Or maybe the newer Blu-ray based Archival Disc https://en.wikipedia.org/wiki/Archival_Disc which ups per-side storage to 150 GB and uses both side. But the pricing and retrieval scheduling strongly suggests LTO tape.

Perhaps this is the case, but I remember digging deep one night and seeing a picture of a StorageTek tape system on what I thought was an AWS page. I can't remember the URL. Oracle is trying to compete in this space, so maybe I'm misremembering.

I'm struggling to see the value proposition there. I would imagine you'd get way higher data / rack density using even plain old hard disk drives than you would with bluray disc libraries.

The only way to get close to that would be to have some insanely complicated automated jukebox/storage mechanism, because you'd be relying on essentially piling up disks in a rack.

Just rough back-of-napkin figures: blu-ray is 4.75" in diameter, and 0.05" in depth. 42U racks: 78″ x 42" x 24″ (more or less.)

Assuming you just pack them all in, in great big tall piles: You could get 5 x 8 columns, 1560 discs deep.

5 * 8 = 40. 40 x 1560 = 62,400 discs in a rack. At 150Gb a disc, that's a total of 9,360,000gb (9.36pb).

Of course you probably need to cut that in half at the very least, to be able to provide some kind of mechanism for extracting and remove the discs, and for safe storage. I'd consider that generous, but it's a good figure to work from.

And of course you have to consider that any jukebox for doing the writing / retrieval is effectively wasted space. The amount of discs you could have stored in transit there aren't likely to be sufficient enough to worth bothering with, so at the very least halve it again. That's also assuming you'd have one jukebox per storage rack.

So... 9.36 / 2 / 2 = 2.34 petabytes average rack density.

Current backblaze PODs come in at https://www.backblaze.com/blog/open-source-data-storage-serv... 480 TB per 4U. Rack I used for disc storage is the standard 42U. I'm going to go assume you'd lose 6U for top of rack switches, power, etc. so 9 servers per 42U. 9 * 480tb = 4.32 petabytes.

So even just with Backblaze PODs you'd get more than double the data density, and it would all be on-line and retrievals could be nearly instantaneous. Plus you'd be dealing with tried and true technology that is likely to be way more reliable and have much less unknowns, comprising of fairly easy to replace storage media, vs relatively new technology with less certain supply lines and specialised devices associated with it.

edit: I see below that bluray archive disks are reaching 150gb per side, so 300gb total. So the rack data density probably reaches 4.68 petabytes, just slightly over that of the hard disks, but still my final point remains. Choosing a newer technology vs well known hard disks and supply chains would be crazy, unless there was a strong advantage that it supplied. I don't see 0.36pb per rack as being that significant an advantage.