Object-based cloud storage has been widely adopted for their agility in deploying storage with a very low up-front cost. However, enterprises currently use them to store secondary data and not for expensive primary data. The driving reason is performance; most enterprises conclude that storing primary data in the cloud will not deliver the performance needed to serve typical workloads. Our analysis of real-world traces shows that certain primary data sets can reside in the cloud with its working set cached locally, using a cloud gateway that acts as a caching bridge between local data centers and the cloud. We use a realistic cloud gateway simulator to study the performance and cost of moving such workloads to different cloud backends (like Amazon S3). We find that when equipped with the right techniques, cloud gateways can provide competitive performance and price compared to on-premise storage. We also provide insights on how to build such cloud gateways, especially with respect to caching and prefetching techniques.

We introduce a new reliability infrastructure for file systems called I/O shepherding. I/O shepherding allows a file system developer to craft nuanced reliability policies to detect and recover from a wide range of storage system failures. We incorporate shepherding into the Linux ext3 file system through a set of changes to the consistency management subsystem, layout engine, disk scheduler, and buffer cache. The resulting file system, CrookFS, enables a broad class of policies to be easily and correctly specified. We implement numerous policies, incorporating data protection techniques such as retry, parity, mirrors, checksums, sanity checks, and data structure repairs; even complex policies can be implemented in less than 100 lines of code, confirming the power and simplicity of the shepherding framework. We also demonstrate that shepherding is properly integrated, adding less than 5% overhead to the I/O path.