The exokernel operating system architecture safely gives untrusted software efficient control over hardware and software resources by separating management from protection. This paper describes an exokernel system that allows specialized applications to achieve high performance without sacrificing the performance of unmodified UNIX programs. It evaluates the exokernel architecture by measuring end-to-end application performance on Xok, an exokernel for Intel x86-based computers, and by comparing Xok’s performance to the performance of two widely-used 4.4BSD UNIX systems (Free-BSD and OpenBSD). The results show that common unmodified UNIX applications can enjoy the benefits of exokernels: applications either perform comparably on Xok/ExOS and the BSD UNIXes, or perform significantly better. In addition, the results show that customized applications can benefit substantially from control over their resources (e.g., a factor of eight for a Web server). This paper also describes insights about the exokernel approach gained through building three different exokernel systems, and presents novel approaches to resource multiplexing. 1

Plutus is a cryptographic storage system that enables secure file sharing without placing much trust on the file servers. In particular, it makes novel use of cryptographic primitives to protect and share files. Plutus features highly scalable key management while allowing individual users to retain direct control over who gets access to their files. We explain the mechanisms in Plutus to reduce the number of cryptographic keys exchanged between users by using filegroups, distinguish file read and write access, handle user revocation efficiently, and allow an untrusted server to authorize file writes. We have built a prototype of Plutus on OpenAFS. Measurements of this prototype show that Plutus achieves strong security with overhead comparable to systems that encrypt all network traffic.

Track-aligned extents (traxtents) utilize disk-specific knowledge to match access patterns to the strengths of modern disks. By allocating and accessing related data on disk track boundaries, a system can avoid most rotational latency and track crossing overheads. Avoiding these overheads can increase disk access efficiency by up to 50 % for mid-sized requests (100-500 KB). This paper describes traxtents, algorithms for detecting track boundaries, and some uses of traxtents in file systems and video servers. For large-file workloads, a version of FreeBSD's FFS implementation that exploits traxtents reduces application run times by up to 20 % compared to the original version. A video server using traxtent-based requests can support 56 % more concurrent streams at the same startup latency and buffer space. For LFS, 44 % lower overall write cost for track-sized segments can be achieved.

Disk performance is increasingly limited by its head positioning latencies, i.e., seek time and rotational delay. To reduce the head positioning latencies, we propose a novel technique that dynamically places copies of data in file system’s free blocks according to the disk access patterns observed at runtime. As one or more replicas can now be accessed in addition to their original data block, choosing the “nearest ” replica that provides fastest access can significantly improve performance for disk I/O operations. We implemented and evaluated a prototype based on the popular Ext2 file system. In our prototype, since the file system layout is modified only by using the free/unused disk space (hence the name Free Space File System, or FS 2), users are completely oblivious to how the file system layout is modified in the background; they will only notice performance improvements over time. For a wide range of workloads running under Linux, FS 2 is shown to reduce disk access time by 41–68 % (as a result of a 37–78% shorter seek time and a 31–68 % shorter rotational delay) making a 16–34 % overall user-perceived performance improvement. The reduced disk access time also leads to a 40–71 % energy savings per access.

The UNIX Fast File System (FFS) is probably the most widely-used file system for performance comparisons. However, such comparisons frequently overlook many of the performance enhancements that have been added over the past decade. In this paper, we explore the two most commonly used approaches for improving the performance of meta-data operations and recovery: journaling and Soft Updates. Journaling systems use an auxiliary log to record meta-data operations and Soft Updates uses ordered writes to ensure meta-data consistency. The commercial sector has moved en masse to journaling file systems, as evidenced by their presence on nearly every server platform available today: Solaris, AIX, Digital UNIX, HP-UX, Irix, and Windows NT. On all but Solaris, the default file system uses journaling. In the meantime, Soft Updates holds the promise of providing stronger reliability guarantees than journaling, with faster recovery and superior performance in certain boundary cases. In this paper, we explore the benefits of Soft Updates and journaling, comparing their behavior on both microbenchmarks and workload-based macrobenchmarks. We find that journaling alone is not sufficient to “solve ” the meta-data update problem. If synchronous semantics are required (i.e., meta-data operations are durable once the system call returns), then the journaling systems cannot realize their full potential. Only when this synchronicity requirement is relaxed can journaling systems approach the performance of systems like Soft Updates (which also relaxes this requirement). Our asynchronous journaling and Soft Updates systems perform comparably in most cases. While Soft Updates excels in some meta-data intensive microbenchmarks, the macrobenchmark results are more ambiguous. In three cases Soft Updates and journaling are comparable. In a file intensive news workload, journaling prevails, and in a small ISP workload, Soft Updates prevails. 2

Most modern I/O systems treat each file access independently. However, events in a computer system are driven by programs. Thus, accesses to files occur in consistent patterns and are by no means independent. The result is that modern I/O systems ignore useful information. Using traces of file system activity we show that file accesses are strongly correlated with preceding accesses. In fact, a simple last-successor model (one that predicts each file access will be followed by the same file that followed the last time it was accessed) successfully predicted the next file 72% of the time. We examine the ability of two previously proposed models for file access prediction in comparison to this baseline model and see a stark contrast in accuracy and high overheads in state space. We then enhance one of these models to address the issues of model space requirements. This new model is able to improve an additional 10% on the accuracy of the last-successor model, while working within a state...

Delta compression, which consists of compactly encoding one le version as the result of changes to another, can improve eciency in the use of network and disk resources. Delta compression techniques are readily available and can result in compression factors of ve to ten on typical data. Managing delta-compressed storage, however, is a dicult task. I will present a system that attempts to isolate the complexity of delta-compressed storage management by separating the task of version labeling from performance issues. I will show how the system integrates delta-compressed transport with delta-compressed storage. Existing tools for managing delta-compressed storage suer from weak le system support. Lack of transaction support is responsible for inecient application behavior. The only atomic operation in the traditional le system forces unnecessary disk activity due to copying costs. I will demonstrate that transaction support can improve application performance and extensibility wit...

In petabyte-scale distributed file systems that decouple read and write from metadata operations, behavior of the metadata server cluster will be critical to overall system performance and scalability. We present a dynamic subtree partitioning and adaptive metadata management system designed to efficiently manage hierarchical metadata workloads that evolve over time. We examine the relative merits of our approach in the context of traditional workload partitioning strategies, and demonstrate the performance, scalability and adaptability advantages in a simulation environment.

Benchmarks are important because they provide a means for users and researchers to characterize how their workloads will perform on different systems and different system architectures. The field of file system design is no different from other areas of research in this regard, and a variety of file system benchmarks are in use, representing a wide range of the different user workloads that may be run on a file system. A realistic benchmark, however, is only one of the tools that is required in order to understand how a file system design will perform in the real world. The benchmark must also be executed on a realistic file system. While the simplest approach may be to measure the performance of an empty file system, this represents a state that is seldom encountered by real users. In order to study file systems in more representative conditions, we present a methodology for aging a test file system by replaying a workload similar to that experienced by a real file system over a period of many months, or even years. Our aging tools allow the same aging workload to be applied to multiple versions of the same file system, allowing scientific evaluation of the relative merits of competing file system designs. In addition to describing our aging tools, we demonstrate their use by applying them to evaluate two enhancements to the file layout policies of the UNIX fast file system.

We describe a novel on-line file access predictor, Recent Popularity, capable of rapid adaptation to workload changes while simultaneously predicting more events with greater accuracy than prior efforts. We distinguish the goal of predicting the most events accurately from the goal of offering the most accurate predictions (when declining to offer a prediction is acceptable). For this purpose we present two distinct measures of accuracy, general and specific accuracy, corresponding to these goals. We describe how our new predictor and an earlier effort, Noah, can trade the number of events predicted for prediction accuracy by modifying simple parameters. When prediction accuracy is strictly more important than the number of predictions offered, trace-based evaluation demonstrates error rates as low as 2%, while offering predictions for more than 60% of all file access events.