How to migrate linux between different hardware

Any experienced systems administrator must have come across this issue in his life. Your server became outdated 3 years ago with all RAM and disk slots allready filled. You need more power. Gotta buy a new server, set it up, install the software, configure it and run the services on the new server.

But wait a minute... The old redhat 3.x you are using presently is hardly available now. What about the code your fellow admin wrote 3 years ago and left soon afterwards (code still works but you don't have idea about its dependancies etc.).

The best idea is to clone the server and deploy it on the new one. But this is usually hardware dependant - which means you cannot redeploy the clone on servers which require different drivers).

Fortunately, linux handles hardware changes quite nicely and it is easy to setup hardware independent imaging.

However some conditions apply:

-> on the new server, the only thing that will change is probably modules used, along with initrd. These things are hardware dependant.

-> you will be running grub1 on the destination server (grub2 is not supported by systemimager AFAIK - see later)

-> you might experience some minor problems with udev which may fail to start during boot. From my experience it turns out, that this is usually not a major problem.

Software:

-> systemimager
-> an iso of your linux distribution

Hardware:
-> old server (ServerA)
-> new server (ServerB)
-> a third server (ServerC)to run systemimager-server - just for migration time

Procedure overview:

We first get the image from ServerA and transfer it to systemimager-server on ServerC. We also install a basic, plain operating system on serverB.
Then we overwrite serverB with ServerA's image excluding parts which are hardware dependent (modprobe.conf, modprobe.d). We regenerate initrd and reboot ;-)


Diving in

-> install your linux distro onto ServerB from iso

-> On ServerC download and install systemimager-server package (instructions available on the systemimager website. My ubuntu has it out-of-the-box in apt). The server on which you install should have enough disk capacity to hold ServerA's filesystem. Start systemimager-server-rsyncd service.

-> download and install systemimager-client on ServerA and ServerB

-> on serverA turn off firewall, shut down production services (possibly - as many daemons as you can). Then run:

si_prepareclient --server serverC

-> on serverC:

si_getimage --image image-serverA --golden-client serverA

this will start image retrieval which is done with rsync

-> while the image is being cloned, login to serverB and say which files will not be overwritten. On serverB - edit the file /etc/systemimager/updateclient.local.exclude and add the following lines:

/etc/modprobe.d/

/etc/modprobe.conf

-> when image retrieval is finished, on serverB run:

si_updateclient --server serverC --image image-serverA --no-bootloader

this will transfer the image from serverC onto your new server serverB

-> to hold new hardware on serverB, you need to regenerate initrd and look if all grub entries are correct. mkinitrd reads /etc/modprobe* to determine which hardware modules are needed to start the system. They are not overwritten by cloning because you excluded them earlier. (redhat provides easy command for the whole process called new-kernel-pkg). On redhat it also works to reinstall the kernel with rpm --force option.

-> after this is finished, reboot.

Conclusion

That's it. In practice this has worked for me several times. I would mostly reimage IBM eServer (from /dev/sda) on an HP ProLiant (on /dev/cciss/c0d0) and vice-versa. It worked well on dell blades. I successfully done V2V and P2V migrations on vmware/xen,kvm. The systems were redhat 5 and 4, debian.
However I expect some problems might arise if you tried to advance too much in kernel versions (for example run ancient redhat with a modern kernel on new hardware). An idea to handle this is to exclude not only modprobe* stuff from imaging but also whole /boot partition and /lib/modules/*. This is still to be tested ;-)