Virtualization systems should be responsible for satisfying the service level objectives (SLOs) for each VM. Performance SLOs, in particular, are generally achieved by isolating the underlying hardware resources among the VMs. In this paper, we show through empirical evaluation that performance SLOs cannot be satisfied with current commercial SSDs. We show that garbage collection is the source of this problem and that this cannot be easily controlled because of the interaction between VMs. To control the effect of garbage collection on VMs, we propose a scheme called OPS isolation. OPS isolation allocates flash memory blocks so that blocks of one VM do not interfere with blocks of other VMs during garbage collection. Experimental results show that performance SLO can be achieved through OPS isolation.