FBA vs CKD Overview

Parameter	Open System	IBM z System (Mainframe)
Storage Type	FBA (Fixed Block Architecture)	CKD (Count Key Data)
Connectivity	Fibre Channel/SCSI etc	ESCON/FICON
Character sets	ASCII	EBCDIC
Extent Size	16 MiB, 1GiB	21 cylinders (or) 1113 cylinders
Base	Used by many OEMs	IBM proprietary

Note: Whether Flash or HDD, it’s still an emulated track for zOS or FB environments