eMMC support & concerns
dillon at backplane.com
Sat Feb 18 10:15:13 PST 2023
Just as a side note, it is totally possible to mount root read-only and
then use tmpfs/copied mounts for the directories that the system needs to
write to. Example included below (this is what our release image uses).
Basically you specify a read-only root mount in /etc/fstab and then a
bunch of rw tmpfs mounts using the -C option, which causes tmpfs to copy
the underlying read-only filesystem onto the rw tmpfs filesystem.
# Device Mountpoint FStype Options Dump
/dev/blahblah / ufs ro 0 0
dummy /root tmpfs rw,-C 0 0
dummy /var tmpfs rw,-C 0 0
dummy /tmp tmpfs rw,-C 0 0
dummy /etc tmpfs rw,-C 0 0
dummy /usr/obj tmpfs rw 0 0
proc /proc procfs rw 0 0
If you need to make adjustments to the root filesystem you can manually
change it to a rw mount via 'mount -u -o rw /', and then you can use a null
mount to access the portions hidden by the tmpfs mounts without having to
unmount the tmpfs mounts. e.g. 'mount_null / /mnt' like that.
A couple of other things you might want to do for a small read-only system
is turn off syslogd, e.g. in /etc/rc.conf:
Some additional work might be necessary but those are the basics of working
with a read-only root filesystem.
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