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Selective Page Updating

IP.com Disclosure Number: IPCOM000040180D
Original Publication Date: 1987-Oct-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 2 page(s) / 38K

Publishing Venue

IBM

Related People

Clark, BE: AUTHOR [+4]

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.

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Selective Page Updating

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. Similarly 4K granularity means one page of Bit Map will represent 16 MB segments of data. Each of the bits in the Bit Map represent a specific range in the file. For example, for 4KB granularity, the 4th bit of the 20th byte represents a starting offset of 'OA3000'X((20*8+4-1)*4K) and ending offset of 'OA3FFF'X (starting offset + 4K - 1). Another simple example is: the first bit of the first byte of the Bit Map represents the range of data at starting offset of '000000'X and ending offset of '000FFF'X. Immediately preceding data modification, a bit in the Bit Map representing the address range of the data being modified is turned ON. If the start address...