Enable multiattach of volumes

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The ability to attach a volume to multiple hosts/servers simultaneously is a use case that has been asked for over the years. The implementation of the Attach workflow in Cinder however made this somewhat difficult and less than robust.

With the new Cinder Attachment API’s [1] this is a bit easier because Attachments are treated as independent objects.

This spec proposes a set of changes to enable and control the use of multiattach volumes from the Cinder side.

Problem description

Currently we refuse to attach a volume that is already attached (in-use). There are situations where a cloud user may have deployed a clustered file-system on a Cinder volume and they would like the ability to attach a volume to multiple hosts/servers at the same time.

Note that Cinder is not providing any sort of Filesystem checks or sharing overlays on the volume, it’s completely up to the user to handle this and determine if they can in fact attach a volume to multiple locations without incurring filesystem/data corruption.

The problem that we are proposing to solve with this spec is simply to enable the ability to attach a volume to multiple locations, and provide the cloud operator with policies to enable/disable the functionality for the cloud. We do NOT offer any mechanism to keep a user from corrupting their data or doing something terrible.


Multi-Attach RO:

The ability to attach a volume as one would normally do (Read Write access) and then to attach that volume to another host(s)/server(s) in Read Only mode. Note, this is actually tricky because this typically will require that the consumer (ie Nova/KVM) knows how to set and enforce Read Only on an attached volume.

Multi-Attach RW:

The ability to attach a volume as one would normally do (Read Write access) and then to attach that volume to another host(s)/server(s). In this case there is no differentiation between the initial attachment and any subsequent attachments. They are all treated as independent items and are all Read Write volumes with no real distinction beyond what’s provided with a single attachment.

The ability to set Read Only options on attachments will be handled as seperate work in the from of an argument to attachment-create. That feature is not considered part of the scope of this spec and will be handled independently.

Multi-Attach Capable Backend:

Some backend devices just flat out may not support this. Those that do are considered to be “Multi-Attach Capable” backend devices and need to report this in their capabilities.

Example Volume Types to communicate Multi Path info:

{ ‘multiattach’: ‘<is> True’, }

It will be up to the Cinder scheduler to determine if the policy allows these types of volumes to be created, and it will then be up to the driver to form the corresponding Connection and Volume attributes appropriately in it’s model returns.

Again NOTE we will NOT allow retype of multiattachment setting for an in-use volume.

Use Cases

  • There are products that can be used like Oracle RAC that would make things like H/A databases via a shared Block device possible which is something of interest.

  • The other use case that’s been proposed is the ability to have things like passive stand-by servers/volumes.

  • Analyzing/Fleecing an Instance from another Instance or host

Proposed change

MultiAttach policy

Create a Cinder policy that enables/disables the ability to multi-attach a volume. This policy is a global policy for the endpoint and simply either enables or disables the capability.

Additionally we’ll want a specific policy to enable/disable the ability to multiattach bootable volumes (volume with the bootable parameter set True).

MultiAttach capable volume type

In order to ensure that a volume is created on a backend that supports multiple attachments, it will need to be created on a backend that allows this. The only way to safely control this is be requiring that a type of “multiattach” is created and used on volume creation.

If an additional attach request is made to a volume that is NOT of the type multi-attach enabled the request should fail as the volume is already in-use, and either a new volume should be created of the correct type, or the volume should be retyped. Of course if the policy allows and the type is correct the additional attachment request should be processed and the existing attachment(s) are left active and NOT interrupted.

To avoid various corner cases and confusion for users in what’s allowed and what’s not, this spec proposes we set a hard-code rule that disallows the retyping of attachment parameters of an in-use volume. Regardless of what those changes are; multi–>non-multi, non-multi–>multi.

Any retype command should inspect the multiattach keys and if there is any change being requested in said keys the retype should fail immediately at the API service.

NOTE that the default will be multiattach:False

There is an additional required task, the –allow-multiattach parameter included with volume create needs to be deprecated and it needs to be very explicit that that flag is NOT relevant to the implementation of multi-attach that is being proposed in this spec and implemented in Cinder API microversion xyz.

Special considerations for force-detach; recent changes have been added that treat things like detach of attachments with no connector as force detach operations. This should be fine in a multi-attach scenario because the force detach is ONLY issued against that instantiation of an attachment. In other words the only thing force detach does is ignore state checks and disconnects the specified session, leaving others intact. The process on the Cinder side should follow the standard multi-attach detach process with the exception of ignoring the status of the volume.

FWIW that call rarely works as users expect anyway, so maybe this will be a good time to revisit it. Perhaps deprecate it, and include a force parameter to the attachment-delete API call itself?


  • Be cloudy and write clustered apps

  • Use things like independent data instances

  • Get over it, you probably don’t really need this

Data model impact

N/A All of the needed changes to the data model should already be in place. Inparticular the existing multiattach column on the volume object, which will signal the consumer that they’re working with a multiattach capable volume. So, if a volume of type multiattach:True is requested/created, it’s multiattach column is set to True, else False.

To elaborate on the create flow: Admin creates a volume type multiattach with extra-specs: { ‘multiattach’: ‘<is> True’ }

If a user desires a multiattach volume, he/she issues a create call specifying the multiattach type: cinder create –volume-type multiattach –name my-mavol 20

If there are NO backends reporting multiattach: True then scheduling will fail. Unfortunatly this process includes the task-flow retries and the object will be created, the caller will get a 202 response, but after taskflow and the scheduler finish retries and fail to find a backend the volume status will be set to error. It may be possible to speed this up if we need to and put capability checks into the API layer so that we could respond immediately with an Invalid Request. We do provide an API call now that pulls capabilities from the system, so we could make that check upon receipt of the create request and make sure it’s possible to fulfill the request. It would probably be wise to cache this info based on the periodic capability reporting instead of fetching it each time.

REST API impact

This has a number of impacts on the API, the most obvious of which is the fact that you can attach a volume multiple times. The other changes that may not be so obvious are things like representing a volume’s attachment status, and managing state changes when a secondary attachment is processed or removed.

The current representation of volume-status is likely to be insufficent once multiple attachments are enabled.

Security impact


Notifications impact


Other end user impact

User can potentially attach a volume to multiple servers, and corrupt their data.

Performance Impact


Developer impact

Drivers will need to add a capabilities field “multiattach: True/False”, and do any special handling on their end for connecting/disconnecting volumes in this category.



Primary assignee:


Other contributors:


Work Items

  • Update Cinder attach/detach to accomodate shared targets (most of this is being done independently, see Dependencies section for more info)

  • Implement policy changes

  • Implement changes to API to allow/ignore existing attachments on attachment-create calls.

  • Update the attach/detach volume-status transitions to be multiattach aware and make sure they reflect the correct values.

  • Update the detail volume view and possibly the summary view to clearly inform the user when a volume is attached to multiple servers.




New unit tests will be added to test the changed code and functional testing will need to be added as well.

Documentation Impact

This will require updates to both deployment guides as well as end-user guides.


Related Nova spec: https://specs.openstack.org/openstack/nova-specs/specs/queens/approved/cinder-volume-multi-attach.html