Well you have to come up with some way to measure it. How about "time I expect this data to be retrievable"? So you can measure memory and process durability

Well you have to come up with some way to measure it. How about "time I expect this data to be retrievable"? So you can measure memory and process durability in hours to days, and spinning disk durability in years. I don't know how long SSDs last yet. Say, (just making up numbers) you expect your servers to run 10 days without a reboot or crash, and your disks to last 5 years before failing. That means writing to memory is about 0.005479 as durable. So it's not great, but it's not zero either. An interesting thought experiment: how many machines replicating data only in RAM does it take to be more durable than one machine with a spinning disk?