This article presents the first large-scale field study of NAND-based SSDs in enterprise storage systems (in contrast to drives in distributed data center storage systems). The study is based on a very comprehensive set of field data, covering 1.6 million SSDs of a major storage vendor (NetApp). The drives comprise three different manufacturers, 18 different models, 12 different capacities, and all major flash technologies (SLC, cMLC, eMLC, 3D-TLC). The data allows us to study a large number of factors that were not studied in prior works, including the effect of firmware versions, the reliability of TLC NAND, and the correlations between drives within a RAID system. This article presents our analysis, along with a number of practical implications derived from it.

As solid state drives (SSDs) are increasingly replacing hard disk drives, the reliability of storage systems depends on the failure modes of SSDs and the ability of the file system layered on top to handle these failure modes. While the classical paper on IRON File Systems provides a thorough study of the failure policies of three file systems common at the time, we argue that 13 years later it is time to revisit file system reliability with SSDs and their reliability characteristics in mind, based on modern file systems that incorporate journaling, copy-on-write, and log-structured approaches and are optimized for flash. This article presents a detailed study, spanning ext4, Btrfs, and F2FS, and covering a number of different SSD error modes. We develop our own fault injection framework and explore over 1,000 error cases. Our results indicate that 16% of these cases result in a file system that cannot be mounted or even repaired by its system checker. We also identify the key file system metadata structures that can cause such failures, and, finally, we recommend some design guidelines for file systems that are deployed on top of SSDs.

E-voting systems have emerged as a powerful technology for improving democracy by reducing election cost, increasing voter participation, and even allowing voters to directly verify the entire election procedure. Prior internet voting systems have single points of failure, which may result in the compromise of availability, voter secrecy, or integrity of the election results. In this paper, we present the design, implementation, security analysis, and evaluation of D-DEMOS, a complete e-voting system that is distributed, privacy-preserving and end-to-end verifiable. Our system includes a fully asynchronous vote collection subsystem that provides immediate assurance to the voter her vote was recorded as cast, without requiring cryptographic operations on behalf of the voter. We also include a distributed, replicated and fault-tolerant Bulletin Board component, that stores all necessary election-related information, and allows any party to read and verify the complete election process. Finally, we also incorporate trustees, i.e., individuals who control election result production while guaranteeing privacy and end-to-end-verifiability as long as their strong majority is honest. Our system is the first e-voting system whose voting operation is human verifiable, i.e., a voter can vote over the web, even when her web client stack is potentially unsafe, without sacrificing her privacy, and still be assured her vote was recorded as cast. Additionally, a voter can outsource election auditing to third parties, still without sacrificing privacy. Finally, as the number of auditors increases, the probability of election fraud going undetected is diminished exponentially. We provide a model and security analysis of the system. We implement a prototype of the complete system, we measure its performance experimentally, and we demonstrate its ability to handle large-scale elections.

Interactive consistency is the problem in which n nodes, where up to t may be byzantine, each with its own private value, run an algorithm that allows all non-faulty nodes to infer the values of each other node. This problem is relevant to critical applications that rely on the combination of the opinions of multiple peers to provide a service. Examples include monitoring a content source to prevent equivocation or to track variability in the content provided, and resolving divergent state amongst the nodes of a distributed system. Previous works assume a fully synchronous system, where one can make strong assumptions such as negligible message delivery delays and/or detection of absent messages. However, practical, real-world systems are mostly asynchronous, i.e., they exhibit only some periods of synchrony during which message delivery is timely, thus requiring a different approach. In this paper, we present a thorough study on practical interactive consistency. We leverage the vast prior work on broadcast and byzantine consensus algorithms to design, implement and evaluate a set of algorithms, with varying timing assumptions and message complexity, that can be used to achieve interactive consistency in real-world distributed systems. We provide a complete, open-source implementation of each proposed interactive consistency algorithm by building a multi-layered stack of protocols that include several broadcast protocols, as well as a binary and a multi-valued consensus protocol. Most of these protocols have never been implemented and evaluated in a real system before. We analyze the performance of our suite of algorithms experimentally by engaging in both single instance and multiple parallel instances of each alternative.