When talking about attaching a Cinder volume there are three steps that must happen before the volume is available in the host:
- Retrieve connection information from the host where the volume is going to be attached. Here we would be getting iSCSI initiator name, IP, and similar information.
- Use the connection information from step 1 and make the volume accessible to it in the storage backend returning the volume connection information. This step entails exporting the volume and initializing the connection.
- Attaching the volume to the host using the data retrieved on step 2.
If we are running cinderlib and doing the attach in the same host, then all steps will be done in the same host. But in many cases you may want to manage the storage backend in one host and attach a volume in another. In such cases, steps 1 and 3 will happen in the host that needs the attach and step 2 on the node running cinderlib.
Projects in OpenStack use the OS-Brick library to manage the attaching and detaching processes. Same thing happens in cinderlib. The only difference is that there are some connection types that are handled by the hypervisors in OpenStack, so we need some alternative code in cinderlib to manage them.
Connection objects’ most interesting attributes are:
- connected: Boolean that reflects if the connection is complete.
- volume: The Volume to which this instance holds the connection information.
- protocol: String with the connection protocol for this volume, ie: iscsi, rbd.
- connector_info: Dictionary with the connection information from the host that is attaching. Such as it’s hostname, IP address, initiator name, etc.
- conn_info: Dictionary with the connection information the host requires to do the attachment, such as IP address, target name, credentials, etc.
- device: If we have done a local attachment this will hold a dictionary with all the attachment information, such as the path, the type, the scsi_wwn, etc.
- path: String with the path of the system device that has been created when the volume was attached.
Once we have created a volume with cinderlib doing a local attachment is really simple, we just have to call the attach method from the Volume and we’ll get the Connection information from the attached volume, and once we are done we call the detach method on the Volume.
vol = lvm.create_volume(size=1) attach = vol.attach() with open(attach.path, 'w') as f: f.write('*' * 100) vol.detach()
This attach method will take care of everything, from gathering our local connection information, to exporting the volume, initializing the connection, and finally doing the local attachment of the volume to our host.
The detach operation works in a similar way, but performing the exact opposite steps and in reverse. It will detach the volume from our host, terminate the connection, and if there are no more connections to the volume it will also remove the export of the volume.
The Connection instance returned by the Volume attach method also has a detach method, but this one behaves differently than the one we’ve seen in the Volume, as it will just perform the local detach step and not the termiante connection or the remove export method.
For a remote connection it’s a little more inconvenient at the moment, since you’ll have to manually use the OS-Brick library on the host that is going to do the attachment.
THIS SECTION IS INCOMPLETE
First we need to get the connection information on the host that is going to do the attach:
import os_brick # Retrieve the connection information dictionary
Then we have to do the connection
# Create the connection attach = vol.connect(connector_dict) # Return the volume connection information
import os_brick # Do the attachment
If we want to use multipathing for local attachments we must let the Backend know when instantiating the driver by passing the use-multipath_for_image_xfer=True:
import cinderlib lvm = cinderlib.Backend( volume_driver='cinder.volume.drivers.lvm.LVMVolumeDriver', volume_group='cinder-volumes', iscsi_protocol='iscsi', iscsi_helper='lioadm', volume_backend_name='lvm_iscsi', use-multipath_for_image_xfer=True, )
Multi attach support has just been added to Cinder in the Queens cycle, and it’s not currently supported by cinderlib.
All other methods available in the Snapshot class will be explained in their relevant sections:
- load will be explained together with json, jsons, dump, and dumps properties, and the to_dict method in the Serialization section.
- refresh will reload the volume from the metadata storage and reload any lazy loadable property that has already been loaded. Covered in the Serialization and Resource tracking sections.