Thank you for sharing that. Reading material for tonight ;) I googled ECKD and it seems it is still in use and support by IBM (maybe through emulation).
The original indexed file format under MVS, ISAM, heavily relied on the physical hardware keys on (E)CKD disks. In the 1970s, IBM replaced ISAM with VSAM, which didn't use that physical key feature and handled all the keys in software – and it was actually faster. (I don't know if the reason for it being faster was due to some fundamental flaw with the underlying idea, or just due to limitations of the particular implementation.) (Despite being deprecated, IBM continued to support ISAM until the mid-2000s, when they finally removed it from the operating system.)
IIRC, the legacy PDS (partitioned data set) format also uses it, but not the newer PDSE 1.0 or PDSE 2.0. (PDSE = partitioned data set extended). (PDS is like an archive file format, for object code it functions similar to .a files on Unix, for source code it was used to make up for the fact that originally MVS didn't really have the concept of directories, so keeping all your program's source modules in a single dataset was more convenient.)
I think the mainframe filesystem (VTOC) also uses it a bit.
There's probably a few other random things in z/OS that still need it, but the newer stuff (VSAM, PDSE, HFS/zFS, etc) doesn't really use it. I think if IBM really wanted to, they could add support to z/OS for running on industry-standard FBA disks instead of ECKD. (They did the same thing to VSE all the way back in the 1980s.) I think the main reason they don't do it, is not technical, but commercial – mainframes needing special SANs helps keep their storage business alive, if mainframes could work on industry standard SANs, they'd face more competition in that area.
The original indexed file format under MVS, ISAM, heavily relied on the physical hardware keys on (E)CKD disks. In the 1970s, IBM replaced ISAM with VSAM, which didn't use that physical key feature and handled all the keys in software – and it was actually faster. (I don't know if the reason for it being faster was due to some fundamental flaw with the underlying idea, or just due to limitations of the particular implementation.) (Despite being deprecated, IBM continued to support ISAM until the mid-2000s, when they finally removed it from the operating system.)
IIRC, the legacy PDS (partitioned data set) format also uses it, but not the newer PDSE 1.0 or PDSE 2.0. (PDSE = partitioned data set extended). (PDS is like an archive file format, for object code it functions similar to .a files on Unix, for source code it was used to make up for the fact that originally MVS didn't really have the concept of directories, so keeping all your program's source modules in a single dataset was more convenient.)
I think the mainframe filesystem (VTOC) also uses it a bit.
There's probably a few other random things in z/OS that still need it, but the newer stuff (VSAM, PDSE, HFS/zFS, etc) doesn't really use it. I think if IBM really wanted to, they could add support to z/OS for running on industry-standard FBA disks instead of ECKD. (They did the same thing to VSE all the way back in the 1980s.) I think the main reason they don't do it, is not technical, but commercial – mainframes needing special SANs helps keep their storage business alive, if mainframes could work on industry standard SANs, they'd face more competition in that area.