Root Device Hints

Bare-metal machines often have more than one block device, and in many cases a user will want to specify, which of them to use as the root device. Root device hints allow selecting one device or a group of devices to choose from. You can provide the hints via the spec.rootDeviceHints field on your BareMetalHost:

  # ...
    wwn: "0x55cd2e415652abcd"

Hint: root device hints in Metal3 are closely modeled on the Ironic’s root device hints, but there are important differences in available hints and the comparison operators they use.

Warning: the default root device depends on the hardware profile as explained below. Currently, /dev/sda path is used when no hints are specified. This value is not going to work for NVMe storage. Furthermore, Linux does not guarantee the block device names to be consistent across reboots.

RootDeviceHints format

One or more hints can be provided, the chosen device will need to match all of them. Available hints are:

  • deviceName – A string containing a canonical Linux device path like /dev/vda or a by-path alias like /dev/disk/by-path/pci-0000:04:00.0.

    Warning: as mentioned above, block device names are not guaranteed to be consistent across reboots. If possible, choose a more reliable hint, such as wwn or serialNumber.

    Hint: only by-path aliases are supported, other aliases, such as by-id or by-uuid, cannot currently be used.

  • hctl – A string containing a SCSI bus address like 0:0:0:0.

  • model – A string containing a vendor-specific device identifier. The hint can be a substring of the actual value.

  • vendor – A string containing the name of the vendor or manufacturer of the device. The hint can be a substring of the actual value.

  • serialNumber – A string containing the device serial number.

  • minSizeGigabytes – An integer representing the minimum size of the device in Gigabytes.

  • wwn – A string containing the unique storage identifier.

  • wwnWithExtension – A string containing the unique storage identifier with the vendor extension appended.

  • wwnVendorExtension – A string containing the unique vendor storage indentifier.

  • rotational – A boolean indicating whether the device must be a rotating disk (true) or not (false). Examples of non-rotational devices include SSD and NVMe storage.

Finding the right hint value

Since the root device hints are only required for provisioning, you can use the results of inspection to get an overview of available storage devices:

kubectl get hardwaredata/<BMHNAME> -n <NAMESPACE> -o jsonpath='{}' | jq .

This commands produces a JSON output, where you can find all necessary fields to populate the root device hints before provisioning. For example, on a virtual testing environment:

    "alternateNames": [
    "hctl": "0:0:0:0",
    "model": "QEMU HARDDISK",
    "name": "/dev/disk/by-path/pci-0000:03:00.0-scsi-0:0:0:0",
    "rotational": true,
    "serialNumber": "drive-scsi0-0-0-0",
    "sizeBytes": 32212254720,
    "type": "HDD",
    "vendor": "QEMU"

Interaction with hardware profiles

Hardware profiles are a deprecated concept that was introduced to describe homogenous types of hardware. The default hardware profile is unknown, which implies using /dev/sda as the root device.

In a future version of BareMetalHost API, the hardware profile concept will be disabled, and Metal3 will default to having no root device hints by default. In this case, the default logic in Ironic will apply: the smaller block device that is at least 4 GiB. If you want this logic to apply in the current verson of the API, use the empty profile:

  # ...
  hardwareProfile: empty

In all other cases, use explicit root device hints.