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从我厂自制的OS 7u2升级转换到CentOS 7u2操作系统，总是发生内核一旦升级，就无法正常启动。这个问题困扰了我很久，往往退而求其次，避开升级内核。但是，伪处女座，不能忍啊！

从带外检查启动输出显示

         Starting udev Kernel Device Manager...
[  OK  ] Started udev Kernel Device Manager.
         Starting udev Coldplug all Devices...
         Mounting Configuration File System...
[  OK  ] Mounted Configuration File System.
[  OK  ] Started udev Coldplug all Devices.
[  OK  ] Reached target System Initialization.
[    5.378473] megasas:IOC Init cmd success
         Starting drac[  OK  ] Started Show Plymouth Boot Screen.
[  OK  ] Reached target Paths.
[  OK  ] Reached target Basic System.
[    5.425533] megasas: INIT adapter done
[  OK  ] Found device P[    5.843292] systemd-fsck[522]: Failed to detect devic/
ERC_H710P /.
 [    5.852940] systemd[1]: Failed to start File System Check on /dev/disk/by-l.
        Starting[    5.862951] systemd[1]: Dependency failed for /sysroot.
 File System Che[    5.870309] systemd[1]: Dependency failed for Initrd Root Fi.
ck on /dev/disk/[    5.879141] systemd[1]: Dependency failed for Reload Configu.
by-label/\x2f...
[FAILED] Failed to start File System Check on /dev/disk/by-label/\x2f.
See 'systemctl status sys
Generating "/run/initramfs/rdsosreport.txt"


Entering emergency mode. Exit the shell to continue.
Type "journalctl" to view system logs.
You might want to save "/run/initramfs/rdsosreport.txt" to a USB stick or /boot
after mounting them and attach it to a bug report.

报错显示和磁盘的label有关，也就是升级以后，设备文件无法找到原先按照label标记的磁盘设备。我遇到过以往升级后，磁盘设备完全是被为UUID的情况，所以推测有可能类似情况又发生了。

  • 切换到磁盘操作系统
mkdir /mnt
mount /dev/sda2 /mnt
mount /dev/sda1 /mnt/boot
mount -t proc proc /mnt/proc
mount --rbind /sys /mnt/sys
mount --make-rslave /mnt/sys
mount --rbind /dev /mnt/dev
mount --make-rslave /mnt/dev
chroot /mnt /bin/bash
source /etc/profile
  • 检查系统设备
cd /dev/disk/by-label
ls -lh

显示输出

total 0
lrwxrwxrwx 1 root 0 10 Jul 16 10:52 SWAP -> ../../sda3
lrwxrwxrwx 1 root 0 10 Jul 16 10:52 \x2f -> ../../sda2
lrwxrwxrwx 1 root 0 10 Jul 16 10:52 \x2fboot -> ../../sda1

看来是设备文件by-label识别的时候原先的/符号被识别成了\x2f - 通过挂载/dev/sda2(挂载命令 mkdir /mnt;mount /dev/sda2 /mnt)检查这个磁盘设备下的文件 etc/fstab 可以看到，原先设置是

LABEL=/boot		/boot		ext4		defaults	1 2
LABEL=/		/		ext4		defaults	1 1
LABEL=SWAP		swap		swap		defaults	0 0

凡是带/开头的符号都被识别成了\x2f，所以需要修改磁盘挂载。

可以采用以下方法之一：

  • 使用旧版本内核启动，然后从新对磁盘分区进行label，去除掉不兼容的/符号。然后再重启到新内核
  • 直接修改etc/fstab，将LABEL=识别磁盘的方式更改为使用UUID识别方式

label标记分区

参考 Label a Linux Partition 使用 e2label 命令

  • 显示分区标记
cd /sbin
./e2label /dev/sda1

输出显示

/boot
  • 修改分区标记
./e2label /dev/sda1 BOOT

同样再修改 /dev/sda2

./e2label /dev/sda2 ROOT
  • 修改etc/fstab配置如下
LABEL=BOOT		/boot		ext4		defaults	1 2
LABEL=ROOT		/		ext4		defaults	1 1
LABEL=SWAP		swap		swap		defaults	0 0

但是很奇怪，重启依然没有找到by-label设备。

汗，忘记修改 grub 启动中的配置了！原先的配置中还遗留着对/dev/disk/by-label/\x2f的引用！

编辑 /mnt/boot/grub2/grub.cfg 可以看到，这个文件不是直接编辑的，是通过编辑模板/etc/grub.d目录下模板和 /etc/default/grub后，然后通过grub2-mkconfig生成的。在斌即过/etc/fstab之后（更换label或者从label转换到uuid或相反），需要执行一次grub2-mkconfig来生成新的配置文件。

检查 /boot/grub2/grub.cfg 可以看到如下配置显示是采用root=LABEL=/

#grep "root=LABEL" grub.cfg
	linux16 /vmlinuz-0-rescue-0b784259b076423083282fb71ecc7fa9 root=LABEL=/ ro crashkernel=auto vconsole.font=latarcyrheb-sun16 vconsole.keymap=us biosdevname=0 ipv6.disable=1 console=tty0 console=ttyS0,115200 scsi_mod.scan=sync LANG=en_US.UTF-8
	linux16 /vmlinuz-3.10.0-327.22.2.el7.x86_64 root=LABEL=/ ro crashkernel=auto vconsole.font=latarcyrheb-sun16 vconsole.keymap=us biosdevname=0 ipv6.disable=1 console=tty0 console=ttyS0,115200 scsi_mod.scan=sync LANG=en_US.UTF-8

上述两行就是新安装内核自动添加按照磁盘LABEL=/来挂载启动盘。虽然后面订正了/etc/fstab，但是grub.cfg没有订正。

cp grub.cfg grub.cfg.bak
grub2-mkconfig

不过，再次启动，依然存在问题，显示

[    5.468314] megasas: INIT adapter done
[ TIME    94.982368] systemd[1]: Timed out waiting for device dev-disk-by\x2dlabel-\x5cx2f.device.
0m] Timed out wa[   94.982696] systemd[1]: Dependency failed for /sysroot.
iting for device dev-disk-by\x2d[   95.000182] systemd[1]: Dependency failed for Initrd Root File System.
label-\x5cx2f.de[   95.010574] systemd[1]: Dependency failed for Reload Configuration from the Real Root.
vice.
[[   95.020962] systemd[1]: Dependency failed for File System Check on /dev/disk/by-label/\x2f.
DEPEND] Dependency failed foWarning: /dev/disk/by-label/x2f does not exist

Generating "/run/initramfs/rdsosreport.txt"


Entering emergency mode. Exit the shell to continue.
Type "journalctl" to view system logs.
You might want to save "/run/initramfs/rdsosreport.txt" to a USB stick or /boot
after mounting them and attach it to a bug report.
...

[  128.769627] dracut-initqueue[466]: Warning: dracut-initqueue timeout - starting timeout scripts
[  129.292098] dracut-initqueue[466]: Warning: dracut-initqueue timeout - starting timeout scripts
...
[  189.812708] dracut-initqueue[466]: Warning: Could not boot.
[  189.815089] dracut-initqueue[466]: Warning: /dev/disk/by-label/x2f does not exist
[  189.826322] dracut-initqueue[466]: Failed to start dracut-emergency.service: Transaction is destructive.
[  189.829207] dracut-initqueue[466]: Warning: Not all disks have been found.
[  189.829534] dracut-initqueue[466]: Warning: You might want to regenerate your initramfs.

这步不需要：按照提示我尝试创建initramfs文件来修正磁盘启动 - 对于RHEL/CentOS 6/7 ，需要使用 dracut 工具(不过这个步骤看来无效，应该还是需要手工订正/boot/grub2/grub.cfg)

cd /boot
cp initramfs-3.10.0-327.22.2.el7.x86_64.img initramfs-3.10.0-327.22.2.el7.x86_64.img.bak
dracut -f /boot/initramfs-3.10.0-327.22.2.el7.x86_64.img 3.10.0-327.22.2.el7.x86_64

如果是ubuntu/debian，则参考 UsingUUID，编辑 /etc/initramfs-tools/conf.d/resume 将这个文件中的UUID或者label订正成正确的值并保存。再执行命令 sudo update-initramfs -u

不过，执行之后依然没有解决系统启动后还是在寻找/dev/disk/by-label/x2f，报错依然相同。我本来以为grub2-mkconfig命令会自动按照/etc/fstab中设置来创建/boot/grub2/grub.cfg，但是发现执行后grub.cfg中依然是保留了原来的root=LABEL=/，所以需要手工订正这个配置文件，将 root=LABEL=/ 修改成 root=LABEL=ROOT。

果然，最终是手工通过修正/boot/grub2/grub.cfg来解决启动时候启动分区识别

UUID

除了上述通过e2label修正磁盘设备的label，另一种方法是使用磁盘设备的UUID。参考 How To Use UUID To Mount Partitions / Volumes Under Ubuntu Linux：

  • 检查磁盘UUID
cd /sbin
./blkid

显示输出

/dev/sda1: LABEL="BOOT" UUID="ac7453fb-aa2a-4bc2-9114-78bd9fb62cb5" TYPE="ext4"
/dev/sda2: LABEL="ROOT" UUID="37d96a70-ae3f-4591-91de-e29145faad8d" TYPE="ext4"
/dev/sda3: LABEL="SWAP" UUID="23e9c8b6-22fd-46b4-8e2f-78eb33127dcd" TYPE="swap"
  • 修改etc/fstab如下
UUID=ac7453fb-aa2a-4bc2-9114-78bd9fb62cb5  /boot  ext4  defaults  1 2
UUID=37d96a70-ae3f-4591-91de-e29145faad8d  /      ext4	defaults  1 1
UUID=23e9c8b6-22fd-46b4-8e2f-78eb33127dcd  swap   swap  defaults  0 0
  • 同样，需要手工修改 /boot/grub2/grub.cfg 将原先启动参数 root=LABEL=/ 修改成 root=UUID=37d96a70-ae3f-4591-91de-e29145faad8d

小结

当修改磁盘的DISK label时，需要修改2个位置：

  • /etc/fstab 对应更改LABEL或者UUID
  • 修改/boot/grub2/grub.cfg将启动参数中root=LABEL=/修改成都应的LABEL或者UUID