Described herein is a storage system having hierarchical levels of storage functions. The storage system may comprise one or more hierarchical levels, each hierarchical level comprising a server pool of one or more physical servers. Each hierarchical level may be assigned to perform a particular set of storage functions. Each physical server may implement one or more VMs, each VM executing a storage operating system that provides an overall set of storage functions. Each VM is configured to perform only the set of storage functions assigned to the hierarchical level on which the VM executes. The set of storage functions assigned to a VM may comprise a sub-set of the overall storage functions of the storage operating system. In some embodiments, VMs of a first hierarchical level provide storage transaction functions and VMs of a second hierarchical level provide storage access functions.