I'm trying to have this done on a debian 9 based embedded system. The fs that come for root is mounted as such in /etc/fstab:
# /etc/fstab: static file system information.
#
/dev/mmcblk1p1 / ext4 noatime,errors=remount-ro 0 1
debugfs /sys/kernel/debug debugfs defaults 0 0
My current file system looks like this:
Filesystem Size Used Avail Use% Mounted on
udev 215M 0 215M 0% /dev
tmpfs 49M 6.0M 43M 13% /run
/dev/mmcblk1p1 3.5G 1.9G 1.4G 59% /
tmpfs 242M 0 242M 0% /dev/shm
tmpfs 5.0M 0 5.0M 0% /run/lock
tmpfs 242M 0 242M 0% /sys/fs/cgroup
tmpfs 49M 0 49M 0% /run/user/1000
Now what I'm required to do is use unionfs (or aufs) + ext4 to create a more resilient mounting system because /ext4 might fail due to power outage and file corruption might occur. The idea here is to have a readonly file layer at the bottom with a writable one on top for /home/debian as well as any other that might require system writing to them (like var/log dir). What I was told is that ext4 working with an embedded system that might loss power unexpectedly on regular basis, would cause series disk issues/corruption especially since we are going to be writing to disk alot while our Application is running.(Link). IS this completely true even with the presence of Journaling in ext4?
After hours and hours of researching I found that it is suggested to go through the /usr/share/initramfs-tools by creating hooks/scripts and such. After creating new script and booting my system I was able to get something similar to what I had wanted:
Filesystem Size Used Avail Use% Mounted on
udev 215M 0 215M 0% /dev
tmpfs 49M 6.0M 43M 13% /run
/dev/mmcblk1p1 3.5G 1.9G 1.4G 59% /ro
root.rw 242M 8.5M 234M 4% /rw
root.union 242M 8.5M 234M 4% /
tmpfs 242M 0 242M 0% /dev/shm
tmpfs 5.0M 0 5.0M 0% /run/lock
tmpfs 242M 0 242M 0% /sys/fs/cgroup
tmpfs 49M 0 49M 0% /run/user/1000
&& using mount I get the following details for newly added ro && rw file systems:
/dev/mmcblk1p1 on /ro type ext4 (ro,relatime,data=ordered)
root.rw on /rw type tmpfs (rw,relatime)
root.union on / type aufs (rw,relatime,si=587d3414)
Now this has worked in terms of making my initial / read only system that is in /ro dir && I have installed on top a writable layer in /rw. The issue here is that if I create new files/dir they are all gone after booting the device. Looking deep into this I suspect the fact that I was using tmpfs might have caused that since it is used normally for RAM, but I have tough using aufs on top (root.union) would solce tis issue but it didnt.
How can I solve this issue by making the /rw layer retain any new information/files created even after boot? Keep in mind my system is read only now so everytime I try to go into my script and mount /rw as aufs it is reverted back after booting the device.
Note: Does my current set up mean that logs from my current App are not logged to my /var/log/... and are lost after booting device? some Apps would usually log to /var/log/syslog which is in /rw directory but I can't seem to confirm if they are retaining new logs after boot up or just reverting to what they had at start up in them?
