an OS question

[cellio]

While waiting for assorted software updates to install today I found myself wondering... Mac OS and Windows usually need to reboot your machine to install updates. Yet I have, several times, seen Unix machines that I believe were being maintained with uptimes of more than a year. What's the deal? Is Unix just better able to support hot-fixes, or

Comments

[geekosaur]

OSX / Darwin has an additional little feature: it prelinks shared objects so they can be fast-loaded at non-conflicting addresses for most processes. The gotcha is, if a prelinked object is modified and then used before the prelink cache is generated again (as by an install while the system is running), Bad Things happen. (It's been known to require a complete reinstall to recover.) So starting in 10.5 Apple made most installs require reboots, specifically to insure that the prelink cache remains valid.

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Many Linux/Unix admins don't want to reboot, hoping that the occasional (or not-so-occasional) kernel-based security hole won't affect them. I tend to consider that a bad idea these days.

That said, there's a (for-pay, although IIRC some distribution (Ubuntu?) has a reduced functionality version available to all registered users) technology called Ksplice that can patch a running kernel in many (most? all? haven't looked closely but I think the latter is impossible for logistical reasons) cases. It's been around for long enough that there could well be some Linux boxes with uptime > 1 year that use it to keep the kernel up to date on patches.