We introduce Shear, a user-level software tool that characterizes RAID storage arrays. Shear employs a set of controlled algorithms combined with statistical techniques to automatically determine the important properties of a RAID system, including the number of disks, chunk size, level of redundancy, and layout scheme. We illustrate the correctness of Shear by running it upon numerous simulated configurations, and then verify its real-world applicability by running Shear on both software-based and hardware-based RAID systems. Finally, we demonstrate the utility of Shear through three case studies. First, we show how Shear can be used in a storage management environment to verify RAID construction and detect failures. Second, we demonstrate how Shear can be used to extract detailed characteristics about the individual disks within an array. Third, we show how an operating system can use Shear to automatically tune its storage subsystems to specific RAID configurations.

We describe an evolutionary path that allows operating systems to be used in a more flexible and appropriate manner by higher-level services. An infokernel exposes key pieces of information about its algorithms and internal state; thus, its default policies become mechanisms, which can be controlled from user-level. We have implemented two prototype infokernels based on the linuxtwofour and netbsdver kernels, called infolinux and infobsd, respectively. The infokernels export key abstractions as well as basic information primitives. Using infolinux, we have implemented four case studies showing that policies within Linux can be manipulated outside of the kernel. Specifically, we show that the default file cache replacement algorithm, file layout policy, disk scheduling algorithm, and TCP congestion control algorithm can each be turned into base mechanisms. For each case study, we have found that infokernel abstractions can be implemented with little code and that the overhead and accuracy of synthesizing policies at user-level is acceptable.