Volume multi-attach: Enable attaching a volume to multiple servers

The ability to attach a volume to multiple hosts/servers simultaneously is a use case desired for active/active or active/standby scenarios.

Support was added in both Cinder and Nova in the Queens release to volume multi-attach with read/write (RW) mode.

Warning

It is the responsibility of the user to ensure that a multiattach or clustered file system is used on the volumes. Otherwise there may be a high probability of data corruption.

In Cinder the functionality is available from microversion ‘3.50’ or higher.

As a prerequisite new Attach/Detach APIs were added to Cinder in Ocata to overcome earlier limitations towards achieving volume multi-attach.

In case you use Cinder together with Nova, compute API calls were switched to using the new block storage volume attachment APIs in Queens, if the required block storage API microversion is available.

For more information on using multiattach volumes with the compute service, refer to the corresponding compute admin guide section.

How to create a ‘multiattach’ volume

In order to be able to attach a volume to multiple server instances you need to have the ‘multiattach’ flag set to ‘True’ in the volume details. Please ensure you have the right role and policy settings before performing the operation.

The only way to create a multiattach volume is by creating a multiattach volume type and using it to create the volume.

Note

For information on back ends that provide the functionality see Back end support.

Multiattach volume type

Starting from the Queens release the ability to attach a volume to multiple hosts/servers requires that the volume is of a special type that includes an extra-spec capability setting of multiattach=<is> True. You can create the volume type the following way:

$ cinder type-create multiattach
$ cinder type-key multiattach set multiattach="<is> True"

Note

Creating a new volume type is an admin-only operation by default. You can change the settings in the cinder policy file if needed. For more information about configuring cinder policies, see Policy configuration.

To create the volume you need to use the volume type you created earlier, like this:

$ cinder create <volume_size> --name <volume_name> --volume-type <volume_type_uuid>

In addition, it is possible to retype a volume to be (or not to be) multiattach capable. Currently however we only allow retyping a volume if its status is available.

The reasoning behind the limitation is that some consumers/hypervisors need to make special considerations at attach-time for multiattach volumes (like disable caching) and there’s no mechanism currently to update a currently attached volume in a safe way while keeping it attached the whole time.

RO / RW caveats (the secondary RW attachment issue)

By default, secondary volume attachments are made in read/write mode which can be problematic, especially for operations like volume migration.

There might be improvements to provide support to specify the attach-mode for the secondary attachments, for the latest information please take a look into Cinder’s specs list for the current release.

Back end support

In order to have the feature available, multi-attach needs to be supported by the chosen back end which is indicated through capabilities in the corresponding volume driver.

The reference implementation is available on LVM in the Queens release. You can check the Driver Support Matrix for further information on which back end provides the functionality.

Policy rules

You can control the availability of volume multi-attach through policies that you can configure in the cinder policy file. For more information about the cinder policy file, including how to generate a sample file so you can view the default policy settings, see Policy configuration.

Multiattach policy

The general policy rule to allow the creation or retyping of multiattach volumes is named volume:multiattach.

Multiattach policy for bootable volumes

This is a policy to disallow the ability to create multiple attachments on a volume that is marked as bootable with the name volume:multiattach_bootable_volume.

Known issues and limitations

• Retyping an in-use volume from a multiattach-capable type to a non-multiattach-capable type, or vice-versa, is not supported.

• It is not recommended to retype an in-use multiattach volume if that volume has more than one active read/write attachment.

• Encryption is not supported with multiattach-capable volumes.