SE Minneapolis , MN 55455 - 0159 USA TR 07 - 009 Practical Techniques for Eliminating Storage of Deleted Data

The layered design of modern file systems hides the liveness of data from the underlying storage systems. In this paper, we define a generic “purge” operation that can be used by a file system to pass liveness information to the storage system with minimal changes in the layer interfaces. We present three approaches for implementing such a purge operation: direct call, zero pages, and flagged writes. We demonstrate the feasibility of these techniques through a reference implementation in User-mode Linux to dynamically manage a copy-onwrite (COW) data store. Performance results obtained from this reference implementation show that these techniques can achieve significant storage savings with a reasonable execution time overhead. Our results demonstrate that passing liveness information across the file system-block layer interface with minimal changes is not only feasible but practical.