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Contiguous File Allocation Algorithm for the File Allocation System

IP.com Disclosure Number: IPCOM000115255D
Original Publication Date: 1995-Apr-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 2 page(s) / 48K

Publishing Venue

IBM

Related People

Misuraca, CM: AUTHOR

Abstract

Disclosed is an enhancement to a disk space allocation algorithm for the File Allocation Table (FAT) Physical File System (PFS), allowing the FAT to allocate files contiguously while maintaining backward compatibility with older FAT PFS implementation.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 68% of the total text.

Contiguous File Allocation Algorithm for the File Allocation System

      Disclosed is an enhancement to a disk space allocation
algorithm for the File Allocation Table (FAT) Physical File System
(PFS), allowing the FAT to allocate files contiguously while
maintaining backward compatibility with older FAT PFS implementation.

      Without this enhancement, a conventional FAT PFS allocates
clusters (disk space) from the FAT in a pseudo-random order.  A file
may be placed in contiguous or non-contiguous physical disk space,
creating a problem for an advanced operating system which has a need
to allocate contiguous disk space for paging and caching algorithms.
Furthermore, performance problems are caused by non-contiguous file
allocation.  As additional files are created and deleted, a physical
disk becomes increasingly fragmented, causing delays in data
retrieval with the disk read/write head latency recovery time.  With
increased fragmentation of the disk, longer and more frequent head
moves cause greater delays in data access.  As the size of a file
increases, these data retrieval delays become more pronounced.

      This enhancement is provided with a new allocation algorithm
using a bitmap of the FAT to map all used and unused clusters.  The
bitmap allows the FAT PFS to search the FAT quickly for contiguous
disk space.  If insufficient disk space is available, the standard
cluster allocation algorithm is used.

      The new allocation algorithm co...