Browse Prior Art Database

Disk Cache Read Prefetch Command Function for Personal Systems

IP.com Disclosure Number: IPCOM000120668D
Original Publication Date: 1991-May-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 4 page(s) / 135K

Publishing Venue

IBM

Related People

McNeill, AB: AUTHOR [+2]

Abstract

Described is a cache read prefetch command function for use in personal computer systems to provide a "read-ahead" function from a hard disk without requiring the use of system memory buffers or requiring the system bus overhead to transfer prefetched data into system memory. The function improves overall cache operation and increases system performance.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Disk Cache Read Prefetch Command Function for Personal Systems

      Described is a cache read prefetch command function for
use in personal computer systems to provide a "read-ahead" function
from a hard disk without requiring the use of system memory buffers
or requiring the system bus overhead to transfer prefetched data into
system memory.  The function improves overall cache operation and
increases system performance.

      Typically, personal computer systems which utilize the Micro
Channel* environment perform bus master operations by using an
internal microprocessor to handle multiple requests and command
interpretation and execution operations.  An adapter containing an
intelligent disk cache is used in this function and is controlled by
a microcode program executed by a microprocessor.  Caching algorithms
improve effective hard disk performance by keeping frequently
accessed data in the cache for immediate access by the system and by
performing physical prefetch functions.

      When a disk read request is received, the cache algorithm
determines if the requested data is in the cache. If the requested
data is in the cache it is known as a cache hit and data is
transferred to the system memory and not read from the hard disk
device.  This operation saves time since accessing data from a hard
disk is usually slow when compared with accessing data from a cache
memory.

      When data requested by the system from a hard disk is not found
in the cache memory it is known as a cache miss. Data is read from
the hard disk and will usually read additional sectors physically
adjacent to the requested data and place the data in the cache for
possible future access. This is known as a prefetch function and is
performed because many application programs access data in a
sequential manner from a hard disk.  This operation ultimately saves
time such that if the prefetched sectors are requested in the future,
many of the requests will be read hits and disk performance will be
improved.

      Although data stored on a hard disk may be sequentially
organized, it may not always be stored in physically sequential
sectors on the hard disk.  When a logically sequential data file is
not stored in...