The Indirection Map

 

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:

• scanning large portions of the disk for recovery,
• reliance on NVRAM, which is not always available,
• excessive overhead in terms of space and extra I/O's needed to maintain the map, and
• altering the physical block format on disks to include, for example, a self-identifying header.