In　this　paper,　we　study　the　problem　of　designing　new　mechanisms　for　selling　reserved　instances　(also　referred　to　as　virtual　machines)　in　cloud　computing.　Unlike　the　practice　in　today＇s　clouds　in　which　users　only　have　a　few　predefined　options　to　reserve　instances　(i.e.,　either　1-year　reservation　or　3-year　reservation),　we　allow　users　to　reserve　resources　for　any　length　and　from　any　time　point　in　the　future.　Our　goal　is　to　maximize　the　social　welfare.　We　propose　two　mechanisms,　one　for　the　case　where　all　the　jobs　are　tight　(their　lengths　are　exactly　their　reservation　time　intervals),　and　the　other　for　the　more　general　case　where　jobs　are　delayable　and　have　some　flexibility　on　their　reservations.　Both　of　the　mechanisms　are　prompt　in　the　sense　that　the　acceptance　and　the　payment　for　a　job　is　determined　at　the　very　moment　of　its　arrival.　We　use　competitive　analysis　to　evaluate　the　performance　of　our　mechanisms,　and　show　that　both　of　the　mechanisms　have　a　competitive　ratio　of　O(In(kT))　under　some　mild　assumption,　where　k　(res.　T)　is　the　maximum　ratio　between　per-instance-hour　valuation　(res.　length)　of　any　two　jobs.　We　then　prove　that　no　algorithm　can　achieve　a　competitive　ratio　better　than　In(2kT)　under　the　same　assumption.　Therefore,　our　mechanisms　are　optimal　within　a　constant　factor.