alt Hacker NewsDASDs supported ISAM.
And they're not strictly just a disk. It's more like a complex multiplexing system for an array of disks. It has interesting capabilities like "channel programs" that persist to this day which allow you to send miniature programs to the disk controller to have it seek out the precise record you're looking for in one of several access modes.
IBM still provides almost the entirety of it's OS documentation online:
https://www.ibm.com/docs/en/zos-basic-skills?topic=set-what-...
> send miniature programs to the disk controller to have it seek out the precise record you're looking for
A very long time ago, a guy I used to work with was porting a sales and stock control database he'd written on the Commodore PET to a PC. By then he had a 286 with a 20MB hard disk and 2MB of RAM to play with - whopping stuff! - but his original program would assemble up a query routine, and transmit it to the 6502 in the PET disk drives over HPIB. Then it would chunter away discovering the records it needed to construct a reply while the host computer could continue working as normal. It was absolutely genius stuff, and it made the whole system seem really responsive even though in reality it was pretty slow.