OverlayFS
An overlay-filesystem, commonly known as OverlayFS tries to present a filesystem which is the result of overlaying one filesystem on top of the other. This is also sometimes referred to as union-filesystems.
An overlay filesystem combines two filesystems - an ‘upper’ filesystem and a ‘lower’ filesystem. When a name exists in both filesystems, the object in the ‘upper’ filesystem is visible while the object in the ‘lower’ filesystem is either hidden or, in the case of directories, merged with the ‘upper’ object.
Overlay root filesystem
Overlay root filesystem can be enabled by adding the option overlaytmpfs=yes to the Kernel command-line parameters. This will cause the underlying root filesystem to be mounted read-only, with an overlayed tmpfs for modifications which will be discarded at shutdown. This option will avoid constant writing to the disk.
In pi, adding the parameter to /boot/cmdline.txt file saves constant writing to the sd-card. The /boot/cmdline.txt file appears as follows
Contents of /boot/cmdline.txt
The below output shows how the filesystem is mounted when Overlay root filesystem is enabled:
# df -m Filesystem 1M-blocks Used Available Use% Mounted on ... /dev/mmcblk0p2 59555 326 56172 1% /media/root-ro root-tmpfs 225 13 211 6% /media/root-rw overlayfs 225 13 211 6% / ... # mount ... /dev/mmcblk0p2 on /media/root-ro type ext4 (ro,relatime) root-tmpfs on /media/root-rw type tmpfs (rw,relatime,mode=755) overlayfs on / type overlay (rw,relatime,lowerdir=/media/root-ro,upperdir=/media/root-rw/root,workdir=/media/root-rw/work,uuid=on) ...
Removing and adding back the overlaytmpfs=yes option in the /boot/cmdline.txt file using a toggle script will allow easy maintenance on the pi and any software/configuration can be easily added/removed.
Loopback image with overlayfs
When you install Alpine in diskless mode, the entire system is loaded into memory at boot. If you want additional storage (for example, if you need more space than offered by your RAM) we need to create loop-back storage onto the SD card or any writable storage mounted with overlayfs.
First, make the SD card writable again and change fstab to always do so:
mount /media/mmcblk0p1 -o rw,remount sed -i 's/vfat\ ro,/vfat\ rw,/' /etc/fstab
Create the loop-back file, this example is 1 GB:
dd if=/dev/zero of=/media/mmcblk0p1/persist.img bs=1024 count=0 seek=1048576
Install the ext4 utilities:
apk add e2fsprogs
Format the loop-back file:
mkfs.ext4 /media/mmcblk0p1/persist.img
Mount the storage:
echo "/media/mmcblk0p1/persist.img /media/persist ext4 rw,relatime,errors=remount-ro 0 0" >> /etc/fstab mkdir /media/persist mount -a
Make the overlay folders, we are using the /usr directory here, but you can use /home or anything else.
mkdir /media/persist/usr mkdir /media/persist/.work_usr echo "overlay /usr overlay lowerdir=/usr,upperdir=/media/persist/usr,workdir=/media/persist/.work_usr 0 0" >> /etc/fstab mount -a
Your /etc/fstab file should look something like this:
Contents of /etc/fstab
Now commit the changes: (optionally remove the e2fsprogs, but it does contain repair tools)
lbu_commit -d
Remember, with this setup if you install things and you have done this overlay for /usr, you must not commit the 'apk add', otherwise, while it boots it will try and install it to memory, not to the persistent storage.
If you do want to install something small at boot, you can use apk add and lbu commit -d.
If it is something a bit bigger, then you can use apk add but then not commit it. It will be persistent (in /usr), but be sure to check everything you need is in that directory and not in folders you have not made persistent.