Think of it this way: How will devices in 20 years try to interact with my server?
back
1 comments
10base-T is 32 years old and still works (there may be some advanced ethernet switches with rj45 that don't stoop so low as to link at 10M anymore, but most will), gigE will probably work just fine in 20 years for slow legacy devices. :P
To answer the original question... your server design should have quality capacitors, good passive cooling (put fans in, but know they're likely to fail over 20 years; make it easy to swap them, but try to do ok if the fans all failed), and underclocked. Storage is tricky and may or may not last 20 years. Run with several storage devices mirrored or raidz3, and make it easy to replace failed storage devices. Also, make sure any batteries are remote mounted where leakage won't damage important circuit boards.
Most devices fail over the long term because of storage issues, heat stress/cycling, capacitor failure either directly or because the electrolyte leakage damages other components, or because of battery leakage. Some of these modes reinforce each other --- fan failure brings more heat which makes capacitors fail, etc.
I’d never have thought of remote mounted CMOS battery
CMOS batteries are commonly lithium cells which usually work for 7-10 years. It's almost certainly going to fail electrically in a 20 year system, even if it's power continuously (but there's likely to be blips here and there), but the seal may or may not fail too. I've seen a couple crusty coin cells; it's not super common, but it can happen, and battery leakage is not kind to circuit boards.