HA cluster on Proxmox 6.x (Debian 10) with multipath iSCSI

I build a highly available cluster on Proxmox 6.x (Debian 10) that uses multipath iSCSI for shared storage, backed by an HPE MSA array.
What the article covers
- On three Proxmox nodes: a management bridge, a VLAN-aware corporate bridge and a bond for iSCSI (active-backup or LACP), with an isolated subnet for iSCSI
- Forming the cluster over the management network after the first node is installed
- On the HPE MSA: registering each node's iSCSI initiator name, then pools, disk groups, virtual volumes, LUN IDs and mappings
- Connecting through iSCSI from the Proxmox GUI, with at least two IPs per node
- open-iscsi startup and session timeout settings; installing multipath-tools
- Reading LUN WWIDs with scsi_id; a multipath config tuned for ALUA, round-robin and failback
- Confirming with multipath -ll that one path group is active and another is standby
- LVM volume group on the multipath device, added as shared storage at the Datacenter level