what is the downtime of mongodb cluster when upgrade to 3.0 I am following  http://docs.mongodb.org/manual/release-notes/3.0-upgrade/ to upgrade mongodb cluster from 2.6 to 3.0. The downtime is less than 1 second? because it takes less than 1 sec to elect one secondary to become primary. Am I right? I need to double check the whole process and the downtime...

thanks.
If you follow the upgrade process it is designed to avoid downtime -- for example using rs.stepDown() to gracefully step down the replica set primaries rather than abruptly triggering an election by shutting down the server.

Of course there is no guarantee if something doesn't go to plan, but you should test your upgrade in a QA/staging environment before upgrading production.