Looking for a sanity check from the folk how know more of systemd than do I. Not looking for someone else to solve my problem, but could use a clue.
Trying to figure out how to get an overlayfs root mounted early in boot.
Building an embedded system that must not store any sensitive data when powered down. Found this was referred to on this list more recently as a "stateless" system. (And Tobias Hunger seems to have the most to say. :) ) We are using Centos8, with systemd 239. Starting point is a vanilla Centos8 server installation to a small (8 or 32GB) flash volume.
Had the (possibly) clever notion of using an overlayfs as the root mount, with a tmpfs as the upper, and the usual persistent volume as the lower. The initial round of configuration and test would just be against a stock Centos8 install. Once fully configured and tested, would add a default boot menu item to boot with root mounted as an overlayfs, with the fully configured root volume as the read-only lower.
Updates would be accomplished by booting from the original boot menu entry. (This is slightly complicated by the fact the target systems' computers do not have a console - but figure I can script altering the default boot.)
Have what I think is a properly configured overlayfs root. Currently hung up on getting that root mounted early enough(?) in boot. Working down a list of possibilities. :)
Have a script to repeatably build/rebuild the overlayfs root. Relevant commands (w/o supporting logic):
----
BASE=/run/overlay_root
OVERLAY=$BASE/merged
ROOT_MOUNT=$OVERLAY/mnt/root
mount -t tmpfs root-base $BASE ; mkdir $BASE/{lower,upper,work,merged}
mount --bind / $BASE/lower
mount -t overlayfs root-overlay $OVERLAY -olowerdir=$BASE/lower,upperdir=$BASE/upper,workdir=$BASE/work
for d in boot dev proc run sys ; do mount --bind /$d $OVERLAY/$d ; done
mkdir $ROOT_MOUNT ; mount --bind / $ROOT_MOUNT
----
This all seems to work. The overlayfs prevents writes to persistent media in usual places. Have a path to write to persistent media. The special directories (that do not work from mounts to lower on an overlayfs) work as expected.
What I do not have as yet is a means to get the overlayfs root mounted early in boot.
1. Does not look like /etc/fstab can create the overlayfs.
2. Tried the "systemd.volatile=yes" kernel command line, w/o luck. Substantial commits between 239 and 245 around this.
3. See systemd "pivot_root" and "switch_root", but not yet puzzled out usage.
Looking for where/how to swap in the overlayfs root early in boot. Or tell me I'm an idiot, and this will not work. :/
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