使用overlayfs打造一个只读的不怕意外关机的树莓派Raspberry Pi

树莓派的本领就不多说了。但是在树莓派的应用场合，关机的时候还是显得尴尬，先不说执行 sudo halt 要么需要ssh上去，要么需要有键盘和显示器，更不要说，有的场景可能连网络和显示器都没有，真正的 headless。 但是如果不执行sudo halt直接关电源，那么有很大的概率会损坏SD卡上的文件系统，甚至损坏SD卡。

overlayfs是linux系统下的一种影子文件系统，它可以把真正的存储文件系统作为只读挂载，而把所有文件改动都存放在RAM中，关机或者重启就失效，真正的保护了存储文件系统。  下面就说一下我在树莓派中实施overlayfs的方法，其实很简单。

我参考了这个帖子中介绍的方法：

https://www.raspberrypi.org/forums/viewtopic.php?t=173063#p1151405

1. 首先，禁用交换空间，毕竟树莓派里的交换空间也是占用RAM（tempfs）的，所以如果使用了overlayfs后，交换空间就显得没有意义了。

执行如下3条命令：

  sudo dphys-swapfile swapoff
  sudo dphys-swapfile uninstall
  sudo update-rc.d dphys-swapfile remove

2. 更新系统，保证系统为最新状态

sudo apt-get update
sudo apt-get upgrade

3. 保存如下脚本为 overlayRoot.sh

注(2020-11-22)：如下脚本已过时，最新脚本请前往链接【 http://wiki.psuter.ch/doku.php?id=solve_raspbian_sd_card_corruption_issues_with_read-only_mounted_root_partition#the_script 】

#!/bin/sh
#  Read-only Root-FS for Raspian using overlayfs
#  Version 1.0
#
#  Created 2017 by Pascal Suter @ DALCO AG, Switzerland
#  to work on Raspian as custom init script
#  (raspbian does not use an initramfs on boot)
#
#  Modified 2017-Apr-21 by Tony McBeardsley 
#
#  This program is free software: you can redistribute it and/or modify
#  it under the terms of the GNU General Public License as published by
#  the Free Software Foundation, either version 3 of the License, or
#  (at your option) any later version.
#
#  This program is distributed in the hope that it will be useful,
#  but WITHOUT ANY WARRANTY; without even the implied warranty of
#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#  GNU General Public License for more details.
#
#    You should have received a copy of the GNU General Public License
#    along with this program.  If not, see
#    <http://www.gnu.org/licenses/>.
#
#
#  Tested with Raspbian mini, 2017-01-11
#
#  This script will mount the root filesystem read-only and overlay it with a temporary tempfs 
#  which is read-write mounted. This is done using the overlayFS which is part of the linux kernel 
#  since version 3.18. 
#  when this script is in use, all changes made to anywhere in the root filesystem mount will be lost 
#  upon reboot of the system. The SD card will only be accessed as read-only drive, which significantly