1. A system comprising: a cluster of storage servers coupled to a network, the storage servers attached to storage devices organized as a replication group; and a host computer coupled to the cluster of storage servers over the network, the host computer including a processor connected to a storage class memory (SCM), the processor configured to execute an operating system kernel, an application and a user-level library stored in the SCM, the user-level library cooperating with the application to control access to data stored in a region of the SCM and to change the data of the region at a byte-addressable granularity, the user-level library replicating the changed data of the region to the replication group, the user-level library when executed operable to: define a storage subspace based on a predetermined number of low bits of a multi-bit memory address of an address space of the region; assign the storage subspace to the storage devices of the replication group; and select a storage device of the replication group to receive the changed data to enable replication of the changed data at the byte-addressable granularity.