What backup method would ensure the shortest return to a working state after an OS upgrade failure on a VM disk stored on a SAN?

A storage-level snapshot made on the SAN provides the shortest return to a working state after an operating system upgrade failure on a virtual machine (VM) disk. This is because a storage-level snapshot captures the entire state of the virtual machine’s disk at a specific moment in time, allowing for instant restoration to that exact state without requiring a lengthy restoration process.

When using storage-level snapshots, the data is saved at the block level, which makes it extremely efficient. If an OS upgrade fails, the snapshot can be quickly reverted to restore the VM to its previous, stable state. This process is typically much faster than restoring from a full file-system backup, which may involve copying large amounts of data back to the operational VM and can introduce downtime.

Other backup methods, like a full file-system backup stored on the SAN or on LTO tape, involve more steps and can take considerable time to retrieve and restore the data. A system recovery point made with the OS may also take longer, as it depends on the OS's management of recovery processes and might require more significant preparation or configuration before a rollback can occur. Therefore, for rapid recovery from an upgrade failure, utilizing a storage-level snapshot from the SAN is clearly the best choice.