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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: .\"    @(#)2.t 4.3 (Berkeley) 2/1/86
        !             6: .\"
        !             7: .ds RH Overview of the file system
        !             8: .NH
        !             9: Overview of the file system
        !            10: .PP
        !            11: The file system is discussed in detail in [Mckusick84];
        !            12: this section gives a brief overview.
        !            13: .NH 2
        !            14: Superblock
        !            15: .PP
        !            16: A file system is described by its
        !            17: .I "super-block" .
        !            18: The super-block is built when the file system is created (\c
        !            19: .I newfs (8))
        !            20: and never changes.
        !            21: The super-block
        !            22: contains the basic parameters of the file system,
        !            23: such as the number of data blocks it contains
        !            24: and a count of the maximum number of files.
        !            25: Because the super-block contains critical data,
        !            26: .I newfs
        !            27: replicates it to protect against catastrophic loss.
        !            28: The
        !            29: .I "default super block"
        !            30: always resides at a fixed offset from the beginning
        !            31: of the file system's disk partition.
        !            32: The
        !            33: .I "redundant super blocks"
        !            34: are not referenced unless a head crash
        !            35: or other hard disk error causes the default super-block
        !            36: to be unusable.
        !            37: The redundant blocks are sprinkled throughout the disk partition.
        !            38: .PP
        !            39: Within the file system are files.
        !            40: Certain files are distinguished as directories and contain collections
        !            41: of pointers to files that may themselves be directories.
        !            42: Every file has a descriptor associated with it called an
        !            43: .I "inode".
        !            44: The inode contains information describing ownership of the file,
        !            45: time stamps indicating modification and access times for the file,
        !            46: and an array of indices pointing to the data blocks for the file.
        !            47: In this section,
        !            48: we assume that the first 12 blocks
        !            49: of the file are directly referenced by values stored
        !            50: in the inode structure itself\(dg.
        !            51: .FS
        !            52: \(dgThe actual number may vary from system to system, but is usually in
        !            53: the range 5-13.
        !            54: .FE
        !            55: The inode structure may also contain references to indirect blocks
        !            56: containing further data block indices.
        !            57: In a file system with a 4096 byte block size, a singly indirect
        !            58: block contains 1024 further block addresses,
        !            59: a doubly indirect block contains 1024 addresses of further single indirect
        !            60: blocks,
        !            61: and a triply indirect block contains 1024 addresses of further doubly indirect
        !            62: blocks (the triple indirect block is never needed in practice).
        !            63: .PP
        !            64: In order to create files with up to
        !            65: 2\(ua32 bytes,
        !            66: using only two levels of indirection,
        !            67: the minimum size of a file system block is 4096 bytes.
        !            68: The size of file system blocks can be any power of two
        !            69: greater than or equal to 4096.
        !            70: The block size of the file system is maintained in the super-block,
        !            71: so it is possible for file systems of different block sizes
        !            72: to be accessible simultaneously on the same system.
        !            73: The block size must be decided when
        !            74: .I newfs
        !            75: creates the file system;
        !            76: the block size cannot be subsequently
        !            77: changed without rebuilding the file system.
        !            78: .NH 2
        !            79: Summary information
        !            80: .PP
        !            81: Associated with the super block is non replicated
        !            82: .I "summary information" .
        !            83: The summary information changes
        !            84: as the file system is modified.
        !            85: The summary information contains
        !            86: the number of blocks, fragments, inodes and directories in the file system.
        !            87: .NH 2
        !            88: Cylinder groups
        !            89: .PP
        !            90: The file system partitions the disk into one or more areas called
        !            91: .I "cylinder groups".
        !            92: A cylinder group is comprised of one or more consecutive
        !            93: cylinders on a disk.
        !            94: Each cylinder group includes inode slots for files, a
        !            95: .I "block map"
        !            96: describing available blocks in the cylinder group,
        !            97: and summary information describing the usage of data blocks
        !            98: within the cylinder group.
        !            99: A fixed number of inodes is allocated for each cylinder group
        !           100: when the file system is created.
        !           101: The current policy is to allocate one inode for each 2048
        !           102: bytes of disk space;
        !           103: this is expected to be far more inodes than will ever be needed.
        !           104: .PP
        !           105: All the cylinder group bookkeeping information could be
        !           106: placed at the beginning of each cylinder group.
        !           107: However if this approach were used,
        !           108: all the redundant information would be on the top platter.
        !           109: A single hardware failure that destroyed the top platter
        !           110: could cause the loss of all copies of the redundant super-blocks.
        !           111: Thus the cylinder group bookkeeping information
        !           112: begins at a floating offset from the beginning of the cylinder group.
        !           113: The offset for
        !           114: the
        !           115: .I "i+1" st
        !           116: cylinder group is about one track further
        !           117: from the beginning of the cylinder group
        !           118: than it was for the
        !           119: .I "i" th
        !           120: cylinder group.
        !           121: In this way,
        !           122: the redundant
        !           123: information spirals down into the pack;
        !           124: any single track, cylinder,
        !           125: or platter can be lost without losing all copies of the super-blocks.
        !           126: Except for the first cylinder group,
        !           127: the space between the beginning of the cylinder group
        !           128: and the beginning of the cylinder group information stores data.
        !           129: .NH 2
        !           130: Fragments
        !           131: .PP
        !           132: To avoid waste in storing small files,
        !           133: the file system space allocator divides a single
        !           134: file system block into one or more
        !           135: .I "fragments".
        !           136: The fragmentation of the file system is specified
        !           137: when the file system is created;
        !           138: each file system block can be optionally broken into
        !           139: 2, 4, or 8 addressable fragments.
        !           140: The lower bound on the size of these fragments is constrained
        !           141: by the disk sector size;
        !           142: typically 512 bytes is the lower bound on fragment size.
        !           143: The block map associated with each cylinder group
        !           144: records the space availability at the fragment level.
        !           145: Aligned fragments are examined
        !           146: to determine block availability.
        !           147: .PP
        !           148: On a file system with a block size of 4096 bytes
        !           149: and a fragment size of 1024 bytes,
        !           150: a file is represented by zero or more 4096 byte blocks of data,
        !           151: and possibly a single fragmented block.
        !           152: If a file system block must be fragmented to obtain
        !           153: space for a small amount of data,
        !           154: the remainder of the block is made available for allocation
        !           155: to other files.
        !           156: For example,
        !           157: consider an 11000 byte file stored on
        !           158: a 4096/1024 byte file system.
        !           159: This file uses two full size blocks and a 3072 byte fragment.
        !           160: If no fragments with at least 3072 bytes
        !           161: are available when the file is created,
        !           162: a full size block is split yielding the necessary 3072 byte
        !           163: fragment and an unused 1024 byte fragment.
        !           164: This remaining fragment can be allocated to another file, as needed.
        !           165: .NH 2
        !           166: Updates to the file system
        !           167: .PP
        !           168: Every working day hundreds of files
        !           169: are created, modified, and removed.
        !           170: Every time a file is modified,
        !           171: the operating system performs a
        !           172: series of file system updates.
        !           173: These updates, when written on disk, yield a consistent file system.
        !           174: The file system stages
        !           175: all modifications of critical information;
        !           176: modification can
        !           177: either be completed or cleanly backed out after a crash.
        !           178: Knowing the information that is first written to the file system,
        !           179: deterministic procedures can be developed to
        !           180: repair a corrupted file system.
        !           181: To understand this process,
        !           182: the order that the update
        !           183: requests were being honored must first be understood.
        !           184: .PP
        !           185: When a user program does an operation to change the file system,
        !           186: such as a 
        !           187: .I write ,
        !           188: the data to be written is copied into an internal
        !           189: .I "in-core"
        !           190: buffer in the kernel.
        !           191: Normally, the disk update is handled asynchronously;
        !           192: the user process is allowed to proceed even though
        !           193: the data has not yet been written to the disk.
        !           194: The data,
        !           195: along with the inode information reflecting the change,
        !           196: is eventually written out to disk.
        !           197: The real disk write may not happen until long after the
        !           198: .I write
        !           199: system call has returned.
        !           200: Thus at any given time, the file system,
        !           201: as it resides on the disk,
        !           202: lags the state of the file system represented by the in-core information.
        !           203: .PP
        !           204: The disk information is updated to reflect the in-core information
        !           205: when the buffer is required for another use,
        !           206: when a
        !           207: .I sync (2)
        !           208: is done (at 30 second intervals) by
        !           209: .I "/etc/update" "(8),"
        !           210: or by manual operator intervention with the
        !           211: .I sync (8)
        !           212: command.
        !           213: If the system is halted without writing out the in-core information,
        !           214: the file system on the disk will be in an inconsistent state.
        !           215: .PP
        !           216: If all updates are done asynchronously, several serious
        !           217: inconsistencies can arise.
        !           218: One inconsistency is that a block may be claimed by two inodes.
        !           219: Such an inconsistency can occur when the system is halted before
        !           220: the pointer to the block in the old inode has been cleared
        !           221: in the copy of the old inode on the disk,
        !           222: and after the pointer to the block in the new inode has been written out
        !           223: to the copy of the new inode on the disk.
        !           224: Here,
        !           225: there is no deterministic method for deciding
        !           226: which inode should really claim the block.
        !           227: A similar problem can arise with a multiply claimed inode.
        !           228: .PP
        !           229: The problem with asynchronous inode updates
        !           230: can be avoided by doing all inode deallocations synchronously. 
        !           231: Consequently,
        !           232: inodes and indirect blocks are written to the disk synchronously
        !           233: (\fIi.e.\fP the process blocks until the information is
        !           234: really written to disk)
        !           235: when they are being deallocated.
        !           236: Similarly inodes are kept consistent by synchronously
        !           237: deleting, adding, or changing directory entries.
        !           238: .ds RH Fixing corrupted file systems

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