什么是superblock？
可以参考下这个网站：http://homepage.smc.edu/morgan_david/cs40/analyze-ext2.htm
详细的介绍superblock组成 构成
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。
例如我现在/dev/sdb1无法挂载 现象如上，可以这样操作：
[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 ~]#
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。 这么做是因为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。
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 ( 0x00020x0002 ) = 0x1000 = 4096
( [0x18, 0x22) 处值n和blocksize的关系是 blocksize = 0x0400 x 0x0002n)
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 ~]#
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 /]#
当你的系统出现superblock corrupt而无法启动时：
1.用应急盘启动,先看fdisk的结果.如果你的分区表看起来正常,那么恢复的可能性就比较大,如果出现cannot open /dev/sda2的提示,那么想一想你的scsi卡启动没有,如果没有,那么你可以试着用小红帽的安装光盘启动,记住,仅仅是看分区表,千万不要写它.然后把分区情况详细记录下来.
2.试着e2fsck /dev/hda2,(先不要加-p -y 之类的参数,)用手动进行修复,同时也可以了解具体是文件系统的那些地方损坏了,如果你的运气好,e2fsck过去了,/dev/hda2已经基本修复,当然修复的可能是99.9%,也可能是99%这就看文件系统的损坏程度乐,不过现在可以说你的数据已经都找回来了.剩下的事就是mount上把数据备份出来以防万一.
3.如果e2fsck没过去(确保你的硬盘已经正确驱动乐),也不要着急,因为superblock在硬盘中有很多地方有备份,现在你最好把硬盘卸下来挂到另一个好的linux系统上,当然同样要保证硬盘被正确驱动乐.先用e2fsck /dev/hda2,如果结果和前面一样，就用e2fsck -b xxx -f /dev/hda2, xxx是硬盘上superblock的备份块,xxx=n*8192+1,n=1,2,3...一般来讲,如果系统瘫痪的真正原因是superblock损坏，这种办法就应该可以恢复你的数据了。如果执行后的结果还是不能通过,那么往下一步.
4.利用dd命令.先dd if=/dev/hda2 of=/tmp/rescue conv=noerror(/tmp/rescue是一个文件),把重要的数据拷出来,当然,这个盘要比你损坏的盘大一点,否则拷不下.另外,上面的dd命令在不同的境况下if和of应作相应的修改，写在这里只是一个例子，总之在用dd之前最好先看看man.刚才你已经看到你的分区表了,现在找一个和你的硬盘一样的硬盘,应该是一摸一样（大小，型号),在这块硬盘上按照坏盘上的分区表分区，分的区也应该是也是一模一样然后用dd命令把坏盘上superblock location后的东东全部拷到好盘的superblock location后，上帝保佑你，当你再次启动系统时就可以看到熟悉的数据了,有人用这种方法恢复了99%以上的数据,不过好在这种方法(包括前面的方法)没有动那块坏盘上的数据,如果还是没有恢复,那没你还有最后一种选择.
5. 在手册页里称这种方法为last-ditch recovery method,就是说这是最后的恢复方法，只有当你已经尝试了其他的方法,都没有能恢复你的数据的情况下才用,因为这需要冒一定的风险.
把你的硬盘挂在一台好的linux box上，运行：#mke2fs -S /dev/hda2(如果你的数据在hda2里）这条命令只重建superblock，而不碰inode表，不过这仍有一定的风险。good luck to you all.当时也有人建议我如果实在不行的话就重装系统（不动分区也不格式化），这也可能有效，但你也应该清楚这种方法就像mke2fs -S /dev/hd*一样是有风险的。
一点建议：
如果你的硬盘不是可以轻易就重做的，最好在建立一个新的系统后：
1。拿出笔和纸,把你的分区信息详细记录下来.
2. 用mkbootdisk做好现在这个系统的启动盘并测试.特别是如果你用的硬盘是scsi的。
3. 在用mke2fs建立一个文件系统后将屏幕上的superblock所在位置记录下来。
4. 用crontab对重要数据进行备份。ext2文件系统（包括其他的unix文件系统）是很强壮的，但你仍然应该小心。
RedHat官方解释：
解决方法:
通常在作磁盘操作之前应该备份磁盘的数据，在作这个操作之前也应该把磁盘上的所有内容备份到另一个磁盘中。就是说如果这个故障盘是20g的话，就需要一个20G的备份空间。备份的命令如下：
#dd if=/dev/baddrive of=/storagearea
然后可以在已经卸载的故障盘上运行如下命令找到备份的superblock.
#mke2fs -n /dev/badparition
再运行mke2fs命令的时候需要把参数设置成为文件系统创建时所用的参数。如果当初使用的是默认值， 那就可以使用如下命令：
#mke2fs -n -b 4000 /dev/hdb1
可以看到有如下的输出:
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
122400 inodes, 488848 blocks
24442 blocks (5.00%) reserved for the super user
First data block=1
60 block groups
8192 blocks per group, 8192 fragments per group
2040 inodes per group
Superblock backups stored on blocks:
8193, 24577, 40961, 57345, 73729, 204801, 221185, 401409
从输出可知superblock存在于: 8193, 24577, 40961, 57345, 73729, 204801, 221185, 401409.
