1. A method comprising: tracking 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; indicating 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; transitioning 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 a client node for the client node to write to the first set of the cluster storage units; transitioning the first set of the cluster storage units to committed to represent that data written to the first set of the cluster storage units are visible to nodes of the cluster and that checksums for the data have been recorded in metadata for the clustered file system, if a failure does not interrupt updating of the metadata for the clustered file system; and returning the first set of the cluster storage units to free if a failure occurs before at least one of updating of the metadata completes and the data written to the cluster storage units are visible.