How volume guarantees work with FlexVol volumes

Volume guarantees (sometimes called space guarantees) determine how space for a volume is allocated from its containing aggregate--whether the space is preallocated for the entire volume or for only the reserved files or LUNs in the volume, or whether space for user data is not preallocated.

The guarantee is an attribute of the volume.

Volume guarantee types can be volume (the default type), file, or none

•	A guarantee type of volume allocates space in the aggregate for the volume when you create the volume, regardless of whether that space is used for data yet.

This approach to space management is called thick provisioning. The allocated space cannot be provided to or allocated for any other volume in that aggregate.

When you use thick provisioning, all of the space specified for the volume is allocated from the aggregate at volume creation time. The volume cannot run out of space before the amount of data it contains (including Snapshot copies) reaches the size of the volume. However, if your volumes are not very full, this comes at the cost of reduced storage utilization.

•	A guarantee type of file allocates space for the volume in its containing aggregate so that any reserved LUN or file in the volume can be completely rewritten, even if its blocks are being retained on disk by a Snapshot copy.

However, writes to any file in the volume that is not reserved could run out of space.

Before configuring your volumes with a guarantee of file, you should refer to Technical Report 3965.

•	A guarantee of none allocates space from the aggregate only as it is needed by the volume.

This approach to space management is called thin provisioning. The amount of space consumed by volumes with this guarantee type grows as data is added instead of being determined by the initial volume size, which might leave space unused if the volume data does not grow to that size. The maximum size of a volume with a guarantee of none is not limited by the amount of free space in its aggregate.

Writes to LUNs or files (including space-reserved files) contained by that volume could fail if the containing aggregate does not have enough available space to accommodate the write. If you configure your volumes with a volume guarantee of none, you should refer to Technical Report 3965 for information about how doing so can affect storage availability.

When space in the aggregate is allocated for the guarantee for an existing volume, that space is no longer considered free in the aggregate. Operations that consume free space in the aggregate, such as creation of aggregate Snapshot copies or creation of new volumes in the containing aggregate, can occur only if there is enough available free space in that aggregate; these operations are prevented from using space already allocated to another volume.

When the free space in an aggregate is exhausted, only writes to volumes or files in that aggregate with preallocated space are guaranteed to succeed.

Guarantees are honored only for online volumes. If you take a volume offline, any allocated but unused space for that volume becomes available for other volumes in that aggregate. When you try to bring that volume back online, if there is insufficient available space in the aggregate to fulfill its guarantee, it will remain offline. You must force the volume online, at which point the volume's guarantee will be disabled.