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IP.com Disclosure Number: IPCOM000040180D
Original Publication Date: 1987-Oct-01
Included in the Prior Art Database: 2005-Feb-02

Publishing Venue

IBM

Related People

Authors:
Clark, BE Kurtz, JP Passe, PB Singh, SS [+details]

Abstract

A Bit Map is used to track changed pages in a file. Before a record is updated, its corresponding bit in the Bit Map is set. When the file is forced to disk, the Bit Map is used to identify those address ranges containing updated records so that only those ranges are forced to the non-volatile storage. The figure shows the Bit Map and file. A Bit Map is a cluster of bits where each bit identifies a range of bytes/pages in an associated file. A small amount of storage is reserved for maintaining the Bit Map associated with the file. This space is dependent upon the size of the file and the range size chosen to represent each bit. For example, if a bit represents 1K bytes of data, then one page (512 bytes) of Bit Map will be enough to represent four MB of data. This is called 1K granularity.