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.
You set the guarantee when you create a new volume; you can also change the guarantee for an existing volume by using the vol options command with the guarantee option. You can view the guarantee type and status by using the vol status command.
Volume guarantee types can be volume (the default type), file, or none.
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.
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: Thin Provisioning Deployment and Implementation Guide. You should also be aware that volume guarantees of type file will not be supported in a future release of Data ONTAP.
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. It is possible for the total size of all volumes associated with an aggregate to exceed the amount of free space for the aggregate.
Writes to LUNs or files (including space-reserved LUNs and 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: Thin Provisioning Deployment and Implementation Guide for information about how doing so can affect storage availability.
When space in the aggregate is allocated for a volume or file guarantee for an existing volume, that space is no longer considered free in the aggregate, even if the volume is not yet using the space. 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.
