263 lines
10 KiB
Perl
263 lines
10 KiB
Perl
.\" Copyright (c) 1982, 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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.\" 4. 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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.\" $FreeBSD$
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.\" @(#)2.t 8.1 (Berkeley) 6/5/93
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.\"
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.ds RH Overview of the file system
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.NH
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Overview of the file system
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.PP
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The file system is discussed in detail in [Mckusick84];
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this section gives a brief overview.
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.NH 2
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Superblock
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.PP
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A file system is described by its
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.I "super-block" .
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The super-block is built when the file system is created (\c
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.I newfs (8))
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and never changes.
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The super-block
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contains the basic parameters of the file system,
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such as the number of data blocks it contains
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and a count of the maximum number of files.
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Because the super-block contains critical data,
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.I newfs
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replicates it to protect against catastrophic loss.
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The
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.I "default super block"
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always resides at a fixed offset from the beginning
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of the file system's disk partition.
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The
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.I "redundant super blocks"
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are not referenced unless a head crash
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or other hard disk error causes the default super-block
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to be unusable.
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The redundant blocks are sprinkled throughout the disk partition.
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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 collections
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of 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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The inode contains information describing ownership of the file,
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time stamps indicating modification and access times for the file,
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and an array of indices pointing to the data blocks for the file.
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In this section,
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we assume that the first 12 blocks
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of the file are directly referenced by values stored
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in the inode structure itself\(dg.
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.FS
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\(dgThe 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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The inode structure 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 4096 byte block size, a singly indirect
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block contains 1024 further block addresses,
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a doubly indirect block contains 1024 addresses of further single indirect
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blocks,
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and a triply indirect block contains 1024 addresses of further doubly indirect
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blocks (the triple indirect block is never needed in practice).
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.PP
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In order to create files with up to
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2\(ua32 bytes,
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using only two levels of indirection,
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the minimum size of a file system block is 4096 bytes.
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The size of file system blocks can be any power of two
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greater than or equal to 4096.
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The block size of the file system is maintained in the super-block,
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so it is possible for file systems of different block sizes
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to be accessible simultaneously on the same system.
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The block size must be decided when
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.I newfs
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creates the file system;
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the block size cannot be subsequently
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changed without rebuilding the file system.
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.NH 2
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Summary information
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.PP
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Associated with the super block is non replicated
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.I "summary information" .
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The summary information changes
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as the file system is modified.
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The summary information contains
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the number of blocks, fragments, inodes and directories in the file system.
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.NH 2
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Cylinder groups
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.PP
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The file system partitions the disk into one or more areas called
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.I "cylinder groups".
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A cylinder group is comprised of one or more consecutive
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cylinders on a disk.
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Each cylinder group includes inode slots for files, a
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.I "block map"
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describing available blocks in the cylinder group,
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and summary information describing the usage of data blocks
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within the cylinder group.
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A fixed number of inodes is allocated for each cylinder group
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when the file system is created.
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The current policy is to allocate one inode for each 2048
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bytes of disk space;
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this is expected to be far more inodes than will ever be needed.
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.PP
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All the cylinder group bookkeeping information could be
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placed at the beginning of each cylinder group.
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However if this approach were used,
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all the redundant information would be on the top platter.
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A single hardware failure that destroyed the top platter
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could cause the loss of all copies of the redundant super-blocks.
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Thus the cylinder group bookkeeping information
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begins at a floating offset from the beginning of the cylinder group.
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The offset for
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the
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.I "i+1" st
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cylinder group is about one track further
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from the beginning of the cylinder group
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than it was for the
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.I "i" th
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cylinder group.
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In this way,
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the redundant
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information spirals down into the pack;
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any single track, cylinder,
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or platter can be lost without losing all copies of the super-blocks.
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Except for the first cylinder group,
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the space between the beginning of the cylinder group
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and the beginning of the cylinder group information stores data.
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.NH 2
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Fragments
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.PP
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To avoid waste in storing small files,
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the file system space allocator divides a single
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file system block into one or more
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.I "fragments".
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The fragmentation of the file system is specified
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when the file system is created;
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each file system block can be optionally broken into
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2, 4, or 8 addressable fragments.
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The lower bound on the size of these fragments is constrained
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by the disk sector size;
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typically 512 bytes is the lower bound on fragment size.
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The block map associated with each cylinder group
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records the space availability at the fragment level.
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Aligned fragments are examined
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to determine block availability.
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.PP
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On a file system with a block size of 4096 bytes
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and a fragment size of 1024 bytes,
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a file is represented by zero or more 4096 byte blocks of data,
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and possibly a single fragmented block.
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If a file system block must be fragmented to obtain
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space for a small amount of data,
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the remainder of the block is made available for allocation
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to other files.
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For example,
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consider an 11000 byte file stored on
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a 4096/1024 byte file system.
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This file uses two full size blocks and a 3072 byte fragment.
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If no fragments with at least 3072 bytes
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are available when the file is created,
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a full size block is split yielding the necessary 3072 byte
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fragment and an unused 1024 byte fragment.
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This remaining fragment can be allocated to another file, as needed.
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.NH 2
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Updates to the file system
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.PP
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Every working day hundreds of files
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are created, modified, and removed.
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Every time a file is modified,
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the operating system performs a
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series of file system updates.
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These updates, when written on disk, yield a consistent file system.
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The file system stages
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all modifications of critical information;
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modification can
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either be completed or cleanly backed out after a crash.
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Knowing the information that is first written to the file system,
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deterministic procedures can be developed to
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repair a corrupted file system.
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To understand this process,
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the order that the update
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requests were being honored must first be understood.
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.PP
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When a user program does an operation to change the file system,
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such as a
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.I write ,
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the data to be written is copied into an internal
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.I "in-core"
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buffer in the kernel.
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Normally, the disk update is handled asynchronously;
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the user process is allowed to proceed even though
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the data has not yet been written to the disk.
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The data,
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along with the inode information reflecting the change,
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is eventually written out to disk.
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The real disk write may not happen until long after the
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.I write
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system call has returned.
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Thus at any given time, the file system,
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as it resides on the disk,
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lags the state of the file system represented by the in-core information.
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.PP
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The disk information is updated to reflect the in-core information
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when the buffer is required for another use,
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when a
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.I sync (2)
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is done (at 30 second intervals) by
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.I "/etc/update" "(8),"
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or by manual operator intervention with the
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.I sync (8)
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command.
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If the system is halted without writing out the in-core information,
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the file system on the disk will be in an inconsistent state.
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.PP
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If all updates are done asynchronously, several serious
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inconsistencies can arise.
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One inconsistency is that a block may be claimed by two inodes.
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Such an inconsistency can occur when the system is halted before
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the pointer to the block in the old inode has been cleared
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in the copy of the old inode on the disk,
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and after the pointer to the block in the new inode has been written out
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to the copy of the new inode on the disk.
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Here,
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there is no deterministic method for deciding
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which inode should really claim the block.
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A similar problem can arise with a multiply claimed inode.
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.PP
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The problem with asynchronous inode updates
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can be avoided by doing all inode deallocations synchronously.
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Consequently,
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inodes and indirect blocks are written to the disk synchronously
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(\fIi.e.\fP the process blocks until the information is
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really written to disk)
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when they are being deallocated.
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Similarly inodes are kept consistent by synchronously
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deleting, adding, or changing directory entries.
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.ds RH Fixing corrupted file systems
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