Skip to main content

Enable ONTAP space allocation for SAN protocols

Contributors netapp-aherbin netapp-dbagwell
PDFs

ONTAP space allocation helps you to prevent your LUNs or NVMe namespaces from being taken offline if they run out of space and enables your SAN hosts to reclaim space.

ONTAP support for space allocation is based upon your SAN protocol and your version of ONTAP. Beginning with ONTAP 9.16.1, space allocation is enabled by default for iSCSI, FC, and NVMe protocols for all newly created LUNs and namespaces.

ONTAP version Protocols Space allocation is…​

9.16.1 or later

  • iSCSI

  • FC

  • NVMe

Enabled by default for newly created LUNs and namespaces

9.15.1

  • iSCSI

  • FC

Enabled by default for newly created LUNs

NVMe

Not supported

9.14.1 and earlier

  • iSCSI

  • FC

Disabled by default for newly created LUNs

NVMe

Not supported

When space allocation is enabled:

  • If a LUN or namespace runs out of space, ONTAP communicates to the host that no free space is available for write operations. As a result, the LUN or namespace remains online and read operations continue to be serviced. Depending upon the host configuration, either the host retries write operations until it succeeds or the host filesystem is placed offline. Write operations resume when additional free space becomes available to the LUN or namespace.

    If space allocation is not enabled, when a LUN or namespace runs out of space, all I/O operations fail and the LUN or namespace is taken offline; the space issue must be resolved to resume normal operations. Rescanning LUN devices might also be required on the host to restore paths and devices to an operational state.

  • A host can perform SCSI or NVME UNMAP (sometimes called TRIM) operations. UNMAP operations allow a host to identify blocks of data that are no longer required because they no longer contain valid data. Identification normally happens after file deletion. The storage system can then deallocate those data blocks so that the space can be consumed elsewhere. This deallocation greatly improves overall storage efficiency, especially with filesystems that have data high turnover.

Before you begin

Enabling space allocation requires a host configuration that can correctly handle space allocation errors when a write cannot be completed. Leveraging SCSI or NVME UNMAP requires a configuration that can use logical block provisioning as defined in the SCSI SBC-3 standard.

The following hosts currently support thin provisioning when you enable space allocation:

  • Citrix XenServer 6.5 and later

  • VMware ESXi 5.0 and later

  • Oracle Linux 6.2 UEK kernel and later

  • Red Hat Enterprise Linux 6.2 and later

  • SUSE Linux Enterprise Server 11 and later

  • Solaris 11.1 and later

  • Windows

About this task

When you upgrade your cluster to ONTAP 9.15.1 or later, the space allocation setting for all LUNs created prior to the software upgrade remains the same after the upgrade, regardless of host type. For example, if a LUN was created in ONTAP 9.13.1 for a VMware host with space allocation disabled, space allocation on that LUN remains disabled after upgrading to ONTAP 9.15.1.

Steps

  1. Enable space allocation:

    lun modify -vserver <vserver_name> -volume <volume_name> -lun <lun_name> -space-allocation enabled

  2. Verify that space allocation is enabled:

    lun show -vserver <vserver_name> -volume <volume_name> -lun <lun_name> -fields space-allocation

  3. Verify that space allocation is enabled on the host OS.

    Note Some host configurations, including some versions of VMware ESXi, can automatically recognize the setting change and do not require user intervention. Other configurations might require a device rescan. Some filesystems and volume managers might require additional specific settings to enable space reclamation using SCSI UNMAP. Remounting of filesystems or a full OS reboot might be required. Consult the documentation for your specific host for guidance.

Host configuration for VMware ESXi 8.x and later NVMe hosts

If you have a VMware host running ESXi 8.x or later with the NVMe protocol, after you have enabled space allocation in ONTAP, you should perform the following steps on the hosts.

Steps

  1. On your ESXi host, verify that the DSM is disabled:

    esxcfg-advcfg -g /SCSi/NVmeUseDsmTp4040

    The expected value is 0.

  2. Enable the NVMe DSM:

    esxcfg-advcfg -s 1 /Scsi/NvmeUseDsmTp4040

  3. Verify that the DSM is enabled:

    esxcfg-advcfg -g /SCSi/NVmeUseDsmTp4040

    The expected value is 1.

Related links

Learn more about NVMe-oF host configuration for ESXi 8.x with ONTAP.