The big problem with freezing processes or machine state and restoring
it later is that the context gets partially lost like non-local network
connections. Some code dies because time suddently warps. Or where
to position file pointers when restoring a process? This is usually
trivial when the file hasn't changed since taking the snapshot but
can get very hairy otherwise.
There is some software on the market for IRIX (Hibernator II by Softways)
which can handle many of the problems involved gracefully but a general
solutions isn't possible. Software which needs a nontrivial restoration
process will have to handle that at least in part itself because neither
the kernel nor other software can handle that perfectly.
Ralf