Rails 3 ActiveRecord Session: do I need to sweep the data table periodically? -

Rails 3 ActiveRecord Session: do I need to sweep the data table periodically? -

we're running big site , have rails' default cookiestore in place now. want switch either memcached or activerecord stores several reasons. trying decide better.

in bad old days, recall had run "sweepers" time time clear out expired sessions. cannot find reference in docs , optimistically thinking "it's magic!!" , rails 3 removes expired sessions us. need write periodic task clear expired sessions database?

i know it's bad form inquire 2 questions in 1 so, given have memcached running, can't see reason not utilize it, instead (and automagically expire stuff). reason shouldn't go straight memcached sessions?

ruby-on-rails-3 activerecord cookies memcached session-cookies