I'm trying to figure out if deploying/upgrading solutions and activating features recycles the app pool or interrupts/slows the user's actions in any way. I need to know if these steps can be performed while there is load on the server, or it is best to do an after-hours deployment.
The specific commands I'm using upgradesolution or displaysolution, addsolution, installfeature and activatefeature, but for the sake of completeness, it would be nice to have a comprehensive list.
As a general rule - always do feature / solution upgrades outside of hours. You never know how the change will affect the running application.
The reason activatefeature doesn't is because you are executing the commands contained in the feature e.g. events and schemas. All the others are registering assets for use by sharepoint so require a restart.
For info: The upgradesolution seems to perform a full IISReset not just an App Pool recycle. This is an important distinction if you are running more than one site in your farm. To circumvent this it seems you can pass in the parameter of -local and this does then perform an app pool recycle instead of an IISReset. The downside of this is that you have to perform the command on each server. All depends on your situation but a useful thing to know.