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
先看超级块，因为第一个block(1024字节)是引导块 ，所以我们从 1024 字节 开始
inode count 等都可以对得上 (注意是little endian)
-------------------------------------------------------------
struct ext2_super_block {
__u32 s_inodes_count; /* Inodes count */ // f8 65 00 00 = 26140
__u32 s_blocks_count; /* Blocks count */ // 68 97 01 00 = 104296
......
__u16 s_magic; /* Magic signature */ // ef 53
......
[root@ms3003 ~]# dd if=/dev/hdb1 bs=1 count=1024 skip=1024 | od -t x1 -Ax
1024+0 records in
1024+0 records out
000000 f8 65 00 00 68 97 01 00 5e 14 00 00 72 84 01 00
000010 eb 65 00 00 01 00 00 00 00 00 00 00 00 00 00 00
000020 00 20 00 00 00 20 00 00 d8 07 00 00 e5 43 58 4d
000030 e5 43 58 4d 02 00 22 00 53 ef 00 00 01 00 00 00
000040 67 ec 57 4d 00 4e ed 00 00 00 00 00 01 00 00 00
000050 00 00 00 00 0b 00 00 00 80 00 00 00 10 00 00 00
000060 02 00 00 00 01 00 00 00 9c 0e 70 2c f8 0e 43 82
000070 a9 5d 44 4f af aa b3 4c 00 00 00 00 00 00 00 00
000080 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
......
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
4. 查看根目录的inode
ext2 根目录的inode number = 2 ，所以这个inode 在 group 0 ， 是第二个inode
Group 0: block bitmap at 259, inode bitmap at 260, inode table at 261
每个 inode 是一个 struct ext2_inode ， 128 个字节，所以磁盘上的偏移就是 261*1024 + 128 = 267392
[root@ms3003 ext2]# dd if=/dev/hdb1 bs=1 count=256 skip=267392 | od -t x1 -Ax
256+0 records in
256+0 records out
000000 ed 41 00 00 00 04 00 00 52 13 5b 4d e1 82 58 4d
000010 e1 82 58 4d 00 00 00 00 00 00 03 00 02 00 00 00
000020 00 00 00 00 00 00 00 00 00 02 00 00 00 00 00 00
000030 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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
我们看一下根目录有哪些文件(目录是特殊的文件），和磁盘分区信息对照一下
注意最左边的数字是inode number
[root@ms3003 ext2]# ll -ia
total 23
2 drwxr-xr-x 3 root root 1024 Feb 14 09:18 .
812001 drwxr-xr-x 4 root root 4096 Feb 13 22:36 ..
11 drwx------ 2 root root 12288 Feb 13 22:36 lost+found
15 -rw-r--r-- 1 root root 72 Feb 14 09:18 test
12 -rw-r--r-- 1 root root 69 Feb 13 22:45 test2
[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
