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最近看了些kernel fs code, 从实际例子，简单分析一下ext2文件系统的结构,  希望对大家有帮助

本文涉及到一些结构，主要是：

超级块 struct ext2_super_block { }

组描述    struct ext2_group_desc { }

索引节点    struct ext2_inode { }

目录结构    struct ext2_dir_entry_2 { }

1. 准备工作

为了分析，特地格式化了一个100MB左右的ext2文件系统，block size 1024 Bytes

可以看一下这个分区的主要信息：

debugfs:   stats
Filesystem volume name:   <none>
Last mounted on:          <not available>
Filesystem UUID:          9c0e702c-f80e-4382-a95d-444fafaab34c
Filesystem magic number:  0xEF53
Filesystem revision #:    1 (dynamic)
Filesystem features:      resize_inode filetype sparse_super
Default mount options:    (none)
Filesystem state:         not clean
Errors behavior:          Continue
Filesystem OS type:       Linux
Inode count:              26104
Block count:              104296
Reserved block count:     5214
Free blocks:              99442
Free inodes:              26091
First block:              1
Block size:               1024
Fragment size:            1024
Reserved GDT blocks:      256
Blocks per group:         8192
Fragments per group:      8192
Inodes per group:         2008
Inode blocks per group:   251
Filesystem created:       Sun Feb 13 22:36:23 2011
Last mount time:          Mon Feb 14 04:49:41 2011
Last write time:          Mon Feb 14 04:49:41 2011
Mount count:              2
Maximum mount count:      34
Last checked:             Sun Feb 13 22:36:23 2011
Check interval:           15552000 (6 months)
Next check after:         Fri Aug 12 22:36:23 2011
Reserved blocks uid:      0 (user root)
Reserved blocks gid:      0 (group root)
First inode:              11
Inode size:               128
Default directory hash:   tea
Directory Hash Seed:      de62f19c-eaa2-4cb7-a2c6-84fca69baafd
Directories:              2
Group  0: block bitmap at 259, inode bitmap at 260, inode table at 261
           7665 free blocks, 1995 free inodes, 2 used directories
Group  1: block bitmap at 8451, inode bitmap at 8452, inode table at 8453
           7681 free blocks, 2008 free inodes, 0 used directories
Group  2: block bitmap at 16385, inode bitmap at 16386, inode table at 16387
           7939 free blocks, 2008 free inodes, 0 used directories
Group  3: block bitmap at 24835, inode bitmap at 24836, inode table at 24837
           7681 free blocks, 2008 free inodes, 0 used directories
Group  4: block bitmap at 32769, inode bitmap at 32770, inode table at 32771
           7939 free blocks, 2008 free inodes, 0 used directories
Group  5: block bitmap at 41219, inode bitmap at 41220, inode table at 41221
           7681 free blocks, 2008 free inodes, 0 used directories
Group  6: block bitmap at 49153, inode bitmap at 49154, inode table at 49155
           7939 free blocks, 2008 free inodes, 0 used directories
Group  7: block bitmap at 57603, inode bitmap at 57604, inode table at 57605
           7681 free blocks, 2008 free inodes, 0 used directories
Group  8: block bitmap at 65537, inode bitmap at 65538, inode table at 65539
           7939 free blocks, 2008 free inodes, 0 used directories
Group  9: block bitmap at 73987, inode bitmap at 73988, inode table at 73989
           7681 free blocks, 2008 free inodes, 0 used directories
Group 10: block bitmap at 81921, inode bitmap at 81922, inode table at 81923
           7939 free blocks, 2008 free inodes, 0 used directories
Group 11: block bitmap at 90113, inode bitmap at 90114, inode table at 90115
           7939 free blocks, 2008 free inodes, 0 used directories
Group 12: block bitmap at 98305, inode bitmap at 98306, inode table at 98307
           5738 free blocks, 2008 free inodes, 0 used directories

2. 查看 super block

3. 接下来我们看组描述

再看组描述符 ，32个字节一个，我们可以把debugfs的结果和实际的磁盘信息对照一下

/*
* Structure of a blocks group descriptor
*/
struct ext2_group_desc
{
     __u32     bg_block_bitmap;          /* Blocks bitmap block */
     __u32     bg_inode_bitmap;          /* Inodes bitmap block */
     __u32     bg_inode_table;          /* Inodes table block */
     __u16     bg_free_blocks_count;     /* Free blocks count */
     __u16     bg_free_inodes_count;     /* Free inodes count */
     __u16     bg_used_dirs_count;     /* Directories count */
     __u16     bg_pad;
     __u32     bg_reserved[3];
};

Group  0: block bitmap at 259, inode bitmap at 260, inode table at 261
           7665 free blocks, 1995 free inodes, 2 used directories
Group  1: block bitmap at 8451, inode bitmap at 8452, inode table at 8453
           7681 free blocks, 2008 free inodes, 0 used directories
……

观察磁盘上的信息，都可以对得上
03 01 00 00 = 259
04 01 00 00 = 260
F1 1D          = 7665
cb 07           = 1995
02 00           = 2
03 21 00 00 = 8451
04 21 00 00 = 8452

组描述分布在 super block 后面 (所以从2048开始) ，根据block group数量而有多个，32个字节一组(2行一组)，

磁盘100MB，每个block gourp 物理8192KB, 所以有有13个block group, 32*13=0x1A0 , 所以1A0 后面就都是0了

[root@ms3003 ~]# dd if=/dev/hdb1 bs=1 count=1024 skip=2048 | od -t x1 -Ax
1024+0 records in
1024+0 records out
000000 03 01 00 00 04 01 00 00 05 01 00 00 f1 1d cb 07
000010 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
000020 03 21 00 00 04 21 00 00 05 21 00 00 01 1e d8 07
000030 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
000040 01 40 00 00 02 40 00 00 03 40 00 00 03 1f d8 07
000050 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
000060 03 61 00 00 04 61 00 00 05 61 00 00 01 1e d8 07
000070 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
000080 01 80 00 00 02 80 00 00 03 80 00 00 03 1f d8 07
000090 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0000a0 03 a1 00 00 04 a1 00 00 05 a1 00 00 01 1e d8 07
0000b0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0000c0 01 c0 00 00 02 c0 00 00 03 c0 00 00 03 1f d8 07
0000d0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0000e0 03 e1 00 00 04 e1 00 00 05 e1 00 00 01 1e d8 07
0000f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
000100 01 00 01 00 02 00 01 00 03 00 01 00 03 1f d8 07
000110 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
000120 03 21 01 00 04 21 01 00 05 21 01 00 01 1e d8 07
000130 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
000140 01 40 01 00 02 40 01 00 03 40 01 00 03 1f d8 07
000150 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
000160 01 60 01 00 02 60 01 00 03 60 01 00 03 1f d8 07
000170 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
000180 01 80 01 00 02 80 01 00 03 80 01 00 6a 16 d8 07
000190 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
000400

i_mode = 0x41ed = 0040755 (8进制 ) ， 也就是 目录 + 755 权限(drwxr-xr-x)

block 索引第一个是 00 02 ，就是512块

对照 struct ext2_inode ，

 {
 __u16 i_mode;  /* File mode */
 __u16 i_uid;  /* Low 16 bits of Owner Uid */
 __u32 i_size;  /* Size in bytes */

……

__u32 i_block[EXT2_N_BLOCKS];/* Pointers to blocks */ 从40个字节处开始

…..

}

5. 查看根目录的目录结构

[root@ms3003 ext2]#  dd if=/dev/hdb1 bs=1024 count=1 skip=512 | hexdump -C
1+0 records in
1+0 records out
00000000  02 00 00 00 0c 00 01 02  2e 00 00 00 02 00 00 00  |…………….|
00000010  0c 00 02 02 2e 2e 00 00  0b 00 00 00 14 00 0a 02  |…………….|
00000020  6c 6f 73 74 2b 66 6f 75  6e 64 00 00 0c 00 00 00  |lost+found……|
00000030  10 00 05 01 74 65 73 74  32 2e 73 77 0f 00 00 00  |….test2.sw….|
00000040  c4 03 04 01 74 65 73 74  74 65 73 74 00 00 00 00  |….testtest….|
00000050  b4 03 09 01 2e 74 65 73  74 2e 73 77 70 70 00 00  |…..test.swpp..|
00000060  00 00 00 00 a0 03 05 01  74 65 73 74 7e 2e 73 77  |……..test~.sw|
00000070  70 78 78 00 00 00 00 00  00 00 00 00 00 00 00 00  |pxx………….|
00000080  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |…………….|

对照 ext2_dir_entry_2 分析，注意 这是个可变长度的结构，长度为rec_len , 而名字长度为 name_len
struct ext2_dir_entry_2 {
     __u32     inode;               /* Inode number */
     __u16     rec_len;          /* Directory entry length */
     __u8     name_len;          /* Name length */
     __u8     file_type;
     char     name[EXT2_NAME_LEN];     /* File name */
};

蓝色和红色分别是目录  .  和 .. inode id = 2
绿色是 lost+found , inode id = 11 (0b 00 00 00)
紫色是 test2     , inode id = 12 (0c 00 00 00)
深黄色 是 test     , inode id = 15 (0f 00 00 00)

该块信息和 ext2 根目录文件都可以对应上

6.查看普通文件 test 的内容

test 文件的 inode 是 15，那么我们计算一下它的inode的位置， 是group 0 的 第15个inode, 磁盘偏移是

261*1024 + 128*14 = 269056

[root@ms3003 ~]# dd if=/dev/hdb1 bs=1 count=256 skip=269056 | od -t x1 -Ax      
256+0 records in
256+0 records out
000000 a4 81 00 00 48 00 00 00 df 0d 5b 4d e1 82 58 4d
000010 e1 82 58 4d 00 00 00 00 00 00 01 00 02 00 00 00
000020 00 00 00 00 00 00 00 00 09 14 00 00 00 00 00 00
000030 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

我们找到这个 inode , imode = 0x81a4 = 00100644 (8进制 ) , 也就是普通文件+权限644(-rw-r–r– )

第一个block号码是 0x 14 09 = 5192

那我们查看一下 5192 block 的内容， 和 cat test 的结果是一致的

[root@ms3003 ext2]# cat test
apeonaaaaaaaaaaaaaaaaaaab
bbbbbbbbbbbbbbbbbbbb
ccccccccccccccccccc:q!
\

[root@ms3003 ~]# dd if=/dev/hdb1 bs=1024 count=1 skip=5129 | od -t x1 -aAx
1+0 records in
1+0 records out
000000 61 70 65 6f 6e 61 61 61 61 61 61 61 61 61 61 61
         a   p   e   o   n   a   a   a   a   a   a   a   a   a   a   a
000010 61 61 61 61 61 61 61 61 62 0a 62 62 62 62 62 62
         a   a   a   a   a   a   a   a   b  nl   b   b   b   b   b   b
000020 62 62 62 62 62 62 62 62 62 62 62 62 62 62 0a 63
         b   b   b   b   b   b   b   b   b   b   b   b   b   b  nl   c
000030 63 63 63 63 63 63 63 63 63 63 63 63 63 63 63 63
         c   c   c   c   c   c   c   c   c   c   c   c   c   c   c   c
000040 63 63 3a 71 21 0a 5c 0a 00 00 00 00 00 00 00 00
         c   c   :   q   !  nl   \  nl nul nul nul nul nul nul nul nul
000050 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
       nul nul nul nul nul nul nul nul nul nul nul nul nul nul nul nul

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