To use aggregate overcommitment, you create FlexVol volumes with a space guarantee of none or file. With a space guarantee of none or file, the volume size is not limited by the aggregate size. In fact, each volume could, if required, be larger than the containing aggregate. The storage provided by the aggregate is used up only as LUNs are created or data is appended to files in the volumes.

Note: LUNs in this context refer to the LUNs that Data ONTAP serves to clients, not to the array LUNs used for storage on a storage array.

Note: The aggregate must provide enough free space to hold the metadata for each FlexVol volume it contains. The space required for a FlexVol volume's metadata is approximately 0.5 percent of the volume's nominal size.

This could be useful if you are asked to provide greater amounts of storage than you know will be used immediately. Alternatively, if you have several volumes that sometimes need to grow temporarily, the volumes can dynamically share the available space with each other.

When the aggregate is overcommitted, it is possible for these types of writes to fail due to lack of available space:

•	Writes to any volume with space guarantee of none

•	Writes to any file that does not have space reservations enabled and that is in a volume with space guarantee of file

Therefore, if you have overcommitted your aggregate, you must monitor your available space and add storage to the aggregate as needed to avoid write errors due to insufficient space.

For more information about aggregate overcommitment, see Technical Reports 3563 & 3483.