1. A method comprising: in a SAN-attached clustered storage system that has a plurality of storage server nodes, implementing in each of two or more storage server nodes a logical sub-unit; maintaining, for each said logical sub-unit of the two or more storage server nodes, context information that is consistent with corresponding context information of each other logical sub-unit such that the logical sub-units collectively make up a single distributed logical unit, wherein the maintaining includes communicating, from a first storage server node corresponding to a first logical sub-unit to a second server node corresponding to a second logical sub-unit, an update to a portion of the context information; and associating each logical sub-unit with a single host-visible identifier of the single distributed logical unit so that the host can access a logical sub-unit of the single distributed logical unit, such that the logical sub-units are not visible to the host as entities separate from the single distributed logical unit; wherein maintaining context information that is consistent with the corresponding context information of each other logical sub-unit includes: implementing a separate instance of a global task set for each logical sub-unit of the single distributed logical unit, to queue commands and task management functions that modify the context information; coordinating entry of commands into each instance of the global task set such that all instances of the global task set are consistent with each other; processing a command from a first instance of the global task set; and updating the context information for each logical sub-unit to reflect the processing of the command.