Support for Software RAID

Support for Software RAID

https://storyboard.openstack.org/#!/story/2004581

This spec proposes to add support for the configuration of software RAIDs.

In analogy to the way hardware RAIDs are currently set up, the RAID setup shall be done as part of the cleaning (“clean-time software RAID”). Admin Users define the target RAID config which will be applied whenever the node is cleaned, i.e. before it becomes available for instance creation.

In order to allow the End User to provide details on how the software RAID shall be configured, the RAID setup should eventually become part of the deployment steps. Integrating this into the deployment steps framework, however, is beyond the scope of this spec.

Problem description

As it is hardware agnostic, flexible, reliable, and easy to use, software RAID has become a popular choice to protect against disk device failures - also in production setups. Large deployments, such as the ones at Oath or CERN, rely on software RAID for their various services.

Ironic’s current lack of support for such setups requires Deployers and Admins to withdraw to workarounds in order to provide their End Users with physical instances based on a software RAID configuration. These workarounds may require to maintain an additional installation infrastructure which is then either integrated into the installation process or requires the End User to re-install a machine a second time after it has been already provisioned by Ironic to eventually end up with the desired configuration of the disk devices. This increases the complexity for Deployers and Admins, and can also lead to a decrease of the End Users’ satisfaction with the overall provisioning and installation process.

Proposed change

The proposal is to extend Ironic to support software RAID by:

  • using a node’s target_raid_config to specify the desired s/w RAID layout (with some restrictions, see below);

  • adding support in the ironic-python-agent to understand a software RAID config as specified in a node’s target_raid_config and be able to create and delete such configurations;

  • allow the ironic-python-agent to consider s/w RAID devices for deployment, e.g. via root device hints (considering them at all is already addressed in [1]);

  • adding support in Ironic and the ironic-python-agent to take the necessary steps to boot from a s/w RAID, e.g. installing the boot loader on the correct device(s).

Initially, only the following configurations will be supported for the target_raid_config as to be set by the Admin:

  • a single RAID-1 spanning the available devices and serving as the deploy target device, or

  • a RAID-1 serving as the deploy target device plus a RAID-N where the RAID level N is configurable by the Admin. N can be 0, 1, 5, 6, or 10.

The supported configurations have been limited to these two options in order to avoid issues when booting from RAID devices. Having a (small) RAID-1 device to boot from is a common approach when setting up more advanced RAID configurations: a RAID-1 holder device can look like a standalone disk and does not require the bootloader to have any knowledge or capabilities to understand more complex RAID configurations.

In order to signal that a software RAID configuration is indeed desired (and to protect from a situation where a software RAID is set up accidentally when the configuration passed via the target_raid_config was meant for a hardware RAID setup, for instance), the controller property of all of the logical disks needs to be set to software. Without this setting, the software RAID code in the GenericHardwareManager of the IPA will ignore the given target_raid_config. If it is set on only one of the logical drives, the validation code will raise an error.

The controller property set to software will also be used by the conductor to identify a software RAID and trigger the required installation of the bootloader. While whole-disk images are expected to come with a bootloader configuration as part of the image, for software RAIDs in the current design the image will not be at the start of a real disk, but inside the first partition on top of a software RAID-1. The bootloader must hence be explicitly installed onto the underlying holder disks, and this property will indicate when to do this.

An example of a valid software RAID configuration would hence look like:

{
    "logical_disks": [
        {
            "size_gb": 100,
            "raid_level": "1",
            "controller": "software"
        },
        {
            "size_gb": "MAX",
            "raid_level": "0",
            "controller": "software"
        }
    ]
}

Support for more than one RAID-N, support for the selection of a subset of drives to act as holder devices, support for simultaneous software and hardware RAID devices as well as support to partition the created RAID-N device are left for follow-up enhancements and beyond the scope of this specification.

Also, there is currently no support for partition images, only whole disk images are supported.

A first prototype very close to the proposal is available from [2][3][4].

Alternatives

As mentioned above, the alternative is to use other methods to create s/w RAID setups on physical nodes and integrate these out-of-band approaches into the provisioning workflow of individual deployments. This increases complexity on the Deployer/Admin side and can have a negative impact on the user experience when creating physical instances which need to have a software RAID setup..

Data model impact

None.

State Machine Impact

None.

REST API impact

None.

Client (CLI) impact

None.

“ironic” CLI

None.

“openstack baremetal” CLI

None.

RPC API impact

None.

Driver API impact

The proposed functionality could be consolidated into a new RAID interface.

Nova driver impact

None.

Ramdisk impact

The ironic-python-agent will need to be able to: * setup and clean software RAID devices * consider software RAID devices for deployment * configure the holder devices of the RAID-1 device in a way they are bootable

This functionality could be consolidated in an additional RAID interface.

Security impact

None.

Other end user impact

While the predefined RAID-1 ensures that a system should be able to boot, End Users need to be aware that the kernel of the started image needs to be able to understand software RAID devices.

Scalability impact

None.

Performance Impact

None.

Other deployer impact

Deployers will need to be aware that the configuration and clean up of the RAID-N devices is only done during cleaning, so any changes require the node to be cleaned. Also, the config is not configurable by the End User, but limited to admins (as the target_raid_config) is a node property. All of this, however, already holds true for hardware RAID configurations.

Developer impact

None.

Implementation

An inital proof-of-concept is available from [2][3][4].

Assignee(s)

Primary assignee:

None.

Other contributors:

Arne.Wiebalck@cern.ch (arne_wiebalck)

Work Items

This is to be defined once the overall idea is accepted and there’s agreement on a design.

Dependencies

None.

Testing

TBD

Upgrades and Backwards Compatibility

None.

Documentation Impact

Documentation on how to configure a software RAID along with the limitations outlined in ‘Deployer’s Impact’ need to be documented.

Creative Commons Attribution 3.0 License

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