Failure-atomic msync(): a simple and efficient mechanism for preserving the integrity of durable data

Preserving the integrity of application data across updates is difficult if power outages and system crashes may occur during updates. Existing approaches such as relational databases and transactional key-value stores restrict programming flexibility by mandating narrow data access interfaces. We have designed, implemented, and evaluated an approach that strengthens the semantics of a standard operating system primitive while maintaining conceptual simplicity and supporting highly flexible programming: Failureatomic msync() commits changes to a memory-mapped file atomically, even in the presence of failures. Our Linux implementation of failure-atomic msync() has preserved application data integrity across hundreds of whole-machine power interruptions and exhibits good microbenchmark performance on both spinning disks and solid-state storage. Failure-atomic msync() supports higher layers of fully general programming abstraction, e.g., a persistent heap that easily slips beneath the C++ Standard Template Library. An STL <map> built atop failure-atomic msync() outperforms several local key-value stores that support transactional updates. We integrated failure-atomic msync() into the Kyoto Tycoon key-value server by modifying exactly one line of code; our modified server reduces response times by 26--43% compared to Tycoon's existing transaction support while providing the same data integrity guarantees. Compared to a Tycoon server setup that makes almost no I/O (and therefore provides no support for data durability and integrity over failures), failure-atomic msync() incurs a three-fold response time increase on a fast Flash-based SSD---an acceptable cost of data reliability for many.