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.

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.

Currently you can create a multiattach volume in two ways.

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"

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 <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.