1、ext2产生的历史

　　最早的Linux内核是从MINIX系统过渡发展而来的。Linux最早的文件系统就是MINIX文件系统。MINIX文件系统几乎到处都是bug，采用的是16bit偏移量，最大容量为64M，文件名最长为14字符。
　　Linux内核0.96版本，包含了虚拟文件系统(vitual file system, VFS)，VFS中提供了基础的API用于简化扩展新的文件系统——扩展的文件系统(extended file system, ext)基于VFS API。
　　1992年Linux发布的0.96内核版本拥有了Linux真正意义上的文件系统——扩展的文件系统(The Extended Filesystem, ext),ext采用虚拟文件系统(virtual file system, VFS)即基础的API接口简化扩展的文件系统添加于内核。ext使得单个文件的最大数据容量可达2G，文件名的最大长度扩展到了255个字符。但是ext中不支持文件访问atime,文件内容修改mtime,文件属性修改iNode中的ctime相互独立的时间戳。
　　为了解决文件的时间戳问题，1993年Linux 0.99版本内核推出，Linux推出了两种新的文件系统：xiafs和ext2（The Second Extended Filesystem）.ext2的许多设计理念与Berkeley Fast File System相同，具有扩展的理念，在磁盘数据结构中预留了扩展空间，给将来的新版文件系统的留下了扩展空间。
　　之后，ext2成了VFS API中许多新扩展的测试平台。例如VFS内含的POSIX访问控制协议草案和扩展属性协议最先内嵌于ext2，归功于ext2的易扩展和易于理解的中断机制。到了Linux的2.6.17内核版本，ext2中的文件最大可达2TB。目前，ext2在闪盘和固态硬盘领域其影响仍由于日志文件系统(ext3/ext4)。因为ext2没有ext3那样额外的日志写操作。因为频繁的写操作，会降低磁盘的使用寿命，相反固态硬盘的寿命因此得以延长。闪盘因为在挂载的时候没有访问时间戳atime，寿命也得意延长。

翻译自维基百科：https://en.wikipedia.org/wiki/Ext2

2、ext2文件系统的结构

在磁盘中，ext2文件系统的结构可以用以下图来描述：

注意：以上是一个磁盘分区，第一个block，是系统启动扇区（boot sector），其余的分区空间都给了ext2文件系统，ext2将该磁盘分区分成了若干个块组（block group 0-N）。

以下是ext2数据结构（ext2_fs.h）：

  1.          /*
  2.      2     *  linux/include/linux/ext2_fs.h
  3.      3     *
  4.      4     * Copyright (C) 1992, 1993, 1994, 1995
  5.      5     * Remy Card (card@masi.ibp.fr)
  6.      6     * Laboratoire MASI - Institut Blaise Pascal
  7.      7     * Universite Pierre et Marie Curie (Paris VI)
  8.      8     *
  9.      9     *  from
  10.     10     *
  11.     11     *  linux/include/linux/minix_fs.h
  12.     12     *
  13.     13     *  Copyright (C) 1991, 1992  Linus Torvalds
  14.     14     */
  15.  
  16.         #ifndef _LINUX_EXT2_FS_H
  17.         #define _LINUX_EXT2_FS_H
  18.  
  19.         #include <linux/types.h>
  20.  
  21.         /*
  22.     22     * The second extended filesystem constants/structures
  23.     23     */
  24.  
  25.         /*
  26.     26     * Define EXT2FS_DEBUG to produce debug messages
  27.     27     */
  28.         #undef EXT2FS_DEBUG
  29.  
  30.         /*
  31.     31     * Define EXT2_PREALLOCATE to preallocate data blocks for expanding files
  32.     32     */
  33.         #define EXT2_PREALLOCATE
  34.         #define EXT2_DEFAULT_PREALLOC_BLOCKS    8
  35.  
  36.         /*
  37.     37     * The second extended file system version
  38.     38     */
  39.         #define EXT2FS_DATE        "95/08/09"
  40.         #define EXT2FS_VERSION        "0.5b"
  41.  
  42.         /*
  43.     43     * Debug code
  44.     44     */
  45.         #ifdef EXT2FS_DEBUG
  46.         #    define ext2_debug(f, a...)    { \
  47.                             printk ("EXT2-fs DEBUG (%s, %d): %s:", \
  48.                                 __FILE__, __LINE__, __FUNCTION__); \
  49.                               printk (f, ## a); \
  50.                             }
  51.         #else
  52.         #    define ext2_debug(f, a...)    /**/
  53.         #endif
  54.  
  55.         /*
  56.     56     * Special inode numbers
  57.     57     */
  58.         #define    EXT2_BAD_INO         1    /* Bad blocks inode */
  59.         #define EXT2_ROOT_INO         2    /* Root inode */
  60.         #define EXT2_ACL_IDX_INO     3    /* ACL inode */
  61.         #define EXT2_ACL_DATA_INO     4    /* ACL inode */
  62.         #define EXT2_BOOT_LOADER_INO     5    /* Boot loader inode */
  63.         #define EXT2_UNDEL_DIR_INO     6    /* Undelete directory inode */
  64.  
  65.         /* First non-reserved inode for old ext2 filesystems */
  66.         #define EXT2_GOOD_OLD_FIRST_INO    11
  67.  
  68.         /*
  69.     69     * The second extended file system magic number
  70.     70     */
  71.         #define EXT2_SUPER_MAGIC    0xEF53
  72.  
  73.         /*
  74.     74     * Maximal count of links to a file
  75.     75     */
  76.         #define EXT2_LINK_MAX        32000
  77.  
  78.         /*
  79.     79     * Macro-instructions used to manage several block sizes
  80.     80     */
  81.         #define EXT2_MIN_BLOCK_SIZE        1024
  82.         #define    EXT2_MAX_BLOCK_SIZE        4096
  83.         #define EXT2_MIN_BLOCK_LOG_SIZE          10
  84.         #ifdef __KERNEL__
  85.         # define EXT2_BLOCK_SIZE(s)        ((s)->s_blocksize)
  86.         #else
  87.         # define EXT2_BLOCK_SIZE(s)        (EXT2_MIN_BLOCK_SIZE << (s)->s_log_block_size)
  88.         #endif
  89.         #define EXT2_ACLE_PER_BLOCK(s)        (EXT2_BLOCK_SIZE(s) / sizeof (struct ext2_acl_entry))
  90.         #define    EXT2_ADDR_PER_BLOCK(s)        (EXT2_BLOCK_SIZE(s) / sizeof (__u32))
  91.         #ifdef __KERNEL__
  92.         # define EXT2_BLOCK_SIZE_BITS(s)    ((s)->s_blocksize_bits)
  93.         #else
  94.         # define EXT2_BLOCK_SIZE_BITS(s)    ((s)->s_log_block_size + )
  95.         #endif
  96.         #ifdef __KERNEL__
  97.         #define    EXT2_ADDR_PER_BLOCK_BITS(s)    ((s)->u.ext2_sb.s_addr_per_block_bits)
  98.         #define EXT2_INODE_SIZE(s)        ((s)->u.ext2_sb.s_inode_size)
  99.         #define EXT2_FIRST_INO(s)        ((s)->u.ext2_sb.s_first_ino)
  100.        #else
  101.        #define EXT2_INODE_SIZE(s)    (((s)->s_rev_level == EXT2_GOOD_OLD_REV) ? \
  102.                         EXT2_GOOD_OLD_INODE_SIZE : \
  103.                         (s)->s_inode_size)
  104.        #define EXT2_FIRST_INO(s)    (((s)->s_rev_level == EXT2_GOOD_OLD_REV) ? \
  105.                         EXT2_GOOD_OLD_FIRST_INO : \
  106.                         (s)->s_first_ino)
  107.        #endif
  108.  
  109.        /*
  110.    110     * Macro-instructions used to manage fragments
  111.    111     */
  112.        #define EXT2_MIN_FRAG_SIZE        1024
  113.        #define    EXT2_MAX_FRAG_SIZE        4096
  114.        #define EXT2_MIN_FRAG_LOG_SIZE          10
  115.        #ifdef __KERNEL__
  116.        # define EXT2_FRAG_SIZE(s)        ((s)->u.ext2_sb.s_frag_size)
  117.        # define EXT2_FRAGS_PER_BLOCK(s)    ((s)->u.ext2_sb.s_frags_per_block)
  118.        #else
  119.        # define EXT2_FRAG_SIZE(s)        (EXT2_MIN_FRAG_SIZE << (s)->s_log_frag_size)
  120.        # define EXT2_FRAGS_PER_BLOCK(s)    (EXT2_BLOCK_SIZE(s) / EXT2_FRAG_SIZE(s))
  121.        #endif
  122.  
  123.        /*
  124.    124     * ACL structures
  125.    125     */
  126.        struct ext2_acl_header    /* Header of Access Control Lists */
  127.        {
  128.            __u32    aclh_size;
  129.            __u32    aclh_file_count;
  130.            __u32    aclh_acle_count;
  131.            __u32    aclh_first_acle;
  132.        };
  133.  
  134.        struct ext2_acl_entry    /* Access Control List Entry */
  135.        {
  136.            __u32    acle_size;
  137.            __u16    acle_perms;    /* Access permissions */
  138.            __u16    acle_type;    /* Type of entry */
  139.            __u16    acle_tag;    /* User or group identity */
  140.            __u16    acle_pad1;
  141.            __u32    acle_next;    /* Pointer on next entry for the */
  142.                            /* same inode or on next free entry */
  143.        };
  144.  
  145.        /*
  146.    146     * Structure of a blocks group descriptor
  147.    147     */
  148.        struct ext2_group_desc
  149.        {
  150.            __u32    bg_block_bitmap;        /* Blocks bitmap block */
  151.            __u32    bg_inode_bitmap;        /* Inodes bitmap block */
  152.            __u32    bg_inode_table;        /* Inodes table block */
  153.            __u16    bg_free_blocks_count;    /* Free blocks count */
  154.            __u16    bg_free_inodes_count;    /* Free inodes count */
  155.            __u16    bg_used_dirs_count;    /* Directories count */
  156.            __u16    bg_pad;
  157.            __u32    bg_reserved[];
  158.        };
  159.  
  160.        /*
  161.    161     * Macro-instructions used to manage group descriptors
  162.    162     */
  163.        #ifdef __KERNEL__
  164.        # define EXT2_BLOCKS_PER_GROUP(s)    ((s)->u.ext2_sb.s_blocks_per_group)
  165.        # define EXT2_DESC_PER_BLOCK(s)        ((s)->u.ext2_sb.s_desc_per_block)
  166.        # define EXT2_INODES_PER_GROUP(s)    ((s)->u.ext2_sb.s_inodes_per_group)
  167.        # define EXT2_DESC_PER_BLOCK_BITS(s)    ((s)->u.ext2_sb.s_desc_per_block_bits)
  168.        #else
  169.        # define EXT2_BLOCKS_PER_GROUP(s)    ((s)->s_blocks_per_group)
  170.        # define EXT2_DESC_PER_BLOCK(s)        (EXT2_BLOCK_SIZE(s) / sizeof (struct ext2_group_desc))
  171.        # define EXT2_INODES_PER_GROUP(s)    ((s)->s_inodes_per_group)
  172.        #endif
  173.  
  174.        /*
  175.    175     * Constants relative to the data blocks
  176.    176     */
  177.        #define    EXT2_NDIR_BLOCKS        12
  178.        #define    EXT2_IND_BLOCK            EXT2_NDIR_BLOCKS
  179.        #define    EXT2_DIND_BLOCK            (EXT2_IND_BLOCK + 1)
  180.        #define    EXT2_TIND_BLOCK            (EXT2_DIND_BLOCK + 1)
  181.        #define    EXT2_N_BLOCKS            (EXT2_TIND_BLOCK + 1)
  182.  
  183.        /*
  184.    184     * Inode flags
  185.    185     */
  186.        #define    EXT2_SECRM_FL            0x00000001 /* Secure deletion */
  187.        #define    EXT2_UNRM_FL            0x00000002 /* Undelete */
  188.        #define    EXT2_COMPR_FL            0x00000004 /* Compress file */
  189.        #define EXT2_SYNC_FL            0x00000008 /* Synchronous updates */
  190.        #define EXT2_IMMUTABLE_FL        0x00000010 /* Immutable file */
  191.        #define EXT2_APPEND_FL            0x00000020 /* writes to file may only append */
  192.        #define EXT2_NODUMP_FL            0x00000040 /* do not dump file */
  193.        #define EXT2_NOATIME_FL            0x00000080 /* do not update atime */
  194.        /* Reserved for compression usage... */
  195.        #define EXT2_DIRTY_FL            0x00000100
  196.        #define EXT2_COMPRBLK_FL        0x00000200 /* One or more compressed clusters */
  197.        #define EXT2_NOCOMP_FL            0x00000400 /* Don't compress */
  198.        #define EXT2_ECOMPR_FL            0x00000800 /* Compression error */
  199.        /* End compression flags --- maybe not all used */
  200.        #define EXT2_BTREE_FL            0x00001000 /* btree format dir */
  201.        #define EXT2_RESERVED_FL        0x80000000 /* reserved for ext2 lib */
  202.  
  203.        #define EXT2_FL_USER_VISIBLE        0x00001FFF /* User visible flags */
  204.        #define EXT2_FL_USER_MODIFIABLE        0x000000FF /* User modifiable flags */
  205.  
  206.        /*
  207.    207     * ioctl commands
  208.    208     */
  209.        #define    EXT2_IOC_GETFLAGS        _IOR('f', 1, long)
  210.        #define    EXT2_IOC_SETFLAGS        _IOW('f', 2, long)
  211.        #define    EXT2_IOC_GETVERSION        _IOR('v', 1, long)
  212.        #define    EXT2_IOC_SETVERSION        _IOW('v', 2, long)
  213.  
  214.        /*
  215.    215     * Structure of an inode on the disk
  216.    216     */
  217.        struct ext2_inode {
  218.            __u16    i_mode;        /* File mode */
  219.            __u16    i_uid;        /* Low 16 bits of Owner Uid */
  220.            __u32    i_size;        /* Size in bytes */
  221.            __u32    i_atime;    /* Access time */
  222.            __u32    i_ctime;    /* Creation time */
  223.            __u32    i_mtime;    /* Modification time */
  224.            __u32    i_dtime;    /* Deletion Time */
  225.            __u16    i_gid;        /* Low 16 bits of Group Id */
  226.            __u16    i_links_count;    /* Links count */
  227.            __u32    i_blocks;    /* Blocks count */
  228.            __u32    i_flags;    /* File flags */
  229.            union {
  230.                struct {
  231.                    __u32  l_i_reserved1;
  232.                } linux1;
  233.                struct {
  234.                    __u32  h_i_translator;
  235.                } hurd1;
  236.                struct {
  237.                    __u32  m_i_reserved1;
  238.                } masix1;
  239.            } osd1;                /* OS dependent 1 */
  240.            __u32    i_block[EXT2_N_BLOCKS];/* Pointers to blocks */
  241.            __u32    i_generation;    /* File version (for NFS) */
  242.            __u32    i_file_acl;    /* File ACL */
  243.            __u32    i_dir_acl;    /* Directory ACL */
  244.            __u32    i_faddr;    /* Fragment address */
  245.            union {
  246.                struct {
  247.                    __u8    l_i_frag;    /* Fragment number */
  248.                    __u8    l_i_fsize;    /* Fragment size */
  249.                    __u16    i_pad1;
  250.                    __u16    l_i_uid_high;    /* these 2 fields    */
  251.                    __u16    l_i_gid_high;    /* were reserved2[0] */
  252.                    __u32    l_i_reserved2;
  253.                } linux2;
  254.                struct {
  255.                    __u8    h_i_frag;    /* Fragment number */
  256.                    __u8    h_i_fsize;    /* Fragment size */
  257.                    __u16    h_i_mode_high;
  258.                    __u16    h_i_uid_high;
  259.                    __u16    h_i_gid_high;
  260.                    __u32    h_i_author;
  261.                } hurd2;
  262.                struct {
  263.                    __u8    m_i_frag;    /* Fragment number */
  264.                    __u8    m_i_fsize;    /* Fragment size */
  265.                    __u16    m_pad1;
  266.                    __u32    m_i_reserved2[];
  267.                } masix2;
  268.            } osd2;                /* OS dependent 2 */
  269.        };
  270.  
  271.        #define i_size_high    i_dir_acl
  272.  
  273.        #if defined(__KERNEL__) || defined(__linux__)
  274.        #define i_reserved1    osd1.linux1.l_i_reserved1
  275.        #define i_frag        osd2.linux2.l_i_frag
  276.        #define i_fsize        osd2.linux2.l_i_fsize
  277.        #define i_uid_low    i_uid
  278.        #define i_gid_low    i_gid
  279.        #define i_uid_high    osd2.linux2.l_i_uid_high
  280.        #define i_gid_high    osd2.linux2.l_i_gid_high
  281.        #define i_reserved2    osd2.linux2.l_i_reserved2
  282.        #endif
  283.  
  284.        #ifdef    __hurd__
  285.        #define i_translator    osd1.hurd1.h_i_translator
  286.        #define i_frag        osd2.hurd2.h_i_frag;
  287.        #define i_fsize        osd2.hurd2.h_i_fsize;
  288.        #define i_uid_high    osd2.hurd2.h_i_uid_high
  289.        #define i_gid_high    osd2.hurd2.h_i_gid_high
  290.        #define i_author    osd2.hurd2.h_i_author
  291.        #endif
  292.  
  293.        #ifdef    __masix__
  294.        #define i_reserved1    osd1.masix1.m_i_reserved1
  295.        #define i_frag        osd2.masix2.m_i_frag
  296.        #define i_fsize        osd2.masix2.m_i_fsize
  297.        #define i_reserved2    osd2.masix2.m_i_reserved2
  298.        #endif
  299.  
  300.        /*
  301.    301     * File system states
  302.    302     */
  303.        #define    EXT2_VALID_FS            0x0001    /* Unmounted cleanly */
  304.        #define    EXT2_ERROR_FS            0x0002    /* Errors detected */
  305.  
  306.        /*
  307.    307     * Mount flags
  308.    308     */
  309.        #define EXT2_MOUNT_CHECK        0x0001    /* Do mount-time checks */
  310.        #define EXT2_MOUNT_GRPID        0x0004    /* Create files with directory's group */
  311.        #define EXT2_MOUNT_DEBUG        0x0008    /* Some debugging messages */
  312.        #define EXT2_MOUNT_ERRORS_CONT        0x0010    /* Continue on errors */
  313.        #define EXT2_MOUNT_ERRORS_RO        0x0020    /* Remount fs ro on errors */
  314.        #define EXT2_MOUNT_ERRORS_PANIC        0x0040    /* Panic on errors */
  315.        #define EXT2_MOUNT_MINIX_DF        0x0080    /* Mimics the Minix statfs */
  316.        #define EXT2_MOUNT_NO_UID32        0x0200  /* Disable 32-bit UIDs */
  317.  
  318.        #define clear_opt(o, opt)        o &= ~EXT2_MOUNT_##opt
  319.        #define set_opt(o, opt)            o |= EXT2_MOUNT_##opt
  320.        #define test_opt(sb, opt)        ((sb)->u.ext2_sb.s_mount_opt & \
  321.                             EXT2_MOUNT_##opt)
  322.        /*
  323.    323     * Maximal mount counts between two filesystem checks
  324.    324     */
  325.        #define EXT2_DFL_MAX_MNT_COUNT        20    /* Allow 20 mounts */
  326.        #define EXT2_DFL_CHECKINTERVAL        0    /* Don't use interval check */
  327.  
  328.        /*
  329.    329     * Behaviour when detecting errors
  330.    330     */
  331.        #define EXT2_ERRORS_CONTINUE        1    /* Continue execution */
  332.        #define EXT2_ERRORS_RO            2    /* Remount fs read-only */
  333.        #define EXT2_ERRORS_PANIC        3    /* Panic */
  334.        #define EXT2_ERRORS_DEFAULT        EXT2_ERRORS_CONTINUE
  335.  
  336.        /*
  337.    337     * Structure of the super block
  338.    338     */
  339.        struct ext2_super_block {
  340.            __u32    s_inodes_count;        /* Inodes count */
  341.            __u32    s_blocks_count;        /* Blocks count */
  342.            __u32    s_r_blocks_count;    /* Reserved blocks count */
  343.            __u32    s_free_blocks_count;    /* Free blocks count */
  344.            __u32    s_free_inodes_count;    /* Free inodes count */
  345.            __u32    s_first_data_block;    /* First Data Block */
  346.            __u32    s_log_block_size;    /* Block size */
  347.            __s32    s_log_frag_size;    /* Fragment size */
  348.            __u32    s_blocks_per_group;    /* # Blocks per group */
  349.            __u32    s_frags_per_group;    /* # Fragments per group */
  350.            __u32    s_inodes_per_group;    /* # Inodes per group */
  351.            __u32    s_mtime;        /* Mount time */
  352.            __u32    s_wtime;        /* Write time */
  353.            __u16    s_mnt_count;        /* Mount count */
  354.            __s16    s_max_mnt_count;    /* Maximal mount count */
  355.            __u16    s_magic;        /* Magic signature */
  356.            __u16    s_state;        /* File system state */
  357.            __u16    s_errors;        /* Behaviour when detecting errors */
  358.            __u16    s_minor_rev_level;     /* minor revision level */
  359.            __u32    s_lastcheck;        /* time of last check */
  360.            __u32    s_checkinterval;    /* max. time between checks */
  361.            __u32    s_creator_os;        /* OS */
  362.            __u32    s_rev_level;        /* Revision level */
  363.            __u16    s_def_resuid;        /* Default uid for reserved blocks */
  364.            __u16    s_def_resgid;        /* Default gid for reserved blocks */
  365.            /*
  366.    366         * These fields are for EXT2_DYNAMIC_REV superblocks only.
  367.    367         *
  368.    368         * Note: the difference between the compatible feature set and
  369.    369         * the incompatible feature set is that if there is a bit set
  370.    370         * in the incompatible feature set that the kernel doesn't
  371.    371         * know about, it should refuse to mount the filesystem.
  372.    372         *
  373.    373         * e2fsck's requirements are more strict; if it doesn't know
  374.    374         * about a feature in either the compatible or incompatible
  375.    375         * feature set, it must abort and not try to meddle with
  376.    376         * things it doesn't understand...
  377.    377         */
  378.            __u32    s_first_ino;         /* First non-reserved inode */
  379.            __u16   s_inode_size;         /* size of inode structure */
  380.            __u16    s_block_group_nr;     /* block group # of this superblock */
  381.            __u32    s_feature_compat;     /* compatible feature set */
  382.            __u32    s_feature_incompat;     /* incompatible feature set */
  383.            __u32    s_feature_ro_compat;     /* readonly-compatible feature set */
  384.            __u8    s_uuid[];        /* 128-bit uuid for volume */
  385.            char    s_volume_name[];     /* volume name */
  386.            char    s_last_mounted[];     /* directory where last mounted */
  387.            __u32    s_algorithm_usage_bitmap; /* For compression */
  388.            /*
  389.    389         * Performance hints.  Directory preallocation should only
  390.    390         * happen if the EXT2_COMPAT_PREALLOC flag is on.
  391.    391         */
  392.            __u8    s_prealloc_blocks;    /* Nr of blocks to try to preallocate*/
  393.            __u8    s_prealloc_dir_blocks;    /* Nr to preallocate for dirs */
  394.            __u16    s_padding1;
  395.            __u32    s_reserved[];    /* Padding to the end of the block */
  396.        };
  397.  
  398.        #ifdef __KERNEL__
  399.        #define EXT2_SB(sb)    (&((sb)->u.ext2_sb))
  400.        #else
  401.        /* Assume that user mode programs are passing in an ext2fs superblock, not
  402.    402     * a kernel struct super_block.  This will allow us to call the feature-test
  403.    403     * macros from user land. */
  404.        #define EXT2_SB(sb)    (sb)
  405.        #endif
  406.  
  407.        /*
  408.    408     * Codes for operating systems
  409.    409     */
  410.        #define EXT2_OS_LINUX        0
  411.        #define EXT2_OS_HURD        1
  412.        #define EXT2_OS_MASIX        2
  413.        #define EXT2_OS_FREEBSD        3
  414.        #define EXT2_OS_LITES        4
  415.  
  416.        /*
  417.    417     * Revision levels
  418.    418     */
  419.        #define EXT2_GOOD_OLD_REV    0    /* The good old (original) format */
  420.        #define EXT2_DYNAMIC_REV    1     /* V2 format w/ dynamic inode sizes */
  421.  
  422.        #define EXT2_CURRENT_REV    EXT2_GOOD_OLD_REV
  423.        #define EXT2_MAX_SUPP_REV    EXT2_DYNAMIC_REV
  424.  
  425.        #define EXT2_GOOD_OLD_INODE_SIZE 128
  426.  
  427.        /*
  428.    428     * Feature set definitions
  429.    429     */
  430.  
  431.        #define EXT2_HAS_COMPAT_FEATURE(sb,mask)            \
  432.            ( EXT2_SB(sb)->s_es->s_feature_compat & cpu_to_le32(mask) )
  433.        #define EXT2_HAS_RO_COMPAT_FEATURE(sb,mask)            \
  434.            ( EXT2_SB(sb)->s_es->s_feature_ro_compat & cpu_to_le32(mask) )
  435.        #define EXT2_HAS_INCOMPAT_FEATURE(sb,mask)            \
  436.            ( EXT2_SB(sb)->s_es->s_feature_incompat & cpu_to_le32(mask) )
  437.        #define EXT2_SET_COMPAT_FEATURE(sb,mask)            \
  438.            EXT2_SB(sb)->s_es->s_feature_compat |= cpu_to_le32(mask)
  439.        #define EXT2_SET_RO_COMPAT_FEATURE(sb,mask)            \
  440.            EXT2_SB(sb)->s_es->s_feature_ro_compat |= cpu_to_le32(mask)
  441.        #define EXT2_SET_INCOMPAT_FEATURE(sb,mask)            \
  442.            EXT2_SB(sb)->s_es->s_feature_incompat |= cpu_to_le32(mask)
  443.        #define EXT2_CLEAR_COMPAT_FEATURE(sb,mask)            \
  444.            EXT2_SB(sb)->s_es->s_feature_compat &= ~cpu_to_le32(mask)
  445.        #define EXT2_CLEAR_RO_COMPAT_FEATURE(sb,mask)            \
  446.            EXT2_SB(sb)->s_es->s_feature_ro_compat &= ~cpu_to_le32(mask)
  447.        #define EXT2_CLEAR_INCOMPAT_FEATURE(sb,mask)            \
  448.            EXT2_SB(sb)->s_es->s_feature_incompat &= ~cpu_to_le32(mask)
  449.  
  450.        #define EXT2_FEATURE_COMPAT_DIR_PREALLOC    0x0001
  451.        #define EXT2_FEATURE_COMPAT_IMAGIC_INODES    0x0002
  452.        #define EXT3_FEATURE_COMPAT_HAS_JOURNAL        0x0004
  453.        #define EXT2_FEATURE_COMPAT_EXT_ATTR        0x0008
  454.        #define EXT2_FEATURE_COMPAT_RESIZE_INO        0x0010
  455.        #define EXT2_FEATURE_COMPAT_DIR_INDEX        0x0020
  456.        #define EXT2_FEATURE_COMPAT_ANY            0xffffffff
  457.  
  458.        #define EXT2_FEATURE_RO_COMPAT_SPARSE_SUPER    0x0001
  459.        #define EXT2_FEATURE_RO_COMPAT_LARGE_FILE    0x0002
  460.        #define EXT2_FEATURE_RO_COMPAT_BTREE_DIR    0x0004
  461.        #define EXT2_FEATURE_RO_COMPAT_ANY        0xffffffff
  462.  
  463.        #define EXT2_FEATURE_INCOMPAT_COMPRESSION    0x0001
  464.        #define EXT2_FEATURE_INCOMPAT_FILETYPE        0x0002
  465.        #define EXT3_FEATURE_INCOMPAT_RECOVER        0x0004
  466.        #define EXT3_FEATURE_INCOMPAT_JOURNAL_DEV    0x0008
  467.        #define EXT2_FEATURE_INCOMPAT_ANY        0xffffffff
  468.  
  469.        #define EXT2_FEATURE_COMPAT_SUPP    0
  470.        #define EXT2_FEATURE_INCOMPAT_SUPP    EXT2_FEATURE_INCOMPAT_FILETYPE
  471.        #define EXT2_FEATURE_RO_COMPAT_SUPP    (EXT2_FEATURE_RO_COMPAT_SPARSE_SUPER| \
  472.                             EXT2_FEATURE_RO_COMPAT_LARGE_FILE| \
  473.                             EXT2_FEATURE_RO_COMPAT_BTREE_DIR)
  474.        #define EXT2_FEATURE_RO_COMPAT_UNSUPPORTED    ~EXT2_FEATURE_RO_COMPAT_SUPP
  475.        #define EXT2_FEATURE_INCOMPAT_UNSUPPORTED    ~EXT2_FEATURE_INCOMPAT_SUPP
  476.  
  477.        /*
  478.    478     * Default values for user and/or group using reserved blocks
  479.    479     */
  480.        #define    EXT2_DEF_RESUID        0
  481.        #define    EXT2_DEF_RESGID        0
  482.  
  483.        /*
  484.    484     * Structure of a directory entry
  485.    485     */
  486.        #define EXT2_NAME_LEN 255
  487.  
  488.        struct ext2_dir_entry {
  489.            __u32    inode;            /* Inode number */
  490.            __u16    rec_len;        /* Directory entry length */
  491.            __u16    name_len;        /* Name length */
  492.            char    name[EXT2_NAME_LEN];    /* File name */
  493.        };
  494.  
  495.        /*
  496.    496     * The new version of the directory entry.  Since EXT2 structures are
  497.    497     * stored in intel byte order, and the name_len field could never be
  498.    498     * bigger than 255 chars, it's safe to reclaim the extra byte for the
  499.    499     * file_type field.
  500.    500     */
  501.        struct ext2_dir_entry_2 {
  502.            __u32    inode;            /* Inode number */
  503.            __u16    rec_len;        /* Directory entry length */
  504.            __u8    name_len;        /* Name length */
  505.            __u8    file_type;
  506.            char    name[EXT2_NAME_LEN];    /* File name */
  507.        };
  508.  
  509.        /*
  510.    510     * Ext2 directory file types.  Only the low 3 bits are used.  The
  511.    511     * other bits are reserved for now.
  512.    512     */
  513.        enum {
  514.            EXT2_FT_UNKNOWN,
  515.            EXT2_FT_REG_FILE,
  516.            EXT2_FT_DIR,
  517.            EXT2_FT_CHRDEV,
  518.            EXT2_FT_BLKDEV,
  519.            EXT2_FT_FIFO,
  520.            EXT2_FT_SOCK,
  521.            EXT2_FT_SYMLINK,
  522.            EXT2_FT_MAX
  523.        };
  524.  
  525.        /*
  526.    526     * EXT2_DIR_PAD defines the directory entries boundaries
  527.    527     *
  528.    528     * NOTE: It must be a multiple of 4
  529.    529     */
  530.        #define EXT2_DIR_PAD             4
  531.        #define EXT2_DIR_ROUND             (EXT2_DIR_PAD - 1)
  532.        #define EXT2_DIR_REC_LEN(name_len)    (((name_len) + 8 + EXT2_DIR_ROUND) & \
  533.                             ~EXT2_DIR_ROUND)
  534.  
  535.        #ifdef __KERNEL__
  536.        /*
  537.    537     * Function prototypes
  538.    538     */
  539.  
  540.        /*
  541.    541     * Ok, these declarations are also in <linux/kernel.h> but none of the
  542.    542     * ext2 source programs needs to include it so they are duplicated here.
  543.    543     */
  544.        # define NORET_TYPE    /**/
  545.        # define ATTRIB_NORET  __attribute__((noreturn))
  546.        # define NORET_AND     noreturn,
  547.  
  548.        /* balloc.c */
  549.        extern int ext2_bg_has_super(struct super_block *sb, int group);
  550.        extern unsigned long ext2_bg_num_gdb(struct super_block *sb, int group);
  551.        extern int ext2_new_block (struct inode *, unsigned long,
  552.                       __u32 *, __u32 *, int *);
  553.        extern void ext2_free_blocks (struct inode *, unsigned long,
  554.                          unsigned long);
  555.        extern unsigned long ext2_count_free_blocks (struct super_block *);
  556.        extern void ext2_check_blocks_bitmap (struct super_block *);
  557.        extern struct ext2_group_desc * ext2_get_group_desc(struct super_block * sb,
  558.                                    unsigned int block_group,
  559.                                    struct buffer_head ** bh);
  560.  
  561.        /* dir.c */
  562.        extern int ext2_add_link (struct dentry *, struct inode *);
  563.        extern ino_t ext2_inode_by_name(struct inode *, struct dentry *);
  564.        extern int ext2_make_empty(struct inode *, struct inode *);
  565.        extern struct ext2_dir_entry_2 * ext2_find_entry (struct inode *,struct dentry *, struct page **);
  566.        extern int ext2_delete_entry (struct ext2_dir_entry_2 *, struct page *);
  567.        extern int ext2_empty_dir (struct inode *);
  568.        extern struct ext2_dir_entry_2 * ext2_dotdot (struct inode *, struct page **);
  569.        extern void ext2_set_link(struct inode *, struct ext2_dir_entry_2 *, struct page *, struct inode *);
  570.  
  571.        /* fsync.c */
  572.        extern int ext2_sync_file (struct file *, struct dentry *, int);
  573.        extern int ext2_fsync_inode (struct inode *, int);
  574.  
  575.        /* ialloc.c */
  576.        extern struct inode * ext2_new_inode (const struct inode *, int);
  577.        extern void ext2_free_inode (struct inode *);
  578.        extern unsigned long ext2_count_free_inodes (struct super_block *);
  579.        extern void ext2_check_inodes_bitmap (struct super_block *);
  580.        extern unsigned long ext2_count_free (struct buffer_head *, unsigned);
  581.  
  582.        /* inode.c */
  583.        extern void ext2_read_inode (struct inode *);
  584.        extern void ext2_write_inode (struct inode *, int);
  585.        extern void ext2_put_inode (struct inode *);
  586.        extern void ext2_delete_inode (struct inode *);
  587.        extern int ext2_sync_inode (struct inode *);
  588.        extern void ext2_discard_prealloc (struct inode *);
  589.        extern void ext2_truncate (struct inode *);
  590.  
  591.        /* ioctl.c */
  592.        extern int ext2_ioctl (struct inode *, struct file *, unsigned int,
  593.                       unsigned long);
  594.  
  595.        /* super.c */
  596.        extern void ext2_error (struct super_block *, const char *, const char *, ...)
  597.            __attribute__ ((format (printf, , )));
  598.        extern NORET_TYPE void ext2_panic (struct super_block *, const char *,
  599.                           const char *, ...)
  600.            __attribute__ ((NORET_AND format (printf, , )));
  601.        extern void ext2_warning (struct super_block *, const char *, const char *, ...)
  602.            __attribute__ ((format (printf, , )));
  603.        extern void ext2_update_dynamic_rev (struct super_block *sb);
  604.        extern void ext2_put_super (struct super_block *);
  605.        extern void ext2_write_super (struct super_block *);
  606.        extern int ext2_remount (struct super_block *, int *, char *);
  607.        extern struct super_block * ext2_read_super (struct super_block *,void *,int);
  608.        extern int ext2_statfs (struct super_block *, struct statfs *);
  609.  
  610.        /*
  611.    611     * Inodes and files operations
  612.    612     */
  613.  
  614.        /* dir.c */
  615.        extern struct file_operations ext2_dir_operations;
  616.  
  617.        /* file.c */
  618.        extern struct inode_operations ext2_file_inode_operations;
  619.        extern struct file_operations ext2_file_operations;
  620.  
  621.        /* inode.c */
  622.        extern struct address_space_operations ext2_aops;
  623.  
  624.        /* namei.c */
  625.        extern struct inode_operations ext2_dir_inode_operations;
  626.  
  627.        /* symlink.c */
  628.        extern struct inode_operations ext2_fast_symlink_inode_operations;
  629.  
  630.        #endif    /* __KERNEL__ */
  631.  
  632.        #endif    /* _LINUX_EXT2_FS_H */

ext2_fs.h

2.1 超级块（Super Blook）

超级块的数据结构如下：

  1.        /*
  2.    337     * Structure of the super block
  3.    338     */
  4.        struct ext2_super_block {
  5.            __u32    s_inodes_count;        /* Inodes count */
  6.            __u32    s_blocks_count;        /* Blocks count */
  7.            __u32    s_r_blocks_count;    /* Reserved blocks count */
  8.            __u32    s_free_blocks_count;    /* Free blocks count */
  9.            __u32    s_free_inodes_count;    /* Free inodes count */
  10.            __u32    s_first_data_block;    /* First Data Block */
  11.            __u32    s_log_block_size;    /* Block size */
  12.            __s32    s_log_frag_size;    /* Fragment size */
  13.            __u32    s_blocks_per_group;    /* # Blocks per group */
  14.            __u32    s_frags_per_group;    /* # Fragments per group */
  15.            __u32    s_inodes_per_group;    /* # Inodes per group */
  16.            __u32    s_mtime;        /* Mount time */
  17.            __u32    s_wtime;        /* Write time */
  18.            __u16    s_mnt_count;        /* Mount count */
  19.            __s16    s_max_mnt_count;    /* Maximal mount count */
  20.            __u16    s_magic;        /* Magic signature */
  21.            __u16    s_state;        /* File system state */
  22.            __u16    s_errors;        /* Behaviour when detecting errors */
  23.            __u16    s_minor_rev_level;     /* minor revision level */
  24.            __u32    s_lastcheck;        /* time of last check */
  25.            __u32    s_checkinterval;    /* max. time between checks */
  26.            __u32    s_creator_os;        /* OS */
  27.            __u32    s_rev_level;        /* Revision level */
  28.            __u16    s_def_resuid;        /* Default uid for reserved blocks */
  29.            __u16    s_def_resgid;        /* Default gid for reserved blocks */
  30.            /*
  31.    366         * These fields are for EXT2_DYNAMIC_REV superblocks only.
  32.    367         *
  33.    368         * Note: the difference between the compatible feature set and
  34.    369         * the incompatible feature set is that if there is a bit set
  35.    370         * in the incompatible feature set that the kernel doesn't
  36.    371         * know about, it should refuse to mount the filesystem.
  37.    372         *
  38.    373         * e2fsck's requirements are more strict; if it doesn't know
  39.    374         * about a feature in either the compatible or incompatible
  40.    375         * feature set, it must abort and not try to meddle with
  41.    376         * things it doesn't understand...
  42.    377         */
  43.            __u32    s_first_ino;         /* First non-reserved inode */
  44.            __u16   s_inode_size;         /* size of inode structure */
  45.            __u16    s_block_group_nr;     /* block group # of this superblock */
  46.            __u32    s_feature_compat;     /* compatible feature set */
  47.            __u32    s_feature_incompat;     /* incompatible feature set */
  48.            __u32    s_feature_ro_compat;     /* readonly-compatible feature set */
  49.            __u8    s_uuid[];        /* 128-bit uuid for volume */
  50.            char    s_volume_name[];     /* volume name */
  51.            char    s_last_mounted[];     /* directory where last mounted */
  52.            __u32    s_algorithm_usage_bitmap; /* For compression */
  53.            /*
  54.    389         * Performance hints.  Directory preallocation should only
  55.    390         * happen if the EXT2_COMPAT_PREALLOC flag is on.
  56.    391         */
  57.            __u8    s_prealloc_blocks;    /* Nr of blocks to try to preallocate*/
  58.            __u8    s_prealloc_dir_blocks;    /* Nr to preallocate for dirs */
  59.            __u16    s_padding1;
  60.            __u32    s_reserved[];    /* Padding to the end of the block */
  61.        };

struct ext2_super_block

主要的几项描述有：s_inodes_count / s_blocks_count（该分区inode/block总数），s_free_inodes_count / s_free_blocks_count（该分区空闲inode/block数），s_first_data_block（数据块中的起始块位置），s_log_block_size（块大小），s_blocks_per_group（每个块组的块数），s_magic（魔数）

  1. struct ext2_super_block {
  2. 	__u32	s_inodes_count;		/* Inodes count */
  3. 	__u32	s_blocks_count;		/* Blocks count */
  4. 	...
  5. 	__u32	s_free_blocks_count;	/* Free blocks count */
  6. 	__u32	s_free_inodes_count;	/* Free inodes count */
  7. 	__u32	s_first_data_block;	/* First Data Block */
  8. 	__u32	s_log_block_size;	/* Block size */
  9. 	...
  10. 	__u32	s_blocks_per_group;	/* # Blocks per group */
  11. 	...
  12. 	__u16	s_magic;		/* Magic signature */     # ext2的魔数为0xEF53
  13. 	...

说明：每个块组中都有一份超级块的拷贝。当文件系统挂载时，通常只有块组0中的超级块（主备份）会被读取，其他的块组中的超级块只是作为备份，以防文件系统的崩溃。

2.2 组描述符表（Group Descriptors Table，GDT）

超级块之后就是组描述符表，是由该分区所有的块的组描述符（Group Descriptor）组成的，每个块组描述符记录了本块组的inode/block bitmap和inode table等。块组描述符数据结构如下所示：

  1. struct ext2_group_desc
  2. {
  3. 	__u32	bg_block_bitmap;	/* Blocks bitmap block */
  4. 	__u32	bg_inode_bitmap;	/* Inodes bitmap block */
  5. 	__u32	bg_inode_table;		/* Inodes table block */
  6. 	__u16	bg_free_blocks_count;	/* Free blocks count */
  7. 	__u16	bg_free_inodes_count;	/* Free inodes count */
  8. 	__u16	bg_used_dirs_count;	/* Directories count */
  9. 	__u16	bg_pad;
  10. 	__u32	bg_reserved[3];
  11. };

说明：与超级类似，组描述符表也存在各块组中紧接着超级块，其目的与超级块一样，作为备份，防止文件系统的崩溃。

2.3 块位图和inode位图（block/inode bitmap）

位图（bitmap）是位（bit）的序列，每一个位代表该位图所在块组中的一个特定的数据块（block bitmap）或inode table中一个特定的inode（inode bitmap）。当bit为0时，表示对应的block/inode空闲；为1时，表示已被占用。

位图始终索引其所在的块组，并且block位图和inode位图均为1block大小，从而限制了该块组的大小。例如一般的block大小为1024bytes，因此一个块组总共有1024*8个block。

2.4 inode表（inode table）

inode表由一系列连续的block块组成，每个块中都预定义了一定数量的inode。inode表的起始块位置（块号）存储在组描述符的bg_inode_table字段中。

系统对inode表中的inode进行编号，从1开始，inode的数据结构被定义在ext2_fs.h文件的struct ext2_inode函数中：

  1.        /*
  2.    215     * Structure of an inode on the disk
  3.    216     */
  4.        struct ext2_inode {
  5.            __u16    i_mode;        /* File mode */
  6.            __u16    i_uid;        /* Low 16 bits of Owner Uid */
  7.            __u32    i_size;        /* Size in bytes */
  8.            __u32    i_atime;    /* Access time */
  9.            __u32    i_ctime;    /* Creation time */
  10.            __u32    i_mtime;    /* Modification time */
  11.            __u32    i_dtime;    /* Deletion Time */
  12.            __u16    i_gid;        /* Low 16 bits of Group Id */
  13.            __u16    i_links_count;    /* Links count */
  14.            __u32    i_blocks;    /* Blocks count */
  15.            __u32    i_flags;    /* File flags */
  16.            union {
  17.                struct {
  18.                    __u32  l_i_reserved1;
  19.                } linux1;
  20.                struct {
  21.                    __u32  h_i_translator;
  22.                } hurd1;
  23.                struct {
  24.                    __u32  m_i_reserved1;
  25.                } masix1;
  26.            } osd1;                /* OS dependent 1 */
  27.            __u32    i_block[EXT2_N_BLOCKS];/* Pointers to blocks */
  28.            __u32    i_generation;    /* File version (for NFS) */
  29.            __u32    i_file_acl;    /* File ACL */
  30.            __u32    i_dir_acl;    /* Directory ACL */
  31.            __u32    i_faddr;    /* Fragment address */
  32.            union {
  33.                struct {
  34.                    __u8    l_i_frag;    /* Fragment number */
  35.                    __u8    l_i_fsize;    /* Fragment size */
  36.                    __u16    i_pad1;
  37.                    __u16    l_i_uid_high;    /* these 2 fields    */
  38.                    __u16    l_i_gid_high;    /* were reserved2[0] */
  39.                    __u32    l_i_reserved2;
  40.                } linux2;
  41.                struct {
  42.                    __u8    h_i_frag;    /* Fragment number */
  43.                    __u8    h_i_fsize;    /* Fragment size */
  44.                    __u16    h_i_mode_high;
  45.                    __u16    h_i_uid_high;
  46.                    __u16    h_i_gid_high;
  47.                    __u32    h_i_author;
  48.                } hurd2;
  49.                struct {
  50.                    __u8    m_i_frag;    /* Fragment number */
  51.                    __u8    m_i_fsize;    /* Fragment size */
  52.                    __u16    m_pad1;
  53.                    __u32    m_i_reserved2[];
  54.                } masix2;
  55.            } osd2;                /* OS dependent 2 */
  56.        };

struct ext2_inode

以下是inode数据结构中比较重要的字段：

  1. struct ext2_inode {
  2.         __u16   i_mode;         /* File type and access rights */
  3.         __u16   i_uid;          /* Low 16 bits of Owner Uid */
  4.         __u32   i_size;         /* Size in bytes */
  5.         __u32   i_atime;        /* Access time */
  6.         __u32   i_ctime;        /* Creation time */
  7.         __u32   i_mtime;        /* Modification time */
  8.         __u32   i_dtime;        /* Deletion Time */
  9.         __u16   i_gid;          /* Low 16 bits of Group Id */
  10.         __u16   i_links_count;  /* Links count */
  11.         __u32   i_blocks;       /* Blocks count */
  12.         __u32   i_flags;        /* File flags */
  13. 	...
  14. 	__u32   i_block[EXT2_N_BLOCKS];  /* Pointers to blocks */
  15. 	...
  16. };

i_mode字段中包含了文件的类型和文件的访问权限，被定义在宏文件macro (sys/stat.h)中。

Sign Type Macro
- Regular file S_ISREG(m)
d Directory S_ISDIR(m)
c Character Device   S_ISCHR(m)
b Block Device S_ISBLK(m)
f Fifo S_ISIFO(m)
s Socket S_ISSOCK(m)
l Symbolic Link S_ISLNK(m)
Domain Read Write Exec All
User S_IRUSR S_IWUSR S_IXUSR S_IRWXU
Group S_IRGRP S_IWGRP S_IXGRP S_IRWXG
All S_IROTH S_IWOTH S_IXOTH S_IRWXO 
  1.  

i_blocks是该inode指向的文件已使用的block数量；指向数据块的指针存储于字段i_block[EXT2_N_BLOCKS]的数组结构中。

变量EXT2_N_BLOCKS定义在ext2_fs.h的第177行中：

  1.    174	/*
  2.    175	 * Constants relative to the data blocks
  3.    176	 */
  4.    177	#define	EXT2_NDIR_BLOCKS		12
  5.    178	#define	EXT2_IND_BLOCK			EXT2_NDIR_BLOCKS
  6.    179	#define	EXT2_DIND_BLOCK			(EXT2_IND_BLOCK + 1)
  7.    180	#define	EXT2_TIND_BLOCK			(EXT2_DIND_BLOCK + 1)
  8.    181	#define	EXT2_N_BLOCKS			(EXT2_TIND_BLOCK + 1)

在i_block[]数组中有15个指针，它们所代表的含义如下：

  • i_block[0..11] point directly to the first 12 data blocks of the file.              # 序列号0-11的12个元素（指针）指向文件开头的12个数据块
  • i_block[12] points to a single indirect block                                               # 第13号元素指向单索引间接块
  • i_block[13] points to a double indirect block                                              # 第14号元素指向双索引间接块
  • i_block[14] points to a triple indirect block                                                 # 第15号元素指向三索引间接块

由此我们可以计算出ext2文件系统单个文件的最大容量（假设block大小为1K），数组i_block[]的长度为32bit/8=4byte：

  • 直接索引：12 指针
  • 单间接索引：1024/4=256个直接索引，256 指针
  • 双间接索引：1024/4=256个单间接索引，256*256 指针
  • 三间接索引：1024/4=256个双间接索引，256*256*256 指针

因此可得到12K+256K+64M+16G，即大致为16G。如果block的大小为4K，则文件最大可为4T。（注意：真正决定文件大小的是底层的寄存器，寄存器的位数决定了其寻址的能力）

2.5 inode表中的inode指向的目录文件

inode指向的目录文件需加以注意，我们可以通过测试S_ISDIR(mode) macro来加以识别：

  1. if (S_ISDIR(inode.i_mode)) ...

假设inode指向的块是目录实体/home，目录中的内容包含了一系列的文件名和指向inode表中的对应的，如下图：

目录文件的数据结构如下：

  1. struct ext2_dir_entry_2 {
  2. 	__u32	inode;			/* Inode number */
  3. 	__u16	rec_len;		/* Directory entry length */
  4. 	__u8	name_len;		/* Name length */
  5. 	__u8	file_type;
  6. 	char	name[EXT2_NAME_LEN];	/* File name */
  7. };

字段file_type总共有0-7可能的值，分别代表： 

目录内容中的各项（实体）的大小是非固定的，大小取决于文件名称的长度。文件名称最大长度为EXT2_NAME_LEN的值，一般为255；文件名称的实际长度存放于字段name_len；rec_len存储的是本目录实体的大小，该字段自然也就决定了下一目录实体的位置了。

  Example of EXT2 directory

注意：目录中的inode号，指向的是inode表中的inode，指向data block的是i_block[EXT2_N_BLOCKS]的数组中的指针。

2.6 文件操作（创建、查找、修改、删除）的本质

http://cs.smith.edu/~nhowe/Teaching/csc262/oldlabs/ext2.html

https://www.cnblogs.com/f-ck-need-u/p/7016077.html

待完善...

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