Linux系统灾难恢复技术和方法-[3]

系统 /boot 分区损坏

一般来说系统 /boot 分区损坏，我们会先尝试修复文件系统。如果文件系统损坏不能修复，那么我们可以参照前述的方法来依次新建 /boot 分区，重新安装内核和镜像，然后安装 GURB 再手工编辑引导菜单，以最终来恢复系统可正常引导。通常我们需要按照如下的步骤来恢复。
创建分区
碰到比较严重的情况就是 /boot 分区已经完全损坏，启动时会提示找不到引导设备。
图 8. 引导分区损坏



挂载安装盘后进入援救模式，查看分区情况，发现分区 /dev/sda1 不存在。

bash-4.1#
Disk /dev/sda: 43.0 GB, 42991616000 bytes
255 heads, 63 sectors/track, 5226 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Device Boot      Start         End      Blocks   Id  System
/dev/sda2              17         147     1048576   82  Linux swap / Solaris
Partition 2 does not end on cylinder boundary.
/dev/sda3             147        5227    40803328   83  Linux

Disk /dev/sdb: 2147 MB, 2147483648 bytes
255 heads, 63 sectors/track, 261 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0xcdd48395

Device Boot      Start         End      Blocks   Id  System
/dev/sdb1               1         261     2096451   83  Linux

新建一个分区并且设置它为启动分区。

bash-4.1# fdisk /dev/sda

WARNING: DOS-compatible mode is deprecated. It's strongly recommended to
switch off the mode (command 'c') and change display units to
sectors (command 'u').

Command (m for help): n
Command action
e   extended
p   primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-5226, default 1):
Using default value 1
Last cylinder, +cylinders or +size{K,M,G} (1-16, default 16):
Using default value 16

Command (m for help): a
Partition number (1-4): 1

Command (m for help): p

Disk /dev/sda: 43.0 GB, 42991616000 bytes
255 heads, 63 sectors/track, 5226 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          16      128488+  83  Linux
/dev/sda2              17         147     1048576   82  Linux swap / Solaris
Partition 2 does not end on cylinder boundary.
/dev/sda3             147        5227    40803328   83  Linux

Command (m for help): w
The partition table has been altered!

重启主机以更新分区表，然后进入援救模式，并在我们新创建的分区上创建文件系统。

bash-4.1# mkfs.ext4 /dev/sda1
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
Stride=0 blocks, Stripe width=0 blocks
32128 inodes, 128488 blocks
6424 blocks (5.00%) reserved for the super user
First data block=1
Maximum filesystem blocks=67371008
16 block groups
8192 blocks per group, 8192 fragments per group
2008 inodes per group
Superblock backups stored on blocks:
8193, 24577, 40961, 57345, 73729

Writing inode tables: done
Creating journal (4096 blocks): done
Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 38 mounts or
180 days, whichever comes first.  Use tune2fs -c or -i to override.

安装内核镜像文件
通过前述的方法我们安装内核和镜像文件。

bash-4.1# chroot /mnt/sysimage
sh-4.1# mount /dev/sda1 /boot
sh-4.1# mount – o loop /dev/sr0 /media
sh-4.1# cd /media/Server/Packages
sh-4.1# rpm -ivh --force kernel-2.6.32-71.el6.x86_64.rpm
warning: kernel-2.6.32-71.el6.x86_64.rpm: \
Header V3 RSA/SHA256 Signature, key ID fd431d51: NOKEY
Preparing...                ########################################### [100%]
1:kernel                  ########################################### [100%]

安装 GRUB
我们安装 GRUB 到硬盘设备 sda 上。

sh-4.1# grub-install /dev/sda
Installation finished. No error reported.
This is the contents of the device map /boot/grub/device.map.
Check if this is correct or not. If any of the lines is incorrect,
fix it and re-run the script `grub-install'.

(fd0)   /dev/fd0
(hd0)   /dev/sda
(hd1)   /dev/sdb

编辑引导菜单
由于我们创建了新的分区，其对应的 UUID 会发生变化，可以通过命令 blkid 来查询分区的 UUID。

bash-4.1# blkid
/dev/loop0: TYPE="squashfs"
/dev/sda2: UUID="7b1e0fac-ff06-492c-848d-497e2a38c54e" TYPE="swap"
/dev/sda3: UUID="ef89764e-04ff-4f26-ae82-dcab267ecc66" TYPE="ext4"
/dev/sdb1: UUID="2b824352-df2a-44c6-a547-838d46f526fa" SEC_TYPE="ext2" TYPE="ext3"
/dev/loop1: LABEL="RHEL_6.0 x86_64 Disc 1" TYPE="iso9660"
/dev/sda1: UUID="cec964af-1618-48ff-ac33-4ef71b9d3265" TYPE="ext4"

上述的 sda3 为根分区，编辑 /boot/grub/grub.conf 文件更新其对应的 UUID，其内容如下。

title Red Hat Enterprise Linux 6
root (hd0,0)
kernel /vmlinuz-2.6.32-71.el6.x86_64 \
root=UUID=ef89764e-04ff-4f26-ae82-dcab267ecc66 rhgb quiet
initrd /initramfs-2.6.32-71.el6.x86_64.img

更新 /etc/fstab
类似的我们也需要更新 /etc/fstab 里 /boot 分区对应的新 UUID，其内容如下。

#
# /etc/fstab
# Created by anaconda on Sun Mar 18 04:35:07 2012
#
# Accessible filesystems, by reference, are maintained under '/dev/disk'
# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info
#
UUID=ef89764e-04ff-4f26-ae82-dcab267ecc66 /                  ext4    defaults        1 1
UUID=cec964af-1618-48ff-ac33-4ef71b9d3265 /boot              ext4    defaults        1 2
UUID=7b1e0fac-ff06-492c-848d-497e2a38c54e swap               swap    defaults        0 0
tmpfs                   /dev/shm                tmpfs   defaults        0 0
devpts                  /dev/pts                devpts  gid=5,mode=620  0 0
sysfs                   /sys                     sysfs   defaults        0 0
proc                    /proc                    proc    defaults        0 0

现在我们的恢复步骤已经完成，重启主机后 GRUB 中可见我们配置的系统列表。
图 9. GRUB 菜单



至此 /boot 分区已恢复，系统可正常引导启动。
图 10. 系统启动



总结
本文阐述了常见的 Linux 灾难恢复技术和方法，及其出现严重灾难时应注意的恢复顺序，以确保 Linux 系统在出现灾难时得以安全恢复。