How bad is downtime? ie: 6 hours in one day a month on visitors to a website? Is that bad for visitors? Or is it understandable? Will it drive traffic away? Just happened once but... It just happened. Don't want to lose my members, or visitors, or SE rankings... Was doing maintenance on server, upgrading its RAM to 4GB, but OS is only allowing 3GB for some reason...had problem and wouldn't boot with 4GB.
The effect of downtime varies with the number of visitors you get and how you handle your PR beforehand and afterwards. The kind of visitors you get will also make a difference. If you don't have very loyal visitors anyway then 6 hours downtime means that you only lost 6 hours worth of visits. If you do have very loyal visitors (like you get on a forum) then 6 hours of downtime could mean that some of your visitors won't come back for a few days or even ever. Of course you will also have certain visitors you keep clicking "refresh" until your site comes back up. (You know who you are...) To avoid the negative ramifications, it's best to plan the downtime ahead and notify your visitors of how long it will take before you start. If it's possible, it's nice to put a holding page up while you do the work. If you don't have a spare server, you could always just plug the ethernet cable from the server into your laptop and have Apache running on that just serving a single html page saying that you're "Currently down for planned maintenance." Of course, now that it has already happened, all you can really do is sit back and whatch your traffic to see what happens. Don't forget to explain to your visitors why there was downtime... they will appreciate that you're trying to make the site better for them.