142 lines
6.3 KiB
Perl
142 lines
6.3 KiB
Perl
.\" $NetBSD: 2.t,v 1.3 2003/08/07 10:30:54 agc Exp $
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.\"
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.\" Copyright (c) 1986, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)2.t 8.1 (Berkeley) 6/8/93
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.\"
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.ds RH Old file system
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.NH
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Old File System
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.PP
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In the file system developed at Bell Laboratories
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(the ``traditional'' file system),
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each disk drive is divided into one or more
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partitions. Each of these disk partitions may contain
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one file system. A file system never spans multiple
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partitions.\(dg
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.FS
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\(dg By ``partition'' here we refer to the subdivision of
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physical space on a disk drive. In the traditional file
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system, as in the new file system, file systems are really
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located in logical disk partitions that may overlap. This
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overlapping is made available, for example,
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to allow programs to copy entire disk drives containing multiple
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file systems.
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.FE
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A file system is described by its super-block,
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which contains the basic parameters of the file system.
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These include the number of data blocks in the file system,
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a count of the maximum number of files,
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and a pointer to the \fIfree list\fP, a linked
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list of all the free blocks in the file system.
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.PP
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Within the file system are files.
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Certain files are distinguished as directories and contain
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pointers to files that may themselves be directories.
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Every file has a descriptor associated with it called an
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.I "inode".
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An inode contains information describing ownership of the file,
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time stamps marking last modification and access times for the file,
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and an array of indices that point to the data blocks for the file.
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For the purposes of this section, we assume that the first 8 blocks
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of the file are directly referenced by values stored
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in an inode itself*.
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.FS
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* The actual number may vary from system to system, but is usually in
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the range 5-13.
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.FE
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An inode may also contain references to indirect blocks
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containing further data block indices.
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In a file system with a 512 byte block size, a singly indirect
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block contains 128 further block addresses,
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a doubly indirect block contains 128 addresses of further singly indirect
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blocks,
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and a triply indirect block contains 128 addresses of further doubly indirect
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blocks.
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.PP
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A 150 megabyte traditional UNIX file system consists
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of 4 megabytes of inodes followed by 146 megabytes of data.
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This organization segregates the inode information from the data;
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thus accessing a file normally incurs a long seek from the
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file's inode to its data.
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Files in a single directory are not typically allocated
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consecutive slots in the 4 megabytes of inodes,
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causing many non-consecutive blocks of inodes
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to be accessed when executing
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operations on the inodes of several files in a directory.
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.PP
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The allocation of data blocks to files is also suboptimum.
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The traditional
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file system never transfers more than 512 bytes per disk transaction
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and often finds that the next sequential data block is not on the same
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cylinder, forcing seeks between 512 byte transfers.
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The combination of the small block size,
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limited read-ahead in the system,
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and many seeks severely limits file system throughput.
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.PP
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The first work at Berkeley on the UNIX file system attempted to improve both
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reliability and throughput.
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The reliability was improved by staging modifications
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to critical file system information so that they could
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either be completed or repaired cleanly by a program
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after a crash [Kowalski78].
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The file system performance was improved by a factor of more than two by
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changing the basic block size from 512 to 1024 bytes.
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The increase was because of two factors:
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each disk transfer accessed twice as much data,
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and most files could be described without need to access
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indirect blocks since the direct blocks contained twice as much data.
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The file system with these changes will henceforth be referred to as the
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.I "old file system."
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.PP
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This performance improvement gave a strong indication that
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increasing the block size was a good method for improving
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throughput.
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Although the throughput had doubled,
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the old file system was still using only about
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four percent of the disk bandwidth.
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The main problem was that although the free list was initially
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ordered for optimal access,
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it quickly became scrambled as files were created and removed.
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Eventually the free list became entirely random,
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causing files to have their blocks allocated randomly over the disk.
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This forced a seek before every block access.
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Although old file systems provided transfer rates of up
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to 175 kilobytes per second when they were first created,
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this rate deteriorated to 30 kilobytes per second after a
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few weeks of moderate use because of this
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randomization of data block placement.
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There was no way of restoring the performance of an old file system
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except to dump, rebuild, and restore the file system.
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Another possibility, as suggested by [Maruyama76],
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would be to have a process that periodically
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reorganized the data on the disk to restore locality.
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.ds RH New file system
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.sp 2
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.ne 1i
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