Linux FSCK自动修复文件系统
背景：
Linux系统（Ubuntu）在运行时，断电等非正常关机操作，会导致ext4文件系统数据损坏。严重时会导致系统崩溃。如下log就是系统数据损坏。
[ 7.878756] EXT4-fs error (device mmcblk0p2): ext4_mb_generate_buddy:742: group 0, 14845 clusters in bitmap, 14822 in gd [ 8.484660] init: samba-ad-dc main process (995) terminated with status 1 [ 14.248075] EXT4-fs error (device mmcblk0p2): ext4_mb_generate_buddy:742: group 1, 1 clusters in bitmap, 2 in gd
1、开机log，如上log就是开机时，kernel监测到文件系统错误；
2、比如要检查的分区是/dev/mmcblk0p2，如下红色字体部分就是系统错误的信息。
~# tune2fs -l /dev/mmcblk0p2 tune2fs 1.42.9 (4-Feb-2014) Filesystem volume name: <none> Last mounted on: / Filesystem UUID: ab013911-6048-465f-8a1a-cf1420c7bb01 Filesystem magic number: 0xEF53 Filesystem revision #: 1 (dynamic) Filesystem features: ext_attr resize_inode dir_index filetype extent flex_bg sparse_super large_file huge_file uninit_bg dir_nlink extra_isize Filesystem flags: unsigned_directory_hash Default mount options: user_xattr acl Filesystem state: not clean with errors Errors behavior: Continue Filesystem OS type: Linux Inode count: 393216 Block count: 1572864 Reserved block count: 78643 Free blocks: 289870 Free inodes: 169990 First block: 0 Block size: 4096 Fragment size: 4096 Reserved GDT blocks: 383 Blocks per group: 32768 Fragments per group: 32768 Inodes per group: 8192 Inode blocks per group: 512 Flex block group size: 16 Filesystem created: Sat Sep 12 11:55:02 2015 Last mount time: Mon Oct 5 00:56:20 2015 Last write time: Mon Oct 5 00:56:32 2015 Mount count: 49 Maximum mount count: -1 Last checked: Sat Sep 12 11:55:02 2015 Check interval: 0 (<none>) Lifetime writes: 5936 MB Reserved blocks uid: 0 (user root) Reserved blocks gid: 0 (group root) First inode: 11 Inode size: 256 Required extra isize: 28 Desired extra isize: 28 Default directory hash: half_md4 Directory Hash Seed: 37b6421d-4697-4ff5-a68e-4a1e1ea81c0e Journal backup: inode blocks <span style="color:#ff0000;">FS Error count: 5 First error time: Mon Oct 5 00:52:47 2015 First error function: ext4_mb_generate_buddy First error line #: 742 First error inode #: 0 First error block #: 0 Last error time: Mon Oct 5 00:56:32 2015 Last error function: ext4_mb_generate_buddy Last error line #: 742 Last error inode #: 0 Last error block #: 0</span>
修复方法：
1、手动修复：借助其他完整系统启动，对所在磁盘分区卸载，比如要修复/dev/mmcblk0p2,
执行命令 fsck.ext4 /dev/mmcblk0p2 可检查修复系统；
2、自动修复：
条件：
（1）、 自动修复要保证，bootloader参数bootargs 生命挂载以制度方式挂载根文件系统
console=tty1 console=ttySAC2,115200n8 root=UUID=e139ce78-9841-40fe-8823-96a304a09859 rootwait ro
如果最后ro是rw，将不能完成自动修复。
（2）、 编辑/etc/fstab 挂载最后一个选项设置为1，标明启动时自动检测文件系统，如下：
UUID=e139ce78-9841-40fe-8823-96a304a09859 / ext4 errors=remount-ro,noatime,nodiratime 0 1
（3）、 编辑 /etc/default/rcS 最后一个选项（其他linux系统有区别）
# automatically repair filesystems with inconsistencies during boot
FSCKFIX=yes
然后，可以参考/etc/init/mountall.conf
description "Mount filesystems on boot"
start on startup
stop on starting rcS
expect daemon
task
emits virtual-filesystems
emits local-filesystems
emits remote-filesystems
emits all-swaps
emits filesystem
emits mounting
emits mounted
script
. /etc/default/rcS || true
[ -f /forcefsck ] && force_fsck="--force-fsck"
[ "$FSCKFIX" = "yes" ] && fsck_fix="--fsck-fix"
# Doesn't work so well if mountall is responsible for mounting /proc, heh.
if [ -e /proc/cmdline ]; then
read line < /proc/cmdline
for arg in $line; do
case $arg in
-q|--quiet|-v|--verbose|--debug)
debug_arg=$arg
;;
esac
done < /proc/cmdline
fi
# set $LANG so that messages appearing in plymouth are translated
if [ -r /etc/default/locale ]; then
. /etc/default/locale || true
export LANG LANGUAGE LC_MESSAGES LC_ALL
fi
exec mountall --daemon $force_fsck $fsck_fix $debug_arg end script
post-stop script rm -f /forcefsck 2>dev/null || true end script（4）、 系统检测到分区有问题时，会再根目录下创建一个空文件/forcefsck，重启后，执行mountall，自动进行修复，然后删除forcefsck，也可以手动创建/forcefsck，系统同样会在下次启动时强制检查修复文件系统；
Log：
系统启动检查修复过程的log，不在/var/log/fsck/目录下，而是在/var/log/upstart/目录下，文件为 mountall.log，如下：
# cat mountall.log mount: mount point /media/boot does not exist mountall: mount /media/boot [382] terminated with status 32 mountall: Filesystem could not be mounted: /media/boot Skipping /media/boot at user request Skipping /media/boot at user request Skipping /media/boot at user request fsck from util-linux 2.20.1 e2fsck 1.42.9 (4-Feb-2014) /dev/mmcblk0p2: clean, 223220/393216 files, 1282976/1572864 blocks
其他：
也可以通过设置 系统挂载的次数来自动检查修复文件系统
比如：
tune2fs -c 30 /dev/mmcblk0p2 系统每启动30次，就会检查修复一次。
修复完成后，通过 tune2fs -l /dev/mmcblk0p2看到没有错误信息，如下：
~# tune2fs -l /dev/mmcblk0p2 tune2fs 1.42.9 (4-Feb-2014) Filesystem volume name: <none> Last mounted on: / Filesystem UUID: ab013911-6048-465f-8a1a-cf1420c7bb01 Filesystem magic number: 0xEF53 Filesystem revision #: 1 (dynamic) Filesystem features: ext_attr resize_inode dir_index filetype extent flex_bg sparse_super large_file huge_file uninit_bg dir_nlink extra_isize Filesystem flags: unsigned_directory_hash Default mount options: user_xattr acl Filesystem state: not clean Errors behavior: Continue Filesystem OS type: Linux Inode count: 393216 Block count: 1572864 Reserved block count: 78643 Free blocks: 289888 Free inodes: 169996 First block: 0 Block size: 4096 Fragment size: 4096 Reserved GDT blocks: 383 Blocks per group: 32768 Fragments per group: 32768 Inodes per group: 8192 Inode blocks per group: 512 Flex block group size: 16 Filesystem created: Sat Sep 12 11:55:02 2015 Last mount time: Mon Oct 5 00:58:56 2015 Last write time: Mon Oct 5 00:58:41 2015 Mount count: 1 Maximum mount count: -1 Last checked: Mon Oct 5 00:58:41 2015 Check interval: 0 (<none>) Lifetime writes: 5938 MB Reserved blocks uid: 0 (user root) Reserved blocks gid: 0 (group root) First inode: 11 Inode size: 256 Required extra isize: 28 Desired extra isize: 28 Default directory hash: half_md4 Directory Hash Seed: 37b6421d-4697-4ff5-a68e-4a1e1ea81c0e Journal backup: inode blocks