What I've read, and it seems reasonable to me, is that glacier is mostly just stored on the same HDDs that everything else is stored on.

What you need to understand is that a modern hard drives are severely limited in the amount of IOPS they can perform. Disk size has grown, but read and write speeds have not kept up with it.

In 1990 you could read a whole hard drive in 2 minutes, in 2026 as sizes have grown it takes the entire day.

As a system designer you want the maximum utilisation of your hardware, so you want to be close to saturating the IO that your drives are capable of.

What that means in practice is that only a tiny portion of the drive can be allocated to files that are read often.

That leaves the rest of the drive for files which are not used very often.