Very slow booting and running and braindead OS's 4541

Well, you could do that in Unix, but it's generally not done - the disk(s) with the system data are generally the same disk(s) as have the boot information.

It seems to me there are several things one wants to be able to provide here:

* Defence against hardware failure, typically disk system failure. (Most) Unices already provide this by mirroring system disks. If a disk fails you replace it and rebuild the mirrors. If you have all the system disks in some larger buttembly which fails then you're stuffed obviously. But if you want, for instance, to replace that buttembly, you can do this by attaching a new submirror, then removing all the original mirrors &c. This is fiddly of course, and there are awkward points during the process where if the machine dies you'll have to do some slight hand-holding to get it to boot again (tell it to boot from a non-default device and so on).

* Allowing for hardware changes while the machine is up (other than replacing failed HW). This is done by having loadable drivers for that hardware, which drivers may not exist at the time the machine is originally booted. They could be read from some special boot disk, but typically they live on the same disk(s) as the other system data.

* Allowing for software changes while the machine is up. So long as you can restart the application concerned this works fine - just replace the data (executables) and restart it. Although the old executable will have been replaced the whole memory-mapping thing works OK here (the old data still exists in the FS until the actual running copy goes away). If you *can't* restart the application then this trick won't work, and there are some that can't be restarted with the machine up - notably the OS itself. Depending on the implementation of the application there may be things that can be done (for instance maybe it supports patching of the running application). There is more to be done here I suspect in terms of availability.

Given these things I am not sure what else you would want in terms of replacing system-boot media and the data on it. This isn't to say that Unix (or some implementations thereof) are ideal in terms of availability: they're far from that, but I don't think this is the major issue - the major issues are, I think, better resource management, security and some sort of lightweight virtualisation.

--tim