乐在其中

撰写者信息：

Alin Fang (Fang Yunlin)

MSN: [email protected]

G Talk: [email protected]

Blog: http://www.alinblog.cn/

修改日期：

9 Oct, 2008

第2次修改

版权：

GNU

声明：

本人实验笔记，非权威文档。如有错误请告知。十分感谢！

实验笔记：

首先大家可能遇到过这样的情况：

[root@dhcp-0-142 ~]# mount /dev/sdb1 /mnt/sdb1

mount: wrong fs type, bad option, bad superblock on /dev/sdb1,

missing codepage or other error

In some cases useful info is found in syslog – try

dmesg | tail or so

[root@dhcp-0-142 ~]#

这种情况一般为superblock损坏的概率很大。

superblock是什么？

分区的第一块superblock位于分区的第二块block。

如果分区的blocksize是1024，则superblock位于[1024, 2048)之间。

ext2文件系统的结构请参考这个网站，写的非常清楚：

http://homepage.smc.edu/morgan_david/cs40/analyze-ext2.htm

这个是superblock的结构，摘自：

http://homepage.smc.edu/morgan_david/cs40/analyze-ext2.htm

继续刚才的话题。我/dev/sdb1挂载不上了。

所以我做了如下操作：

[root@dhcp-0-142 ~]# dumpe2fs /dev/sdb1

dumpe2fs 1.39 (29-May-2006)

Filesystem volume name: <none>

Last mounted on: <not available>

Filesystem UUID: c3800df3-5c34-4b53-8144-94029b5736d8

Filesystem magic number: 0xEF53

Filesystem revision #: 1 (dynamic)

Filesystem features: has_journal resize_inode dir_index filetype sparse_super

Default mount options: (none)

Filesystem state: clean with errors

Errors behavior: Continue

Filesystem OS type: Linux

Inode count: 0

Block count: 0

Reserved block count: 0

Free blocks: 20926971

Free inodes: 4705752

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: Tue Oct 7 19:18:08 2008

Last mount time: n/a

Last write time: Tue Oct 7 19:29:39 2008

Mount count: 0

Maximum mount count: 20

Last checked: Tue Oct 7 19:18:08 2008

Check interval: 15552000 (6 months)

Next check after: Sun Apr 5 19:18:08 2009

Reserved blocks uid: 0 (user root)

Reserved blocks gid: 0 (group root)

First inode: 11

Inode size: 128

Journal inode: 8

Default directory hash: tea

Directory Hash Seed: 7f7e1c41-5cae-4f23-9873-877991751ccb

Journal backup: inode blocks

dumpe2fs: Illegal inode number while reading journal inode

[root@dhcp-0-142 ~]#

根据Blocks per group: 8192的信息，我定位了第一个备份superblock的位置为8193。

所以我做如下操作：

[root@dhcp-0-142 ~]# fsck -b 8193 /dev/sdb1

fsck 1.39 (29-May-2006)

e2fsck 1.39 (29-May-2006)

/dev/sdb1 was not cleanly unmounted, check forced.

Pass 1: Checking inodes, blocks, and sizes

Pass 2: Checking directory structure

Pass 3: Checking directory connectivity

Pass 4: Checking reference counts

Pass 5: Checking group summary information

/dev/sdb1: ***** FILE SYSTEM WAS MODIFIED *****

/dev/sdb1: 11/26104 files (9.1% non-contiguous), 8966/104388 blocks

[root@dhcp-0-142 ~]# mount /dev/sdb1 /mnt/sdb1

[root@dhcp-0-142 ~]# ls /mnt/sdb1

lost+found

[root@dhcp-0-142 ~]#

superblock已经修复，文件系统挂载正常。

之所以这么做，是有依据的。

ext2/3文件系统在创建文件系统的时候会创建若干个superblock的备份存放于特定位置。

[root@dhcp-0-142 ~]# dumpe2fs /dev/sdb1 | grep –before-context=1 superblock

dumpe2fs 1.39 (29-May-2006)

Group 0: (Blocks 1-8192)

Primary superblock at 1, Group descriptors at 2-2

—

Group 1: (Blocks 8193-16384)

Backup superblock at 8193, Group descriptors at 8194-8194

—

Group 3: (Blocks 24577-32768)

Backup superblock at 24577, Group descriptors at 24578-24578

—

Group 5: (Blocks 40961-49152)

Backup superblock at 40961, Group descriptors at 40962-40962

—

Group 7: (Blocks 57345-65536)

Backup superblock at 57345, Group descriptors at 57346-57346

—

Group 9: (Blocks 73729-81920)

Backup superblock at 73729, Group descriptors at 73730-73730

[root@dhcp-0-142 ~]#

从上面操作可以看出，在第1、3、4、7、9这几个Block Group上存放有superblock备份。

什么是Block Group？ext2/3文件系统为了提高磁盘寻道效率，把inode table等信息按照Inodes per group分成若干组存放，而没有全部放在一起。

此图摘自：http://homepage.smc.edu/morgan_david/cs40/analyze-ext2.htm

Inodes per group信息相见命令：

[root@dhcp-0-142 ~]# dumpe2fs /dev/sdb1 | grep ‘Inodes per group’

dumpe2fs 1.39 (29-May-2006)

Inodes per group: 2008

[root@dhcp-0-142 ~]#

有些文件系统superblock损坏的很厉害。连dumpe2fs和tune2fs都看不到信息。

[root@dhcp-0-175 ~]# dd if=/dev/zero of=/dev/sdb1 bs=1 count=1024 seek=1024

1024+0 records in

1024+0 records out

1024 bytes (1.0 kB) copied, 0.0228272 seconds, 44.9 kB/s

[root@dhcp-0-175 ~]# dumpe2fs /dev/sdb1

dumpe2fs 1.39 (29-May-2006)

dumpe2fs: Bad magic number in super-block while trying to open /dev/sdb1

Couldn’t find valid filesystem superblock.

[root@dhcp-0-175 ~]# tune2fs -l /dev/sdb1

tune2fs 1.39 (29-May-2006)

tune2fs: Bad magic number in super-block while trying to open /dev/sdb1

Couldn’t find valid filesystem superblock.

[root@dhcp-0-175 ~]#

这时候我们根本无法从dumpe2fs和tune2fs看到Backup superblock的位置。

我们可以尝试从superblock的结构来猜测superblock的位置（superblock结构见上图）。

我们从superblock结构可以知道，卷标volume name存放于superblock中。所以如果文件系统有设置卷标，那么我们可以尝试从卷标来定位superblock。

我们用hexdump把文件系统dump出来：

[root@dhcp-0-175 ~]# hexdump -C /dev/sdb1 > /var/sdb1.hexdump

[root@dhcp-0-175 ~]#

我们已知 /dev/sdb1的卷标是sdb1(如果不知道卷标或者没有设置卷标，那么我就没办法了)。

我们搜索sdb1，搜到结果如下：

我们猜测这里就是备份superblock的位置。

卷标起始位置是0x18000078。由于superblock结构里volume name位于0x78的位置，所以我们可以猜测备份superblock的起始位置是0x18000078 – 0x78 = 0x18000000。

由于blocksize位于superblock的[0x18, 0x22)的位置，这里的值是0x0002，得出blocksize是0x0400 x ( 0x0002^0x0002 ) = 0x1000 = 4096

( [0x18, 0x22) 处值n和blocksize的关系是 blocksize = 0x0400 x 0x0002ⁿ，此公式感谢得新倾情赞助)

而备份superblock的偏移量为offset / blocksize，即0x18000000 / 0x1000 = 0x00018000 = 98304。

因此我们执行：

[root@dhcp-0-175 ~]# fsck.ext3 -b 98304 /dev/sdb1

e2fsck 1.39 (29-May-2006)

sdb1 was not cleanly unmounted, check forced.

Pass 1: Checking inodes, blocks, and sizes

Pass 2: Checking directory structure

Pass 3: Checking directory connectivity

Pass 4: Checking reference counts

Pass 5: Checking group summary information

sdb1: ***** FILE SYSTEM WAS MODIFIED *****

sdb1: 11/123648 files (9.1% non-contiguous), 8298/246991 blocks

[root@dhcp-0-175 ~]#

这样文件系统就有给修复的可能性了。

测试一下：

[root@dhcp-0-175 ~]# dumpe2fs /dev/sdb1

dumpe2fs 1.39 (29-May-2006)

Filesystem volume name: sdb1

Last mounted on: <not available>

Filesystem UUID: 0293bd85-b911-43bf-853e-6588b3eaaf39

Filesystem magic number: 0xEF53

Filesystem revision #: 1 (dynamic)

Filesystem features: has_journal resize_inode dir_index filetype sparse_super large_file

Default mount options: (none)

Filesystem state: clean

Errors behavior: Continue

Filesystem OS type: Linux

Inode count: 123648

Block count: 246991

Reserved block count: 12349

Free blocks: 238693

Free inodes: 123637

First block: 0

Block size: 4096

Fragment size: 4096

Reserved GDT blocks: 60

Blocks per group: 32768

Fragments per group: 32768

Inodes per group: 15456

Inode blocks per group: 483

Filesystem created: Wed Oct 8 12:49:09 2008

Last mount time: n/a

Last write time: Wed Oct 8 12:52:10 2008

Mount count: 0

Maximum mount count: 28

Last checked: Wed Oct 8 12:52:10 2008

Check interval: 15552000 (6 months)

Next check after: Mon Apr 6 12:52:10 2009

Reserved blocks uid: 0 (user root)

Reserved blocks gid: 0 (group root)

First inode: 11

Inode size: 128

Journal inode: 8

Default directory hash: tea

Directory Hash Seed: 2efa124c-dde6-4046-9181-a05b7e6d182a

Journal backup: inode blocks

Journal size: 16M

Group 0: (Blocks 0-32767)

Primary superblock at 0, Group descriptors at 1-1

Reserved GDT blocks at 2-61

Block bitmap at 62 (+62), Inode bitmap at 63 (+63)

Inode table at 64-546 (+64)

28113 free blocks, 15445 free inodes, 2 directories

Free blocks: 4655-32767

Free inodes: 12-15456

Group 1: (Blocks 32768-65535)

Backup superblock at 32768, Group descriptors at 32769-32769

Reserved GDT blocks at 32770-32829

Block bitmap at 32830 (+62), Inode bitmap at 32831 (+63)

Inode table at 32832-33314 (+64)

32221 free blocks, 15456 free inodes, 0 directories

Free blocks: 33315-65535

Free inodes: 15457-30912

Group 2: (Blocks 65536-98303)

Block bitmap at 65536 (+0), Inode bitmap at 65537 (+1)

Inode table at 65538-66020 (+2)

32283 free blocks, 15456 free inodes, 0 directories

Free blocks: 66021-98303

Free inodes: 30913-46368

Group 3: (Blocks 98304-131071)

Backup superblock at 98304, Group descriptors at 98305-98305

Reserved GDT blocks at 98306-98365

Block bitmap at 98366 (+62), Inode bitmap at 98367 (+63)

Inode table at 98368-98850 (+64)

32221 free blocks, 15456 free inodes, 0 directories

Free blocks: 98851-131071

Free inodes: 46369-61824

Group 4: (Blocks 131072-163839)

Block bitmap at 131072 (+0), Inode bitmap at 131073 (+1)

Inode table at 131074-131556 (+2)

32283 free blocks, 15456 free inodes, 0 directories

Free blocks: 131557-163839

Free inodes: 61825-77280

Group 5: (Blocks 163840-196607)

Backup superblock at 163840, Group descriptors at 163841-163841

Reserved GDT blocks at 163842-163901

Block bitmap at 163902 (+62), Inode bitmap at 163903 (+63)

Inode table at 163904-164386 (+64)

32221 free blocks, 15456 free inodes, 0 directories

Free blocks: 164387-196607

Free inodes: 77281-92736

Group 6: (Blocks 196608-229375)

Block bitmap at 196608 (+0), Inode bitmap at 196609 (+1)

Inode table at 196610-197092 (+2)

32283 free blocks, 15456 free inodes, 0 directories

Free blocks: 197093-229375

Free inodes: 92737-108192

Group 7: (Blocks 229376-246990)

Backup superblock at 229376, Group descriptors at 229377-229377

Reserved GDT blocks at 229378-229437

Block bitmap at 229438 (+62), Inode bitmap at 229439 (+63)

Inode table at 229440-229922 (+64)

17068 free blocks, 15456 free inodes, 0 directories

Free blocks: 229923-246990

Free inodes: 108193-123648

[root@dhcp-0-175 ~]# mount /dev/sdb1 /mnt

[root@dhcp-0-175 ~]# ls /mnt

lost+found

[root@dhcp-0-175 ~]#

看来我运气很好。文件系统成功修复。

其实对于这种superblock破坏很严重的文件系统，其实系统已经有了很强大的修复方案：

我们可以用mke2fs -S 来修复superblock。

[root@dhcp-0-175 /]# mount /dev/sdb1 /mnt/

mount: you must specify the filesystem type

[root@dhcp-0-175 /]# mount /dev/sdb1 /mnt/ -t ext3

mount: wrong fs type, bad option, bad superblock on /dev/sdb1,

missing codepage or other error

In some cases useful info is found in syslog – try

dmesg | tail or so

[root@dhcp-0-175 /]# mke2fs -S /dev/sdb1

mke2fs 1.39 (29-May-2006)

Filesystem label=

OS type: Linux

Block size=1024 (log=0)

Fragment size=1024 (log=0)

24480 inodes, 97656 blocks

4882 blocks (5.00%) reserved for the super user

First data block=1

Maximum filesystem blocks=67371008

12 block groups

8192 blocks per group, 8192 fragments per group

2040 inodes per group

Superblock backups stored on blocks:

8193, 24577, 40961, 57345, 73729

Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 37 mounts or

180 days, whichever comes first. Use tune2fs -c or -i to override.

[root@dhcp-0-175 /]# mount /dev/sdb1 /mnt/

[root@dhcp-0-175 /]# cd /mnt

[root@dhcp-0-175 mnt]# ls

file0 file14 file20 file27 file33 file4 file46 file52 file59 file65 file71 file78 file84 file90 file97

file1 file15 file21 file28 file34 file40 file47 file53 file6 file66 file72 file79 file85 file91 file98

file10 file16 file22 file29 file35 file41 file48 file54 file60 file67 file73 file8 file86 file92 file99

file100 file17 file23 file3 file36 file42 file49 file55 file61 file68 file74 file80 file87 file93 lost+found

file11 file18 file24 file30 file37 file43 file5 file56 file62 file69 file75 file81 file88 file94

file12 file19 file25 file31 file38 file44 file50 file57 file63 file7 file76 file82 file89 file95

file13 file2 file26 file32 file39 file45 file51 file58 file64 file70 file77 file83 file9 file96

[root@dhcp-0-175 mnt]#

e2fsck也可以达到同样的效果

[root@dhcp-0-175 /]# mount /dev/sdb1 /mnt/ -t ext3

mount: wrong fs type, bad option, bad superblock on /dev/sdb1,

missing codepage or other error

In some cases useful info is found in syslog – try

dmesg | tail or so

[root@dhcp-0-175 /]# e2fsck /dev/sdb1

e2fsck 1.39 (29-May-2006)

Couldn’t find ext2 superblock, trying backup blocks…

/dev/sdb1 was not cleanly unmounted, check forced.

Pass 1: Checking inodes, blocks, and sizes

Pass 2: Checking directory structure

Pass 3: Checking directory connectivity

Pass 4: Checking reference counts

Pass 5: Checking group summary information

Free blocks count wrong for group #0 (3549, counted=3547).

Fix<y>? yes

Free blocks count wrong (88895, counted=88893).

Fix<y>? yes

Free inodes count wrong for group #0 (2029, counted=1929).

Fix<y>? yes

Free inodes count wrong (24469, counted=24369).

Fix<y>? yes

/dev/sdb1: ***** FILE SYSTEM WAS MODIFIED *****

/dev/sdb1: 111/24480 files (1.8% non-contiguous), 8763/97656 blocks

[root@dhcp-0-175 /]# mount /dev/sdb1 /mnt/ -t ext3

[root@dhcp-0-175 /]# ls /mnt

file0 file15 file21 file28 file34 file40 file47 file53 file6 file66 file72 file79 file85 file91 file98

file1 file16 file22 file29 file35 file41 file48 file54 file60 file67 file73 file8 file86 file92 file99

file10 file17 file23 file3 file36 file42 file49 file55 file61 file68 file74 file80 file87 file93 lost+found

file11 file18 file24 file30 file37 file43 file5 file56 file62 file69 file75 file81 file88 file94

file12 file19 file25 file31 file38 file44 file50 file57 file63 file7 file76 file82 file89 file95

file13 file2 file26 file32 file39 file45 file51 file58 file64 file70 file77 file83 file9 file96

file14 file20 file27 file33 file4 file46 file52 file59 file65 file71 file78 file84 file90 file97

[root@dhcp-0-175 /]#