overlay文件系统切换为根文件系统
内核配置
CONFIG_OVERLAY_FS
配置后编译烧写，可见支持了overlay文件系统
/tmp # cat /proc/filesystems
nodev sysfs
nodev rootfs
nodev ramfs
nodev bdev
nodev proc
nodev cgroup
nodev tmpfs
nodev devtmpfs
nodev sockfs
nodev pipefs
nodev rpc_pipefs
nodev devpts
ext3
ext2
ext4
cramfs
squashfs
vfat
msdos
iso9660
nodev nfs
nodev jffs2
fuseblk
nodev fuse
nodev fusectl
nodev overlay
udf
yaffs
yaffs2
nodev mtd_inodefs
nodev ubifs
正常挂载
/tmp # mount
rootfs on / type rootfs (rw)
/dev/root on / type squashfs (ro,relatime)
proc on /proc type proc (rw,nosuid,nodev,relatime)
sysfs on /sys type sysfs (rw,nosuid,nodev,relatime)
tmpfs on /dev type tmpfs (rw,relatime)
ramfs on /tmp type ramfs (rw,nosuid,nodev,relatime,mode=1777)
/dev/root on /mnt/conf type squashfs (ro,relatime)
devpts on /dev/pts type devpts (rw,relatime,mode=600)
/dev/mtdblock3 on /mnt/conf type jffs2 (rw,relatime)挂载脚本
#!/bin/sh
mkdir -p /mnt/conf
mount -t jffs2 /dev/mtdblock3 /mnt/conf
mkdir -p /mnt/conf/base
mkdir -p /mnt/conf/work
mount -t overlay overlay -o lowerdir=/,upperdir=/mnt/conf/base,workdir=/mnt/conf/work /mnt
mount /proc -o move /mnt/proc
mkdir -p /mnt/rom
pivot_root /mnt /mnt/rom
mount /rom/dev -o move /dev
mount /rom/tmp -o move /tmp
mount /rom/sys -o move /sys
mount /rom/dev/pts -o move /dev/pts
挂载效果
/tmp # mount
rootfs on / type rootfs (rw)
/dev/root on /rom type squashfs (ro,relatime)
proc on /proc type proc (rw,nosuid,nodev,relatime)
sysfs on /sys type sysfs (rw,nosuid,nodev,relatime)
tmpfs on /dev type tmpfs (rw,relatime)
ramfs on /tmp type ramfs (rw,nosuid,nodev,relatime,mode=1777)
devpts on /dev/pts type devpts (rw,relatime,mode=600)
/dev/mtdblock3 on /rom/mnt/conf type jffs2 (rw,relatime)
overlay on / type overlay (rw,relatime,lowerdir=/,upperdir=/mnt/conf/base,workdir=/mnt/conf/work)
/tmp # df -h
Filesystem Size Used Available Use% Mounted on
/dev/root 8.4M 8.4M 0 100% /rom
tmpfs 123.8M 4.0K 123.8M 0% /dev
/dev/mtdblock3 512.0K 200.0K 312.0K 39% /rom/mnt/conf
overlay 512.0K 200.0K 312.0K 39% /