To support location independence, we introduce an indirection map, which maps logical addresses to physical disk addresses, a technique similar to those found in some existing systems [5, 7, 8]. In the rest of this section, we describe how we keep the map persistent, how we perform updates, how we recover the map after a failure, and how file systems can be built using these primitives. Our goal is to avoid some of the inefficiencies and inflexibilities of the previous approaches: