ext4文件系统启动自检的必要性

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最近我们发现多个用户设备掉电后重启，系统不工作。

研究这些返修设备，发现这些设备的表象是网络连接失败，DNS resolve不了。进一步发现/etc/resolv.conf为空，所以应用程序没法进行DNS resolve。但是在同一个路由器下面，其他设备是可以正常获取DNS 服务器信息的，后来检查dhcpc的log，发现他获取了DNS信息，但是写/etc/resolv.conf失败，这样应用程序读取dns server时就会失败。

为啥写文件失败呢？后来我们用df检查根文件系统，发现文件系统已经被占满了。下面显示整个16M的根目录空间都被占了

/ # df

Filesystem           1K-blocks      Used Available Use% Mounted on

overlay                                0 100% /

none                                          % /dev

run                                    % /run

shm                                     % /dev/shm

ubi2:exa_data_                     % /config

ubi2:exa_data_                     % /log

ubi2:exa_data_                     % /tokens

tmpfs                                         % /dev/snd

tmpfs                                         % /dev/input/event0

tmpfs                                         % /dev/hbi

tmpfs                                % /tmp/ubus.sock

tmpfs                                % /run/dbus/system_bus_socket

cgroup_root                               % /sys/fs/cgroup

/ # lsof |grep deleted

     /sbin/rc        /run/openrc/exclusive/bootmisc (deleted)

     /sbin/rc        /run/openrc/exclusive/networking (deleted)

     /sbin/rc        /run/openrc/exclusive/syslog (deleted)

     /sbin/rc        /run/openrc/exclusive/avs-server (deleted)

/ # exit

但是我们到文件系统mount到的目录用du查看时，却发现实际的文件并没有占那么多。下面显示upperdir只占用了7M

[router] /overlay # du -d 1

2       ./workdir

7043    ./upperdir

12      ./lost+found

7058    .

那么多余的空间都跑哪去了呢？接着我们来检查一下文件系统对应的img。发现这个img居然被破坏了，这样就导致我们用df看到的信息不正确。

~$ fsck.ext4 rootfs_overlay.img

e2fsck 1.44.1 (24-Mar-2018)

rootfs_overlay.img contains a file system with errors, check forced.

Pass 1: Checking inodes, blocks, and sizes

Inode 2378, i_blocks is 2, should be 0.  Fix<y>? no

Deleted inode 2381 has zero dtime.  Fix<y>? no

Deleted inode 2386 has zero dtime.  Fix<y>? no

Pass 2: Checking directory structure

Pass 3: Checking directory connectivity

Pass 4: Checking reference counts

Pass 5: Checking group summary information

Block bitmap differences:  +2612 -(8525--8526) -(8708--8720) -8725 -8731 -(8769--8772) -9235 -(9239--9248) -10319 -(10380--10396) -(10781--10785) -10923 -(12353--12356) -(12649--12656) -(13513--13764)

Fix<y>? no

Free blocks count wrong for group #1 (7063, counted=7062).

Fix<y>? no

Free blocks count wrong (8361, counted=8356).

Fix<y>? no

Inode bitmap differences:  -2381 -2386

Fix<y>? no

rootfs_overlay.img: ********** WARNING: Filesystem still has errors **********

rootfs_overlay.img: 351/4096 files (2.3% non-contiguous), 8023/16384 blocks

之后我们用fsck将这个image修复。然后重新mount文件系统，系统这时就工作正常了。

~$ fsck.ext4 rootfs_overlay.img

e2fsck 1.44.1 (24-Mar-2018)

rootfs_overlay.img contains a file system with errors, check forced.

Pass 1: Checking inodes, blocks, and sizes

Inode 2378, i_blocks is 2, should be 0.  Fix<y>? yes

Deleted inode 2381 has zero dtime.  Fix<y>? yes

Deleted inode 2386 has zero dtime.  Fix<y>? yes

Pass 2: Checking directory structure

Pass 3: Checking directory connectivity

Pass 4: Checking reference counts

Pass 5: Checking group summary information

Block bitmap differences:  +2612 -(8525--8526) -(8708--8720) -8725 -8731 -(8769--8772) -9235 -(9239--9248) -10319 -(10380--10396) -(10781--10785) -10923 -(12353--12356) -(12649--12656) -(13513--13764)

Fix<y>? yes

Free blocks count wrong for group #0 (1294, counted=1293).

Fix<y>? yes

Free blocks count wrong for group #1 (7063, counted=7382).

Fix<y>? yes

Free blocks count wrong (8361, counted=8675).

Fix<y>? yes

Inode bitmap differences:  -2381 -2386

Fix<y>? yes

Free inodes count wrong for group #1 (1711, counted=1713).

Fix<y>? yes

Free inodes count wrong (3745, counted=3747).

Fix ('a' enables 'yes' to all) <y>? yes to all

rootfs_overlay.img: ***** FILE SYSTEM WAS MODIFIED *****

rootfs_overlay.img: 349/4096 files (2.3% non-contiguous), 7709/16384 blocks

~$ fsck.ext4 rootfs_overlay.img

e2fsck 1.44.1 (24-Mar-2018)

rootfs_overlay.img: clean, 349/4096 files, 7709/16384 blocks

~$