1. A storage area network (SAN)-attached clustered storage system comprising: a plurality of storage server nodes coupled to communicate with each other through an interconnect, each storage server node of the plurality of storage server nodes including: a protocol module of the storage server node, executed by a hardware processor, to allow a host to access the SAN-attached clustered storage system, wherein the protocol module implements a logical sub-unit of a single distributed logical unit in a SAN environment that is distributed across the cluster, such that the logical sub-unit is not visible to the host as a separate entity from the single distributed logical unit, wherein the single distributed logical unit comprises a plurality of logical sub-units including said logical sub-unit; and a cluster services module, executed by the hardware processor, to maintain consistency between the plurality of logical sub-units that make up the single distributed logical unit, the cluster services module including: a quorum manager, wherein the quorum manager of one of the plurality of storage server nodes is a master quorum manager to monitor an in-quorum/out-of-quorum status of each other quorum manager in the clustered storage system; and a cluster transaction manager to control cluster management operation requests and cluster control operation requests by using a multi-phase voting process, wherein the cluster transaction manager includes a group coordinator to direct the multi-phase voting process in a group to determine whether to commit an operation and to maintain a queue of entities in the group, such that each entity has an opportunity to propose the operation.