Fixed-block architecture

Fixed Block Architecture (FBA) is a disk data architecture for disk storage. Each physical disk record is of the same size. Nearly all modern disk drives use this principle.

FBA is physically addressed by sequential block number. The disk controller computes the cylinder, head, and block on the track to use for the operation.

IBM's mainframe systems had traditionally been count-key-data (CKD). After the introduction of VSAM, IBM attempted to wean the user base away from the expensive and complex CKD storage systems. Only the VSE and VM operating systems had full support for FBA and so adoption was slow.