DASDs 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-...