DISK FORMAT FOR SECONDARY STORAGE SYSTEM

摘要

In a disk type mass storage facility for data processing systems, a disk format which improves handling of defective segments of medium and reduces access time. The format has three layers. A first, physical layer comprises the bytes, sectors and collections of sectors, as well as error detection and correction codes. A second, logical layer is used to address the physical layer and to collect together sectors to form a multiplicity of separately addressable spaces, with each space having a distinct functional utility. At a third, functional layer the use of data fields in each space is specified. This layer governs the handling of bad blocks if required, and the use of certain format information. At the physical layer, bits are collected into sectors, sectors are collected into tracks, tracks are collected into groups, and groups are collected into cylinders. These are logically, rather than physically, defined collections. A group is set of tracks wherein individual tracks in a group can be selected in the inter-sector rotation time. At the logical layer, the disk is divided into three main areas, each separately addressable. The first main area is broken down into two sub-areas. The first sub-area is available to the host computing system for its use; the second sub-area is used for replacement of bad blocks. The second is for format information. The third main area is for diagnostics. Access time latencies may be minimized by proper selection of logical addressing - i.e., by selecting the number of sectors/track, tracks/group and group/cylinderto take advantage of access characteristics. Handling of bad blocks is controlled by a hierarchically layered process. A portion of each disk, distributed across the medium, is reserved as spare sectors to replace defective sectors. After a bad sector is replaced, future attempts to access the bad sector are redirected (i.e., revectored) to the replacement sector. For the simplest revectoring, the bad block is replaced by a replacement block in a known location. If that cannot be done, multiple copies of the replacement block's header are stored in the bad block's data field and the copies are compared to find the replacement address. If the comparison fails, or the header cannot be read, a back-up table is available to match the available replacement addresses with the original address which was replaced. A special code is used to identify blocks wherein the medium is good but the contents of the block are logically corrupted. Various drive parameters are stored in the disk and are supplied to its controller on command, permitting the controller to adapt thereto. These include the starting address for each of the various areas, or spaces, of the disk, for example, its transfer rate, logical addressing structure, etc.