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How host operating systems can automatically reclaim space and keep LUNs online

Starting with Data ONTAP 8.2, you can use the space-allocation option to reclaim space and notify the host when a thinly provisioned LUN cannot accept writes. The space allocation option enables the Logical Block Provisioning feature as defined in the SCSI SBC-3 standard.

When you enable the space-allocation on a thinly provisioned LUN, the following two SCSI features are enabled:
  • Reclaims space automatically when your host deletes data. When a host that does not support the space allocation functionality deletes data on a LUN, the storage system is unaware of the deletion, which results in poor space efficiency. If the host supports this functionality, Data ONTAP reclaims this space automatically.
    Hosts that support automatic space reclamation. The following hosts currently support automatic space reclamation when you enable space allocation:
    • VMware ESX 5.0 and later
    • Red Hat Enterprise Linux 6.2 and later
    • Microsoft Windows 2012

    See the host utilities documentation for more information about which hosts support automatic space reclamation.

    Note: The space reclamation process issues one or more SCSI UNMAP commands to free blocks on the storage system after identifying the blocks that can be freed on the host file system.
  • Notifies the host when the LUN cannot accept writes due to lack of space on the volume. On hosts that do not support the space allocation functionality, when the volume containing LUN runs out of space and cannot automatically grow, Data ONTAP takes the LUN offline. Free space must be available on the volume before you can bring the LUN back online.

    However, when you enable the space-allocation option, Data ONTAP notifies the host when the volume containing the LUN is running out of space and cannot grow. If the LUN cannot accept writes due to the lack of space on the volume, the LUN stays online. The host cannot write to the LUN, but the host can still read the LUN.

    You must add more space to your volume before the LUN can accept writes. You can add more space on your volume in the following ways:
    • Manually add free space to the volume
    • Enable volume autosize command
    • Enable Snapshot autodelete
    • Enable FlexClone autodelete
    Hosts that support out-of-space notifications. The following hosts currently support out-of-space notifications that a LUN cannot accept writes when you enable space allocation:
    • VMware ESX 5.0 and later
    • Red Hat Enterprise Linux 6.2 and later
    • Microsoft Windows 2012

See the host utilities documentation for more information about which hosts support out-of-space notifications.

For more information about the snap autodelete command and the volume autosize command, see the Clustered Data ONTAP Logical Storage Management Guide.