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Directory for Disk With Write-Once Storage Medium

IP.com Disclosure Number: IPCOM000040414D
Original Publication Date: 1987-Nov-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 2 page(s) / 14K

Publishing Venue

IBM

Related People

Elliott, GL: AUTHOR [+3]

Abstract

From a logical standpoint, data on a disk is organized as "files" (or "data sets"). A directory relates this logical organization to the physical location of the data. From a physical standpoint, data is stored along a spiral that starts at the outside edge, for example, of the disk and ends at the inner edge. A "track" is one revolution of the spiral, starting and ending at a particular radius of the disk. A track is subdivided into physical units called "sectors". Each track has the same number of sectors, and corresponding sectors start on the same radius. Some sectors may be unusable because the storage material is defective or because these sectors have been logically erased, but the disk medium cannot actually be erased or rewritten.

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Directory for Disk With Write-Once Storage Medium

From a logical standpoint, data on a disk is organized as "files" (or "data sets"). A directory relates this logical organization to the physical location of the data. From a physical standpoint, data is stored along a spiral that starts at the outside edge, for example, of the disk and ends at the inner edge. A "track" is one revolution of the spiral, starting and ending at a particular radius of the disk. A track is subdivided into physical units called "sectors". Each track has the same number of sectors, and corresponding sectors start on the same radius. Some sectors may be unusable because the storage material is defective or because these sectors have been logically erased, but the disk medium cannot actually be erased or rewritten. If the storage medium cannot be erased and rewritten, changes are made by logically erasing the original sectors and physically writing the changed data in previously unused sectors. Write operations proceed sequentially along the track from the outer edge inward. The directory is updated similarly by logically erasing a previous directory entry and writing a changed directory entry in an unused part of the disk. Directory entries start at the inner edge and progress outward. Thus, the data on the disk expands inwardly from the outside edge, and the directory expands outwardly from the inside edge. A particular file can be scattered on the disk as parts are logically erased and rewritten at the next unused location. The term "extent" describes a set of contiguous sectors (including any unwritten and defective sectors) that store parts of the same file. An extent is written in one I/O operation, typically one track or revolution of the spiral. Extent size is specified in the number of logical sectors. It can physically span more than the number of logical sectors since the drive will ignore blank sectors when reading. This results from the drive's ability to skip bad sectors and relocate to the next good sector automatically when writing. If an error occurs during the write/ read verification of the extent, the entire extent is rewritten. There are no directory pointers to bad extents (similar to logically erasing an extent). Referencing files by extents, as opposed to individual sectors, allows compaction of the directory. An extent size of 1 produces...