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What does the LUN option space_alloc do?

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Applies to

  • FlexPod
  • ONTAP 9

Answer

  • 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 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 Thick Provisioned. 

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