Can we hot-swap from one install of linux to another?

Question

I have several separate installs of Linux on my machine, and reboot to whichever one is appropriate. I'm wondering if it is possible to switch from one to the other without a full reboot by doing something like, say, copying over fstab with the partitions for the new install listed and then mount -a.

Obviously, the kernel would not change, but that's OK, I'm hoping to just change the working partitions. I also take it as obvious that this would have to be done from the command line with nothing running (if that's even possible). Or is the idea null and void and sheer madness from the get-go?

The reason for this is that I normally experiment with radical changes (like moving to the 'amd64' kernel in Debian) in a separate 'experimental' install. Rebooting to start it up is of course just fine, but it would be cool if I could sorta jump over to that install without an entire reboot. You could think of this as just changing the root partition on the fly, but maybe that is not possible at all.

Yup, it's possible, as I learn below. If you don't need to swap out the kernel itself use 'chroot'. If you want to simulate a complete reboot use 'kexec'.

Answer 1

I don't think this is possible.

Theoretically what you'll want to do is:

1. Move the system to the "single" run level by using init 1, where theoretically you only have init and bash running.
2. Unmount all the partitions except / . at this point your system may stop working, depending on whether /usr is on the root partition.
3. Mount your other os's root somewhere.
4. pivot_root to change the root to the new partition.
5. Tell init to start the new system by running init 5 (or 2 or a whatever is appropriate).

The problem is that pivot_root only change the root partition for the current process, which means it will not affect init and when you tell init to start the system, it will start your old one. You'd need some way to tell init to do the pivot_root, which I don't think is possible.

Oh, and let's not get into what happens when your other OS uses a different init service (e.g. Fedora uses systemd while Ubuntu has upstart). You'd need to replace the init process (pid 1) under the kernel and this is really a no-go.

Answer 2

If you want to experiment with different kernels, you'll have to either reboot or run them in a virtual machine. Virtual machines are more convenient, but then you can't test the kernel on your real hardware.

If you want to have different setups with different sets of installed programs, you can also use a virtual machine. If you want to avoid the overhead of a virtual machine, you can install a distribution in a chroot. I often do that to have easy access to 32-bit programs on a 64-bit installation, or to have the latest stuff (like Debian unstable) in addition to a stable relase (like Debian stable). I wrote a guide to setting up a chroot for alternate Debian/Ubuntu releases. See also Lightweight isolated linux environment

Answer 3

If you don't want/need to change kernels, but just want to use the other filesystem(s), as you seem to indicate here:

Obviously, the kernel would not change, but that's OK, I'm hoping to just change the working partitions.

Then perhaps you just need (mount and) chroot, which, to use your words,

You could think of this as just changing the root partition on the fly

I use chroot all the time from rescue systems (like clonezilla) to interact with a filesystem that would normally be used be a different kernel. I don't expect to be able to do things like loading new kernel modules, but it's fine for reading and writing files (fixing fstab, adjusting grub.conf, etc.).

You can use bind mounts to make the chroot environment more useful. I use this all the time to replicate mounted filesystems inside a chroot target:

mount /dev/sda1 /mnt
for d in dev sys proc; do mount -obind /$d /mnt/$d; done
chroot /mnt