Aggregate overcommitment provides flexibility to the storage provider. Using aggregate overcommitment, you can appear to provide more storage than is actually available from a given aggregate. 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.
To use aggregate overcommitment, you create flexible 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.
Of course, 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.
See the Data ONTAP Storage Management Guide for more information about space guarantees and other space management issues.