Resource allocation mechanism plays a critical role towards the success of cloud computing. Existing allocation mechanisms in public cloud is unsuitable for private IaaS cloud because they either cannot maximize the sum of users’ value, or provide no service guarantee. This paper proposes a novel online, model-free mechanism that makes different allocations for flexible jobs and inflexible jobs. Users presenting job are incentivized to be truthful not only about their valuations for VM units, but also about their arrival, departure and the character of jobs (flexible or inflexible). We simulate the proposed online mechanism using data from RICC, showing that, compared with the mechanism which adopts same allocation method for all jobs, using our mechanism leads to high social welfare and percentage of served users.