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1.1     ! root        1: .\" Copyright (c) 1982 Regents of the University of California.
        !             2: .\" All rights reserved.  The Berkeley software License Agreement
        !             3: .\" specifies the terms and conditions for redistribution.
        !             4: .\"
        !             5: .\"    @(#)3.t 4.3 (Berkeley) 2/1/86
        !             6: .\"
        !             7: .ds RH Fixing corrupted file systems
        !             8: .NH
        !             9: Fixing corrupted file systems
        !            10: .PP
        !            11: A file system
        !            12: can become corrupted in several ways.
        !            13: The most common of these ways are
        !            14: improper shutdown procedures
        !            15: and hardware failures.
        !            16: .PP
        !            17: File systems may become corrupted during an
        !            18: .I "unclean halt" .
        !            19: This happens when proper shutdown
        !            20: procedures are not observed,
        !            21: physically write-protecting a mounted file system,
        !            22: or a mounted file system is taken off-line.
        !            23: The most common operator procedural failure is forgetting to
        !            24: .I sync
        !            25: the system before halting the CPU.
        !            26: .PP
        !            27: File systems may become further corrupted if proper startup
        !            28: procedures are not observed, e.g.,
        !            29: not checking a file system for inconsistencies,
        !            30: and not repairing inconsistencies.
        !            31: Allowing a corrupted file system to be used (and, thus, to be modified
        !            32: further) can be disastrous.
        !            33: .PP
        !            34: Any piece of hardware can fail at any time.
        !            35: Failures
        !            36: can be as subtle as a bad block
        !            37: on a disk pack, or as blatant as a non-functional disk-controller.
        !            38: .NH 2
        !            39: Detecting and correcting corruption
        !            40: .PP
        !            41: Normally
        !            42: .I fsck
        !            43: is run non-interactively.
        !            44: In this mode it will only fix
        !            45: corruptions that are expected to occur from an unclean halt.
        !            46: These actions are a proper subset of the actions that 
        !            47: .I fsck
        !            48: will take when it is running interactively.
        !            49: Throughout this paper we assume that 
        !            50: .I fsck 
        !            51: is being run interactively,
        !            52: and all possible errors can be encountered.
        !            53: When an inconsistency is discovered in this mode,
        !            54: .I fsck
        !            55: reports the inconsistency for the operator to
        !            56: chose a corrective action.
        !            57: .PP
        !            58: A quiescent\(dd
        !            59: .FS
        !            60: \(dd I.e., unmounted and not being written on.
        !            61: .FE
        !            62: file system may be checked for structural integrity
        !            63: by performing consistency checks on the
        !            64: redundant data intrinsic to a file system.
        !            65: The redundant data is either read from
        !            66: the file system,
        !            67: or computed from other known values.
        !            68: The file system
        !            69: .B must
        !            70: be in a quiescent state when
        !            71: .I fsck
        !            72: is run,
        !            73: since
        !            74: .I fsck
        !            75: is a multi-pass program.
        !            76: .PP
        !            77: In the following sections,
        !            78: we discuss methods to discover inconsistencies
        !            79: and possible corrective actions
        !            80: for the cylinder group blocks, the inodes, the indirect blocks, and
        !            81: the data blocks containing directory entries.
        !            82: .NH 2
        !            83: Super-block checking
        !            84: .PP
        !            85: The most commonly corrupted item in a file system
        !            86: is the summary information
        !            87: associated with the super-block.
        !            88: The summary information is prone to corruption
        !            89: because it is modified with every change to the file
        !            90: system's blocks or inodes,
        !            91: and is usually corrupted
        !            92: after an unclean halt.
        !            93: .PP
        !            94: The super-block is checked for inconsistencies
        !            95: involving file-system size, number of inodes,
        !            96: free-block count, and the free-inode count.
        !            97: The file-system size must be larger than the
        !            98: number of blocks used by the super-block
        !            99: and the number of blocks used by the list of inodes.
        !           100: The file-system size and layout information
        !           101: are the most critical pieces of information for
        !           102: .I fsck .
        !           103: While there is no way to actually check these sizes,
        !           104: since they are statically determined by
        !           105: .I newfs ,
        !           106: .I fsck
        !           107: can check that these sizes are within reasonable bounds.
        !           108: All other file system checks require that these sizes be correct.
        !           109: If
        !           110: .I fsck 
        !           111: detects corruption in the static parameters of the default super-block,
        !           112: .I fsck
        !           113: requests the operator to specify the location of an alternate super-block.
        !           114: .NH 2
        !           115: Free block checking
        !           116: .PP
        !           117: .I Fsck
        !           118: checks that all the blocks
        !           119: marked as free in the cylinder group block maps
        !           120: are not claimed by any files.
        !           121: When all the blocks have been initially accounted for,
        !           122: .I fsck
        !           123: checks that
        !           124: the number of free blocks
        !           125: plus the number of blocks claimed by the inodes
        !           126: equals the total number of blocks in the file system.
        !           127: .PP
        !           128: If anything is wrong with the block allocation maps,
        !           129: .I fsck
        !           130: will rebuild them,
        !           131: based on the list it has computed of allocated blocks.
        !           132: .PP
        !           133: The summary information associated with the super-block
        !           134: counts the total number of free blocks within the file system.
        !           135: .I Fsck
        !           136: compares this count to the
        !           137: number of free blocks it found within the file system.
        !           138: If the two counts do not agree, then
        !           139: .I fsck
        !           140: replaces the incorrect count in the summary information
        !           141: by the actual free-block count.
        !           142: .PP
        !           143: The summary information
        !           144: counts the total number of free inodes within the file system.
        !           145: .I Fsck
        !           146: compares this count to the number
        !           147: of free inodes it found within the file system.
        !           148: If the two counts do not agree, then
        !           149: .I fsck
        !           150: replaces the incorrect count in the
        !           151: summary information by the actual free-inode count.
        !           152: .NH 2
        !           153: Checking the inode state
        !           154: .PP
        !           155: An individual inode is not as likely to be corrupted as
        !           156: the allocation information.
        !           157: However, because of the great number of active inodes,
        !           158: a few of the inodes are usually corrupted.
        !           159: .PP
        !           160: The list of inodes in the file system
        !           161: is checked sequentially starting with inode 2
        !           162: (inode 0 marks unused inodes;
        !           163: inode 1 is saved for future generations)
        !           164: and progressing through the last inode in the file system.
        !           165: The state of each inode is checked for
        !           166: inconsistencies involving format and type,
        !           167: link count,
        !           168: duplicate blocks,
        !           169: bad blocks,
        !           170: and inode size.
        !           171: .PP
        !           172: Each inode contains a mode word.
        !           173: This mode word describes the type and state of the inode.
        !           174: Inodes must be one of six types:
        !           175: regular inode, directory inode, symbolic link inode,
        !           176: special block inode, special character inode, or socket inode.
        !           177: Inodes may be found in one of three allocation states:
        !           178: unallocated, allocated, and neither unallocated nor allocated.
        !           179: This last state suggests an incorrectly formated inode.
        !           180: An inode can get in this state if
        !           181: bad data is written into the inode list.
        !           182: The only possible corrective action is for
        !           183: .I fsck
        !           184: is to clear the inode.
        !           185: .NH 2
        !           186: Inode links
        !           187: .PP
        !           188: Each inode counts the
        !           189: total number of directory entries
        !           190: linked to the inode.
        !           191: .I Fsck
        !           192: verifies the link count of each inode
        !           193: by starting at the root of the file system,
        !           194: and descending through the directory structure.
        !           195: The actual link count for each inode
        !           196: is calculated during the descent.
        !           197: .PP
        !           198: If the stored link count is non-zero and the actual
        !           199: link count is zero,
        !           200: then no directory entry appears for the inode.
        !           201: If this happens,
        !           202: .I fsck
        !           203: will place the disconnected file in the
        !           204: .I lost+found
        !           205: directory.
        !           206: If the stored and actual link counts are non-zero and unequal,
        !           207: a directory entry may have been added or removed without the inode being
        !           208: updated.
        !           209: If this happens,
        !           210: .I fsck
        !           211: replaces the incorrect stored link count by the actual link count.
        !           212: .PP
        !           213: Each inode contains a list,
        !           214: or pointers to
        !           215: lists (indirect blocks),
        !           216: of all the blocks claimed by the inode.
        !           217: Since indirect blocks are owned by an inode,
        !           218: inconsistencies in indirect blocks directly
        !           219: affect the inode that owns it.
        !           220: .PP
        !           221: .I Fsck
        !           222: compares each block number claimed by an inode
        !           223: against a list of already allocated blocks.
        !           224: If another inode already claims a block number,
        !           225: then the block number is added to a list of
        !           226: .I "duplicate blocks" .
        !           227: Otherwise, the list of allocated blocks
        !           228: is updated to include the block number.
        !           229: .PP
        !           230: If there are any duplicate blocks,
        !           231: .I fsck
        !           232: will perform a partial second
        !           233: pass over the inode list
        !           234: to find the inode of the duplicated block.
        !           235: The second pass is needed,
        !           236: since without examining the files associated with
        !           237: these inodes for correct content,
        !           238: not enough information is available
        !           239: to determine which inode is corrupted and should be cleared.
        !           240: If this condition does arise
        !           241: (only hardware failure will cause it),
        !           242: then the inode with the earliest
        !           243: modify time is usually incorrect,
        !           244: and should be cleared.
        !           245: If this happens,
        !           246: .I fsck
        !           247: prompts the operator to clear both inodes.
        !           248: The operator must decide which one should be kept
        !           249: and which one should be cleared.
        !           250: .PP
        !           251: .I Fsck
        !           252: checks the range of each block number claimed by an inode.
        !           253: If the block number is
        !           254: lower than the first data block in the file system,
        !           255: or greater than the last data block,
        !           256: then the block number is a
        !           257: .I "bad block number" .
        !           258: Many bad blocks in an inode are usually caused by
        !           259: an indirect block that was not written to the file system,
        !           260: a condition which can only occur if there has been a hardware failure.
        !           261: If an inode contains bad block numbers,
        !           262: .I fsck
        !           263: prompts the operator to clear it.
        !           264: .NH 2
        !           265: Inode data size
        !           266: .PP
        !           267: Each inode contains a count of the number of data blocks
        !           268: that it contains.
        !           269: The number of actual data blocks
        !           270: is the sum of the allocated data blocks
        !           271: and the indirect blocks.
        !           272: .I Fsck
        !           273: computes the actual number of data blocks
        !           274: and compares that block count against
        !           275: the actual number of blocks the inode claims.
        !           276: If an inode contains an incorrect count
        !           277: .I fsck
        !           278: prompts the operator to fix it.
        !           279: .PP
        !           280: Each inode contains a thirty-two bit size field.
        !           281: The size is the number of data bytes
        !           282: in the file associated with the inode.
        !           283: The consistency of the byte size field is roughly checked
        !           284: by computing from the size field the maximum number of blocks
        !           285: that should be associated with the inode,
        !           286: and comparing that expected block count against
        !           287: the actual number of blocks the inode claims.
        !           288: .NH 2
        !           289: Checking the data associated with an inode
        !           290: .PP
        !           291: An inode can directly or indirectly
        !           292: reference three kinds of data blocks.
        !           293: All referenced blocks must be the same kind.
        !           294: The three types of data blocks are:
        !           295: plain data blocks, symbolic link data blocks, and directory data blocks.
        !           296: Plain data blocks
        !           297: contain the information stored in a file;
        !           298: symbolic link data blocks
        !           299: contain the path name stored in a link.
        !           300: Directory data blocks contain directory entries.
        !           301: .I Fsck
        !           302: can only check the validity of directory data blocks.
        !           303: .PP
        !           304: Each directory data block is checked for
        !           305: several types of inconsistencies.
        !           306: These inconsistencies include
        !           307: directory inode numbers pointing to unallocated inodes,
        !           308: directory inode numbers that are greater than
        !           309: the number of inodes in the file system,
        !           310: incorrect directory inode numbers for ``\fB.\fP'' and ``\fB..\fP'',
        !           311: and directories that are not attached to the file system.
        !           312: If the inode number in a directory data block
        !           313: references an unallocated inode,
        !           314: then
        !           315: .I fsck
        !           316: will remove that directory entry.
        !           317: Again,
        !           318: this condition can only arise when there has been a hardware failure.
        !           319: .PP
        !           320: If a directory entry inode number references
        !           321: outside the inode list, then
        !           322: .I fsck
        !           323: will remove that directory entry.
        !           324: This condition occurs if bad data is written into a directory data block.
        !           325: .PP
        !           326: The directory inode number entry for ``\fB.\fP''
        !           327: must be the first entry in the directory data block.
        !           328: The inode number for ``\fB.\fP''
        !           329: must reference itself;
        !           330: e.g., it must equal the inode number
        !           331: for the directory data block.
        !           332: The directory inode number entry
        !           333: for ``\fB..\fP'' must be
        !           334: the second entry in the directory data block.
        !           335: Its value must equal the inode number for the
        !           336: parent of the directory entry
        !           337: (or the inode number of the directory
        !           338: data block if the directory is the
        !           339: root directory).
        !           340: If the directory inode numbers are
        !           341: incorrect,
        !           342: .I fsck
        !           343: will replace them with the correct values.
        !           344: If there are multiple hard links to a directory,
        !           345: the first one encountered is considered the real parent
        !           346: to which ``\fB..\fP'' should point;
        !           347: \fIfsck\P recommends deletion for the subsequently discovered names.
        !           348: .NH 2
        !           349: File system connectivity
        !           350: .PP
        !           351: .I Fsck
        !           352: checks the general connectivity of the file system.
        !           353: If directories are not linked into the file system, then
        !           354: .I fsck
        !           355: links the directory back into the file system in the
        !           356: .I lost+found
        !           357: directory.
        !           358: This condition only occurs when there has been a hardware failure.
        !           359: .ds RH "References"
        !           360: .SH
        !           361: \s+2Acknowledgements\s0
        !           362: .PP
        !           363: I thank Bill Joy, Sam Leffler, Robert Elz and Dennis Ritchie 
        !           364: for their suggestions and help in implementing the new file system.
        !           365: Thanks also to Robert Henry for his editorial input to
        !           366: get this document together.
        !           367: Finally we thank our sponsors,
        !           368: the National Science Foundation under grant MCS80-05144,
        !           369: and the Defense Advance Research Projects Agency (DoD) under
        !           370: Arpa Order No. 4031 monitored by Naval Electronic System Command under
        !           371: Contract No. N00039-82-C-0235. (Kirk McKusick, July 1983)
        !           372: .PP
        !           373: I would like to thank Larry A. Wehr for advice that lead
        !           374: to the first version of
        !           375: .I fsck
        !           376: and Rick B. Brandt for adapting
        !           377: .I fsck
        !           378: to
        !           379: UNIX/TS. (T. Kowalski, July 1979)
        !           380: .sp 2
        !           381: .SH
        !           382: \s+2References\s0
        !           383: .LP
        !           384: .IP [Dolotta78] 20
        !           385: Dolotta, T. A., and Olsson, S. B. eds.,
        !           386: .I "UNIX User's Manual, Edition 1.1\^" ,
        !           387: January 1978.
        !           388: .IP [Joy83] 20
        !           389: Joy, W., Cooper, E., Fabry, R., Leffler, S., McKusick, M., and Mosher, D.
        !           390: 4.2BSD System Manual,
        !           391: .I "University of California at Berkeley" ,
        !           392: .I "Computer Systems Research Group Technical Report"
        !           393: #4, 1982.
        !           394: .IP [McKusick84] 20
        !           395: McKusick, M., Joy, W., Leffler, S., and Fabry, R.
        !           396: A Fast File System for UNIX,
        !           397: \fIACM Transactions on Computer Systems 2\fP, 3.
        !           398: pp. 181-197, August 1984.
        !           399: .IP [Ritchie78] 20
        !           400: Ritchie, D. M., and Thompson, K.,
        !           401: The UNIX Time-Sharing System,
        !           402: .I "The Bell System Technical Journal"
        !           403: .B 57 ,
        !           404: 6 (July-August 1978, Part 2), pp. 1905-29.
        !           405: .IP [Thompson78] 20
        !           406: Thompson, K.,
        !           407: UNIX Implementation,
        !           408: .I "The Bell System Technical Journal\^"
        !           409: .B 57 ,
        !           410: 6 (July-August 1978, Part 2), pp. 1931-46.
        !           411: .ds RH Appendix A \- Fsck Error Conditions
        !           412: .bp

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