440 lines
14 KiB
Perl
440 lines
14 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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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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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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.\" @(#)3.t 8.1 (Berkeley) 6/5/93
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.\"
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.ds RH Fixing corrupted file systems
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.NH
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Fixing corrupted file systems
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.PP
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A file system
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can become corrupted in several ways.
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The most common of these ways are
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improper shutdown procedures
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and hardware failures.
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.PP
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File systems may become corrupted during an
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.I "unclean halt" .
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This happens when proper shutdown
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procedures are not observed,
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physically write-protecting a mounted file system,
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or a mounted file system is taken off-line.
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The most common operator procedural failure is forgetting to
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.I sync
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the system before halting the CPU.
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.PP
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File systems may become further corrupted if proper startup
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procedures are not observed, e.g.,
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not checking a file system for inconsistencies,
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and not repairing inconsistencies.
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Allowing a corrupted file system to be used (and, thus, to be modified
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further) can be disastrous.
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.PP
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Any piece of hardware can fail at any time.
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Failures
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can be as subtle as a bad block
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on a disk pack, or as blatant as a non-functional disk-controller.
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.NH 2
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Detecting and correcting corruption
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.PP
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Normally
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.I fsck
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is run non-interactively.
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In this mode it will only fix
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corruptions that are expected to occur from an unclean halt.
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These actions are a proper subset of the actions that
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.I fsck
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will take when it is running interactively.
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Throughout this paper we assume that
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.I fsck
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is being run interactively,
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and all possible errors can be encountered.
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When an inconsistency is discovered in this mode,
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.I fsck
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reports the inconsistency for the operator to
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chose a corrective action.
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.PP
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A quiescent\(dd
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.FS
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\(dd I.e., unmounted and not being written on.
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.FE
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file system may be checked for structural integrity
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by performing consistency checks on the
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redundant data intrinsic to a file system.
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The redundant data is either read from
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the file system,
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or computed from other known values.
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The file system
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.B must
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be in a quiescent state when
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.I fsck
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is run,
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since
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.I fsck
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is a multi-pass program.
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.PP
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In the following sections,
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we discuss methods to discover inconsistencies
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and possible corrective actions
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for the cylinder group blocks, the inodes, the indirect blocks, and
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the data blocks containing directory entries.
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.NH 2
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Super-block checking
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.PP
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The most commonly corrupted item in a file system
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is the summary information
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associated with the super-block.
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The summary information is prone to corruption
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because it is modified with every change to the file
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system's blocks or inodes,
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and is usually corrupted
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after an unclean halt.
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.PP
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The super-block is checked for inconsistencies
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involving file-system size, number of inodes,
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free-block count, and the free-inode count.
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The file-system size must be larger than the
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number of blocks used by the super-block
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and the number of blocks used by the list of inodes.
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The file-system size and layout information
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are the most critical pieces of information for
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.I fsck .
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While there is no way to actually check these sizes,
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since they are statically determined by
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.I newfs ,
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.I fsck
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can check that these sizes are within reasonable bounds.
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All other file system checks require that these sizes be correct.
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If
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.I fsck
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detects corruption in the static parameters of the default super-block,
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.I fsck
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requests the operator to specify the location of an alternate super-block.
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.NH 2
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Free block checking
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.PP
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.I Fsck
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checks that all the blocks
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marked as free in the cylinder group block maps
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are not claimed by any files.
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When all the blocks have been initially accounted for,
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.I fsck
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checks that
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the number of free blocks
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plus the number of blocks claimed by the inodes
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equals the total number of blocks in the file system.
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.PP
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If anything is wrong with the block allocation maps,
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.I fsck
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will rebuild them,
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based on the list it has computed of allocated blocks.
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.PP
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The summary information associated with the super-block
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counts the total number of free blocks within the file system.
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.I Fsck
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compares this count to the
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number of free blocks it found within the file system.
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If the two counts do not agree, then
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.I fsck
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replaces the incorrect count in the summary information
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by the actual free-block count.
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.PP
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The summary information
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counts the total number of free inodes within the file system.
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.I Fsck
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compares this count to the number
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of free inodes it found within the file system.
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If the two counts do not agree, then
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.I fsck
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replaces the incorrect count in the
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summary information by the actual free-inode count.
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.NH 2
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Checking the inode state
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.PP
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An individual inode is not as likely to be corrupted as
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the allocation information.
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However, because of the great number of active inodes,
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a few of the inodes are usually corrupted.
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.PP
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The list of inodes in the file system
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is checked sequentially starting with inode 2
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(inode 0 marks unused inodes;
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inode 1 is saved for future generations)
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and progressing through the last inode in the file system.
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The state of each inode is checked for
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inconsistencies involving format and type,
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link count,
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duplicate blocks,
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bad blocks,
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and inode size.
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.PP
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Each inode contains a mode word.
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This mode word describes the type and state of the inode.
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Inodes must be one of six types:
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regular inode, directory inode, symbolic link inode,
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special block inode, special character inode, or socket inode.
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Inodes may be found in one of three allocation states:
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unallocated, allocated, and neither unallocated nor allocated.
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This last state suggests an incorrectly formated inode.
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An inode can get in this state if
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bad data is written into the inode list.
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The only possible corrective action is for
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.I fsck
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is to clear the inode.
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.NH 2
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Inode links
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.PP
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Each inode counts the
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total number of directory entries
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linked to the inode.
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.I Fsck
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verifies the link count of each inode
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by starting at the root of the file system,
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and descending through the directory structure.
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The actual link count for each inode
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is calculated during the descent.
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.PP
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If the stored link count is non-zero and the actual
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link count is zero,
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then no directory entry appears for the inode.
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If this happens,
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.I fsck
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will place the disconnected file in the
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.I lost+found
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directory.
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If the stored and actual link counts are non-zero and unequal,
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a directory entry may have been added or removed without the inode being
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updated.
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If this happens,
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.I fsck
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replaces the incorrect stored link count by the actual link count.
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.PP
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Each inode contains a list,
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or pointers to
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lists (indirect blocks),
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of all the blocks claimed by the inode.
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Since indirect blocks are owned by an inode,
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inconsistencies in indirect blocks directly
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affect the inode that owns it.
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.PP
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.I Fsck
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compares each block number claimed by an inode
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against a list of already allocated blocks.
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If another inode already claims a block number,
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then the block number is added to a list of
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.I "duplicate blocks" .
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Otherwise, the list of allocated blocks
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is updated to include the block number.
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.PP
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If there are any duplicate blocks,
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.I fsck
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will perform a partial second
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pass over the inode list
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to find the inode of the duplicated block.
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The second pass is needed,
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since without examining the files associated with
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these inodes for correct content,
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not enough information is available
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to determine which inode is corrupted and should be cleared.
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If this condition does arise
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(only hardware failure will cause it),
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then the inode with the earliest
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modify time is usually incorrect,
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and should be cleared.
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If this happens,
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.I fsck
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prompts the operator to clear both inodes.
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The operator must decide which one should be kept
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and which one should be cleared.
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.PP
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.I Fsck
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checks the range of each block number claimed by an inode.
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If the block number is
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lower than the first data block in the file system,
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or greater than the last data block,
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then the block number is a
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.I "bad block number" .
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Many bad blocks in an inode are usually caused by
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an indirect block that was not written to the file system,
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a condition which can only occur if there has been a hardware failure.
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If an inode contains bad block numbers,
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.I fsck
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prompts the operator to clear it.
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.NH 2
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Inode data size
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.PP
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Each inode contains a count of the number of data blocks
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that it contains.
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The number of actual data blocks
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is the sum of the allocated data blocks
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and the indirect blocks.
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.I Fsck
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computes the actual number of data blocks
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and compares that block count against
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the actual number of blocks the inode claims.
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If an inode contains an incorrect count
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.I fsck
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prompts the operator to fix it.
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.PP
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Each inode contains a thirty-two bit size field.
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The size is the number of data bytes
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in the file associated with the inode.
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The consistency of the byte size field is roughly checked
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by computing from the size field the maximum number of blocks
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that should be associated with the inode,
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and comparing that expected block count against
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the actual number of blocks the inode claims.
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.NH 2
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Checking the data associated with an inode
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.PP
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An inode can directly or indirectly
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reference three kinds of data blocks.
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All referenced blocks must be the same kind.
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The three types of data blocks are:
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plain data blocks, symbolic link data blocks, and directory data blocks.
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Plain data blocks
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contain the information stored in a file;
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symbolic link data blocks
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contain the path name stored in a link.
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Directory data blocks contain directory entries.
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.I Fsck
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can only check the validity of directory data blocks.
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.PP
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Each directory data block is checked for
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several types of inconsistencies.
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These inconsistencies include
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directory inode numbers pointing to unallocated inodes,
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directory inode numbers that are greater than
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the number of inodes in the file system,
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incorrect directory inode numbers for ``\fB.\fP'' and ``\fB..\fP'',
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and directories that are not attached to the file system.
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If the inode number in a directory data block
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references an unallocated inode,
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then
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.I fsck
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will remove that directory entry.
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Again,
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this condition can only arise when there has been a hardware failure.
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.PP
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If a directory entry inode number references
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outside the inode list, then
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.I fsck
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will remove that directory entry.
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This condition occurs if bad data is written into a directory data block.
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.PP
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The directory inode number entry for ``\fB.\fP''
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must be the first entry in the directory data block.
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The inode number for ``\fB.\fP''
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must reference itself;
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e.g., it must equal the inode number
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for the directory data block.
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The directory inode number entry
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for ``\fB..\fP'' must be
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the second entry in the directory data block.
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Its value must equal the inode number for the
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parent of the directory entry
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(or the inode number of the directory
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data block if the directory is the
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root directory).
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If the directory inode numbers are
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incorrect,
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.I fsck
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will replace them with the correct values.
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If there are multiple hard links to a directory,
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the first one encountered is considered the real parent
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to which ``\fB..\fP'' should point;
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\fIfsck\fP recommends deletion for the subsequently discovered names.
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.NH 2
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File system connectivity
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.PP
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.I Fsck
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checks the general connectivity of the file system.
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If directories are not linked into the file system, then
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.I fsck
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links the directory back into the file system in the
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.I lost+found
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directory.
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This condition only occurs when there has been a hardware failure.
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.ds RH "References"
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.SH
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\s+2Acknowledgements\s0
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.PP
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I thank Bill Joy, Sam Leffler, Robert Elz and Dennis Ritchie
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for their suggestions and help in implementing the new file system.
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Thanks also to Robert Henry for his editorial input to
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get this document together.
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Finally we thank our sponsors,
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the National Science Foundation under grant MCS80-05144,
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and the Defense Advance Research Projects Agency (DoD) under
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Arpa Order No. 4031 monitored by Naval Electronic System Command under
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Contract No. N00039-82-C-0235. (Kirk McKusick, July 1983)
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.PP
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I would like to thank Larry A. Wehr for advice that lead
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to the first version of
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.I fsck
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and Rick B. Brandt for adapting
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.I fsck
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to
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UNIX/TS. (T. Kowalski, July 1979)
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.sp 2
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.SH
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\s+2References\s0
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.LP
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.IP [Dolotta78] 20
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Dolotta, T. A., and Olsson, S. B. eds.,
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.I "UNIX User's Manual, Edition 1.1\^" ,
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January 1978.
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.IP [Joy83] 20
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Joy, W., Cooper, E., Fabry, R., Leffler, S., McKusick, M., and Mosher, D.
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4.2BSD System Manual,
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.I "University of California at Berkeley" ,
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.I "Computer Systems Research Group Technical Report"
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#4, 1982.
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.IP [McKusick84] 20
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McKusick, M., Joy, W., Leffler, S., and Fabry, R.
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A Fast File System for UNIX,
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\fIACM Transactions on Computer Systems 2\fP, 3.
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pp. 181-197, August 1984.
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.IP [Ritchie78] 20
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Ritchie, D. M., and Thompson, K.,
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The UNIX Time-Sharing System,
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.I "The Bell System Technical Journal"
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.B 57 ,
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6 (July-August 1978, Part 2), pp. 1905-29.
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.IP [Thompson78] 20
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Thompson, K.,
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UNIX Implementation,
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.I "The Bell System Technical Journal\^"
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.B 57 ,
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6 (July-August 1978, Part 2), pp. 1931-46.
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.ds RH Appendix A \- Fsck Error Conditions
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.bp
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