Fixed Block Architecture

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Fixed Block Architecture (FBA) is a disk layout in which each physical record (block) on disk is of the same size. The term fell out of use, since nearly all modern disk drives use this principle, having usually a constant block size of 512 bytes.

IBM mainframe systems' disk layout had traditionally been count-key-data (CKD), which was capable of storing records of varied size. After the introduction of VSAM, IBM attempted to wean the user base away from the expensive and complex CKD storage systems. However, only the VSE and VM operating systems had full support for FBA, so adoption was slow.

In IBM's implementation of FBA, a block was physically addressed by sequential number. ^[1] The disk controller computed the cylinder, head, and sector on the track to use for the operation.

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