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1.1 root 1: .\" Copyright (c) 1986 The Regents of the University of California.
2: .\" All rights reserved.
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4: .\" Redistribution and use in source and binary forms are permitted
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13: .\" IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
14: .\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
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16: .\" @(#)1.t 6.2 (Berkeley) 3/7/89
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18: .ds RH Introduction
19: .NH
20: Introduction
21: .PP
22: This paper describes the changes from the original 512 byte UNIX file
23: system to the new one released with the 4.2 Berkeley Software Distribution.
24: It presents the motivations for the changes,
25: the methods used to effect these changes,
26: the rationale behind the design decisions,
27: and a description of the new implementation.
28: This discussion is followed by a summary of
29: the results that have been obtained,
30: directions for future work,
31: and the additions and changes
32: that have been made to the facilities that are
33: available to programmers.
34: .PP
35: The original UNIX system that runs on the PDP-11\(dg
36: .FS
37: \(dg DEC, PDP, VAX, MASSBUS, and UNIBUS are
38: trademarks of Digital Equipment Corporation.
39: .FE
40: has simple and elegant file system facilities. File system input/output
41: is buffered by the kernel;
42: there are no alignment constraints on
43: data transfers and all operations are made to appear synchronous.
44: All transfers to the disk are in 512 byte blocks, which can be placed
45: arbitrarily within the data area of the file system. Virtually
46: no constraints other than available disk space are placed on file growth
47: [Ritchie74], [Thompson78].*
48: .FS
49: * In practice, a file's size is constrained to be less than about
50: one gigabyte.
51: .FE
52: .PP
53: When used on the VAX-11 together with other UNIX enhancements,
54: the original 512 byte UNIX file
55: system is incapable of providing the data throughput rates
56: that many applications require.
57: For example,
58: applications
59: such as VLSI design and image processing
60: do a small amount of processing
61: on a large quantities of data and
62: need to have a high throughput from the file system.
63: High throughput rates are also needed by programs
64: that map files from the file system into large virtual
65: address spaces.
66: Paging data in and out of the file system is likely
67: to occur frequently [Ferrin82b].
68: This requires a file system providing
69: higher bandwidth than the original 512 byte UNIX
70: one that provides only about
71: two percent of the maximum disk bandwidth or about
72: 20 kilobytes per second per arm [White80], [Smith81b].
73: .PP
74: Modifications have been made to the UNIX file system to improve
75: its performance.
76: Since the UNIX file system interface
77: is well understood and not inherently slow,
78: this development retained the abstraction and simply changed
79: the underlying implementation to increase its throughput.
80: Consequently, users of the system have not been faced with
81: massive software conversion.
82: .PP
83: Problems with file system performance have been dealt with
84: extensively in the literature; see [Smith81a] for a survey.
85: Previous work to improve the UNIX file system performance has been
86: done by [Ferrin82a].
87: The UNIX operating system drew many of its ideas from Multics,
88: a large, high performance operating system [Feiertag71].
89: Other work includes Hydra [Almes78],
90: Spice [Thompson80],
91: and a file system for a LISP environment [Symbolics81].
92: A good introduction to the physical latencies of disks is
93: described in [Pechura83].
94: .ds RH Old file system
95: .sp 2
96: .ne 1i
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