[feature request] iSCSI initiator name export over the relation

[Vladimir Grevtsev]

== Problem statement

There is neither no way to dynamically generate the iSCSI initiator list somewhere externally, nor change it with some predictable value.

== Rationale

I'm working now on the cloud which has external Huawei OceanStor storage array; and the upstream driver doesn't have a default PortGroup option - it has only the "DefaultTargetIP" option. While the option itself could accept multiple IPs:

https://github.com/openstack/cinder/blob/stable/ussuri/cinder/volume/drivers/huawei/huawei_conf.py#L280-L283

text = xml_root.findtext('iSCSI/DefaultTargetIP')
if text:
    iscsi_info['default_target_ips'] = [
        ip.strip() for ip in text.split(';') if ip.strip()]

It's still using only the first one supplied, ignoring others:

https://github.com/openstack/cinder/blob/stable/ussuri/cinder/volume/drivers/huawei/rest_client.py#L1345

default_target_ips = self.iscsi_info['default_target_ips']
if default_target_ips:
    target_ips.append(default_target_ips[0])

The only viable approach currently is to use the PortGroup configuration on the storage array side, since it is supported by the driver: https://github.com/openstack/cinder/blob/stable/ussuri/cinder/volume/drivers/huawei/rest_client.py#L1303

temp_tgt_ips = self._get_tgt_ip_from_portgroup(portgroup_id)
valid_port_info = self._get_tgt_port_ip_from_rest()
valid_tgt_ips = valid_port_info

for ip in temp_tgt_ips:
    if ip in valid_tgt_ips:
        target_ips.append(ip)

From the storage array point of view, PortGroup is a logical grouping of multiple storage controller interfaces, and driver is querying storage array's API to get all of those IPs:

https://github.com/openstack/cinder/blob/stable/ussuri/cinder/volume/drivers/huawei/rest_client.py#L1303

temp_tgt_ips = self._get_tgt_ip_from_portgroup(portgroup_id)
valid_port_info = self._get_tgt_port_ip_from_rest()
valid_tgt_ips = valid_port_info

for ip in temp_tgt_ips:
    if ip in valid_tgt_ips:
        target_ips.append(ip)

This is required in order to let driver initialize multiple iSCSI sessions to the same storage array, enabling the iSCSI multipathing effectively.

However, there is no way to specify a default PortGroup - it could only be specified per-initiator (read: per compute host), requiring an InitiatorName to be provided in the driver config:

https://docs.openstack.org/cinder/ussuri/configuration/block-storage/drivers/huawei-storage-driver.html

"To configure iSCSI Multipathing, follow the steps below:

Add the port group settings in the Huawei-customized driver configuration file and configure the port group name needed by an initiator.

<iSCSI>
   <DefaultTargetIP>x.x.x.x</DefaultTargetIP>
   <Initiator Name="xxxxxx" TargetPortGroup="xxxx" /> <---- "Name" option has to contain initiator name from
</iSCSI>"

Currently, we are workarounding this issue by supplying all of the initiators manually to the charm via the following snippet:

juju run -m openstack --application nova-compute --format=json "sudo cat /etc/iscsi/initiatorname.iscsi | grep InitiatorName=" | jq ".[] | .Stdout" | tr -d '"' | sed -e 's/\\n//g;s/InitiatorName=//g' | paste -sd ";" -

However, it would be nice to have an option to create a relation between cinder backend charm and nova-compute to get this information dynamically.