The space_alloc 7-mode LUN option (space-allocation in clustered Data ONTAP) allows the storage system to notify the host when it is about to run out of space in a thin provisioned environment. When the option is enabled, the storage system notifies the host through special SCSI responses when the space is out or nearly out. This allows a gentle reaction of the host to an out-of-space condition. In addition, once a fully out of space condition exists for the volume containing the LUN, the LUN is left online with this option enabled. The behavior with the space_alloc option disabled is to take the LUN offline.
The space allocation functionality, also known as SCSI thin provisioning, uses the Logical Block Provisioning feature as defined in the SCSI SBC-3 standard. Only hosts that support this standard can use the space allocation functionality in Data ONTAP.
When you enable the space allocation functionality, you turn on the following thin provisioning features for standard SCSI features:
- Unmapping and reporting space usage for space reclamation
- Reporting resource exhaustion errors
- Reporting low space warnings for thin provisioning thresholds
Note: Data ONTAP 8.1 and higher in the 8.1 release family will support most VAAI and some Logical Block Provisioning features such as SCSI unmap, even if space_alloc is disabled. Starting in 8.2 these commands require space_alloc be enabled. LUNs with the option disabled in 8.2 will report as Full Provisioned.