The Concept

Is it possible to boot a working operating system from a read-only disk? Yes it is, but one has to pay attention that some locations on a Unix-system should still be writeable. The following directories are writeable on my example box:

/var/log                Logfiles

/tmp                    Temporary Files

/var/run                Sockets and PID-Files

/var/lock               Lockfiles

/var/spool/postfix      Files for my MTA

Start out by copying your current installation to another partition, change the init-scripts to not remount the root-filesystem (and no other partition, if you use multiple partitions) in read-write mode, and modify or insert your own init-scripts to make the system boot into working^2.7 read-only mode.