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.

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.

Note: The aggregate must be 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% of the volume's logical 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.

Example of an overcommitted aggregate

The following example shows a 1-TB aggregate with two FlexVol volumes: one with a size of 600 GB and the other with a size of 500 GB. The space guarantee for each volume is set to file. 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 files or LUNs in the FlexVol volumes, but write operations to these files or LUNs might fail when the aggregate runs out of free space.

This graphic shows a 1-TB aggregate that contains two volumes, one of size 500 MB and one of size 600 MB. The volume fragments shown are, from bottom to top: 200GB LUN; 200 GB intended reserve for overwrites; 100 GB unprotected space for other data; 200 GB LUN; 200 GB intended reserve for overwrites; 200 GB unprotected space for other data. Half of the top fragment extends above the top of the aggregate to show that the aggregate is overcommitted.

Considerations for bringing a volume online in an overcommited aggregate
When you take a FlexVol volume offline, it releases its allocation of storage space in its containing aggregate. Other volumes in that aggregate might start using that space while the volume is offline. When this happens, you cannot bring the volume back online as you normally would.