In-Place OS Upgrades can ruin your day. I'm not talking about complete failures that trash your system: obviously you made backups and can start over in that case.
No, what I'm talking about is upgrades that appear to be succesful, but have introduced subtle (or not so subtle) problems that can be devilishly hard to resolve.
The problem is two-fold: the humans who write the upgrade procedures can't anticipate every situation they might run into. The second reason is to clean up the cruft that sneaks into any machine over the years.
In Place Upgrades can screw up and leave you with a totally dead machine. If you don't have a good understanding of the things that could go wrong, and aren't prepared to deal with these potential problems, then I would suggest that you are not prepared to do this on any machine that is important. Hire somebody experienced to help, or practice on other machines first. If this is an important server, you do not want to fumble around.
Other problems may be less drastic, but still could cripple your business. An application that won't run can be as bad as a dead machine. Even relatively minor glitches can take more time to figure out, which costs you aggravation and lost time.
Speaking of time, very often doing a fresh install and restoring backups is actually faster than an in place upgrade. That's even counting the extra time you spend configuring the box. The IPU software has to search out all kinds of things to see if they need re-doing; that takes time.
I've done many hundreds, maybe even thousands of upgrades and installs. I installed so many Xenix systems that I could do it without a monitor, and later on most of those got upgraded to Unix and later to OSR5. I've also done my share of Solaris and Linux boxes. Yes, sometimes I have done IPU's, but generally I prefer a fresh install over an in place upgrade.
If you haven't done it before, and we have (many of us earn our livings doing installs and upgrades and troubleshooting), why would you need any more advice? If you still feel you are going to try an upgrade, at least be prepared to do a fresh install if necessary, and keep in mind that the problems that might crop up from an upgrade might not be noticed for days or sometimes weeks if they are subtle or relat to things that don't occur every day.
IPU's of course attempt to preserve previous configuration information. That's the whole point. But sometimes you don't WANT the previous setup. Driver information may be incorrect, configuration file settings may not apply and so on. Some of that will be caught by the IPU, but no amount of engineering foresight will ever catch everything.
Actually, if an IPU produced a list of files it preserved from the old system, I could work from that direction. I'd want a diff listing of all system directories so I could see files that would NOT have been there with a fresh install, and a diff for any system file that would have been put down new but was preserved or merged. If I had that, IPU's would be neat.
But they don't give me that, so my approach is always to install new, and then to restore everything from the old system to a separate directory where files can be examined. It's not hard to write scripts to find what is different, and you can then quickly put in place the old settings that you really do want.
This is actually often quicker than doing an IPU. The fresh install is very much quicker, and it takes very little time to identify changes needed.
You'll want to look for:
Document and archive anything you do not entirely understand. If you are merging or copying files from the old system, save a diff of what you are doing against the virgin file put down by the fresh install if you have any doubts about why you are making the changes. This lets you go back to the (hopefully) stable fresh install configuration if it becomes necessary or if you suspect a problem.
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More Articles by Tony Lawrence © 2011-03-17 Tony Lawrence