cinder/doc/source/devref/attach_detach_conventions_v2.rst

Volume Attach/Detach workflow

Previously there were six API calls associated with attach/detach of volumes in Cinder (3 calls for each operation). As the projects grew and the functionality of simple things like attach/detach evolved things have become a bit vague and we have a number of race issues during the calls that continually cause us some problems.

Additionally, the existing code path makes things like multi-attach extremely difficult to implement due to no real good tracking mechansim of attachment info.

To try and improve this we've proposed a new Attachments Object and API. Now we keep an Attachment record for each attachment that we want to perform as opposed to trying to infer the information from the Volume Object.

Attachment Object

We actually already had a VolumeAttachment Table in the db, however we weren't really using it, or at least using it efficiently. For V2 of attach implementation (V3 API) flow we'll use the Attachment Table (object) as the primary handle for managing attachment(s) for a volume.

In addition, we also introduce the AttachmentSpecs Table which will store the connector information for an Attachment so we no longer have the problem of lost connector info, or trying to reassemble it.

New API and Flow

` attachment_create <instance_uuid> <volume_id>`

The attachment_create call simply creates an empty Attachment record for the specified Volume with an optional Instance UUID field set. This is particularly useful for cases like Nova Boot from Volume where Nova hasn't sent the job to the actual Compute host yet, but needs to make initial preparations to reserve the volume for use, so here we can reserve the volume and indicate that we will be attaching it to <Instance-UUID> in the future.

Alternatively, the caller may provide a connector in which case the Cinder API will create the attachment and perform the update on the attachment to set the connector info and return the connection data needed to make a connection.

The attachment_create call can be used in one of two ways:

1. Create an empty Attachment object (reserve) attachment_create call. In this case the attachment_create call requires an instance_uuid and a volume_uuid, and just creates an empty attachment object and returns the UUID of said attachment to the caller.

2. Create and Complete the Attachment process in one call. The Reserve process is only needed in certain cases, in many cases Nova actually has enough information to do everything in a single call. Also, non-nova consumers typically don't require the granularity of a separate reserve at all.

   To perform the complete operation, include the connector data in the attachment_create call and the Cinder API will perform the reserve and initialize the connection in the single request.

param instance-uuid: The ID of the Instance we'll be attaching to param volume-id: The ID of the volume to reserve an Attachment record for rtyp: string:VolumeAttachmentID

` cinder --os-volume-api-version 3.27 attachment-create --instance <instance-uuid> <volume-id>`

param volume_id: The ID of the volume to create attachment for. parm attachment_id: The ID of a previously reserved attachment.

param connector: Dictionary of connection info param mode: rw or ro (defaults to rw if omitted). param mountpoint: Mountpoint of remote attachment. rtype: VolumeAttachment

Example connector:

{'initiator': 'iqn.1993-08.org.debian:01:cad181614cec',

'ip':'192.168.1.20', 'platform': 'x86_64', 'host': 'tempest-1', 'os_type': 'linux2', 'multipath': False}

` cinder --os-volume-api-version 3.27 attachment-create --initiator iqn.1993-08.org.debian:01:29353d53fa41 --ip 1.1.1.1 --host blah --instance <instance-id> <volume-id>`

Returns a dictionary including the connector and attachment_id:

` +-------------------+-----------------------------------------------------------------------+ | Property | Value | +-------------------+-----------------------------------------------------------------------+ | access_mode | rw | | attachment_id | 6ab061ad-5c45-48f3-ad9c-bbd3b6275bf2 | | auth_method | CHAP | | auth_password | kystSioDKHSV2j9y | | auth_username | hxGUgiWvsS4GqAQcfA78 | | encrypted | False | | qos_specs | None | | target_discovered | False | | target_iqn | iqn.2010-10.org.openstack:volume-23212c97-5ed7-42d7-b433-dbf8fc38ec35 | | target_lun | 0 | | target_portal | 192.168.0.9:3260 | | volume_id | 23212c97-5ed7-42d7-b433-dbf8fc38ec35 | +-------------------+-----------------------------------------------------------------------+`

attachment-delete

` cinder --os-volume-api-version 3.27 attachment-delete 6ab061ad-5c45-48f3-ad9c-bbd3b6275bf2`