1. A computer program product for allocating cluster storage units to cluster nodes for a clustered file system that implements redirect-on-write snapshotting, the computer program product comprising: a computer readable storage medium having program instructions embodied therewith, the program instructions comprising program instructions to, track cluster storage units for a clustered file system of a cluster that implements redirect-on-write snapshotting as free, in-flight, or committed, wherein the cluster storage units comprise logical units of aggregate storage space of storage devices of the cluster; indicate a first set of the cluster storage units as free because the first set of cluster storage units are available for allocation to nodes of the cluster; receive a request for cluster storage units from a client node; in response to reception of the request for cluster storage units from the client node, transition the first set of the cluster storage units from free to in-flight to represent that the first set of cluster storage units are allocated to the client node for the client node to write to the first set of the cluster storage units; and send first indications of the first set of the cluster storage units to the client node, receive, from the client node, second indications of the first set of the cluster storage units, checksums corresponding to the first set of the cluster storage units, and an indication of a fileset, wherein reception of the second indications of the first set of the cluster storage units and the checksums corresponding to the first set of the cluster storage unit indicates that the client node has written data to the first set of the cluster storage units; in response to reception of, from the client node, the second indications of the first set of the cluster storage units and checksums corresponding to the first set of the cluster storage units, write the second indications of the first set of the cluster storage units and the checksums corresponding to the first set of the cluster storage units to a location in metadata for the clustered file system corresponding to the fileset; if a failure does not interrupt a writing of the indications of the first set of the cluster storage units and the checksums corresponding to the first set of the cluster storage units to a location in metadata for the clustered file system corresponding to the fileset, then transition the first set of the cluster storage units to committed; and if a failure interrupts a writing of the indications of the first set of the cluster storage units and the checksums corresponding to the first set of the cluster storage units to a location in metadata for the clustered file system corresponding to the fileset, then return the first set of the cluster storage units to free.