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

EROS is a capability-based operating system for commodity processors which uses a single level storage model. The single level store's persistence is transparent to applications. The performance consequences of support for transparent persistence and capability-based architectures are generally believed to be negative. Surprisingly, the basic operations of EROS (such as IPC) are generally comparable in cost to similar operations in conventional systems. This is demonstrated with a set of microbenchmark measurements of semantically similar operations in Linux. The EROS system achieves its performance by coupling well-chosen abstract objects with caching techniques for those objects. The objects (processes, nodes, and pages) are well-supported by conventional hardware, reducing the overhead of capabilities. Software-managed caching techniques for these objects reduce the cost of persistence. The resulting performance suggests that composing protected subsystems may be less costly than c...

The Log-structured File System (LFS), introduced in 1991 [8], has received much attention for its potential order-of-magnitude improvement in file system performance. Early research results [9] showed that small file performance could scale with processor speed and that cleaning costs could be kept low, allowing LFS to write at an effective bandwidth of 62 to 83% of the maximum. Later work showed that the presence of synchronous disk operations could degrade performance by as much as 62% and that cleaning overhead could become prohibitive in transaction processing workloads, reducing performance by as much as 40% [10]. The same work showed that the addition of clustered reads and writes in the Berkeley Fast File System [6] (FFS) made it competitive with LFS in large-file handling and software development environments as approximated by the Andrew benchmark [4]. These seemingly inconsistent results have caused confusion in the file system research community. This paper presents a detail...

Abstract: One of the fundamental limits to high-performance, high-reliability file systems is memory’s vulnerability to system crashes. Because memory is viewed as unsafe, systems periodically write data back to disk. The extra disk traffic lowers performance, and the delay period before data is safe lowers reliability. The goal of the Rio (RAM I/O) file cache is to make ordinary main memory safe for persistent storage by enabling memory to survive operating system crashes. Reliable memory enables a system to achieve the best of both worlds: reliability equivalent to a write-through file cache, where every write is instantly safe, and performance equivalent to a pure write-back cache, with no reliability-induced writes to disk. To achieve reliability, we protect memory during a crash and restore it during a reboot (a “warm ” reboot). Extensive crash tests show that even without protection, warm reboot enables memory to achieve reliability close to that of a write-through file system while performing 20 times faster. Rio makes all writes immediately permanent, yet performs faster than systems that lose 30 seconds of data on a crash: 35% faster than a standard delayed-write file system and 8 % faster than a system that delays both data and metadata. For applications that demand even higher levels of reliability, Rio’s optional protection mechanism makes memory even safer than a write-through file system while while lowering performance 20 % compared to a pure write-back system. 1

Small file performance in most file systems is limited by slowly improving disk access times, even though current file systems improve on-disk locality by allocating related data objects in the same general region. The key insight for why current file systems perform poorly is that locality is insufficient --- exploiting disk bandwidth for small data objects requires that they be placed adjacently. We describe C-FFS (Co-locating Fast File System), which introduces two techniques, embedded inodes and explicit grouping, for exploiting what disks do well (bulk data movement) to avoid what they do poorly (reposition to new locations). With embedded inodes, the inodes for most files are stored in the directory with the corresponding name, removing a physical level of indirection without sacrificing the logical level of indirection. With explicit grouping, the data blocks of multiple small files named by a given directory are allocated adjacently and moved to and from the disk as a unit in ...

In this paper we describe the architecture and design of a new file system, XFS, for Silicon Graphics’ IRIX operating system. It is a general purpose file system for use on both workstations and servers. The focus of the paper is on the mechanisms used by XFS to scale capacity and performance in supporting very large file systems. The large file system support includes mechanisms for managing large files, large numbers of files, large directories, and very high performance I/O. In discussing the mechanisms used for scalability we include both descriptions of the XFS on-disk data structures and analyses of why they were chosen. We discuss in detail our use of B+ trees in place of many of the more traditional linear file system structures. XFS has been shipping to customers since December of 1994 in a version of IRIX 5.3, and we are continuing to improve its performance and add features in upcoming releases. We include performance results from running on the latest version of XFS to demonstrate the viability of our design.

We propose and evaluate the concept of a semantically-smart disk system (SDS). As opposed to a traditional "smart" disk, an SDS has detailed knowledge of how the file system above is using the disk system, including information about the on-disk data structures of the file system. An SDS exploits this knowledge to transparently improve performance or enhance functionality beneath a standard block read/write interface. To automatically acquire this knowledge, we introduce a tool (EOF) that can discover file-system structure for certain types of file systems, and then show how an SDS can exploit this knowledge on-line to understand file-system behavior. We quantify the space and time overheads that are common in an SDS, showing that they are not excessive. We then study the issues surrounding SDS construction by designing and implementing a number of prototypes as case studies; each case study exploits knowledge of some aspect of the file system to implement powerful functionality beneath the standard SCSI interface. Overall, we find that a surprising amount of functionality can be embedded within an SDS, hinting at a future where disk manufacturers can compete on enhanced functionality and not simply cost-per-byte and performance.

In this paper, we study how to minimize the latency of small synchronous writes to disks. The basic approach is to write to free sectors that are near the current disk head location by leveraging the embedded processor core inside the disk. We develop a number of analytical models to demonstrate the performance potential of this approach. We then present the design of a virtual log, a log whose entries are not physically contiguous, and a variation of the log-structured file system based on this approach. The virtual log based file systems can e#ciently support small synchronous writes without extra hardware support while retaining the advantages of LFS including its potential to support transactional semantics. We compare our approach against traditional update-in-place and logging systems by modifying the Solaris kernel to serve as a simulation engine. Our evaluations show that random synchronous updates on an unmodified UFS execute up to an order of magnitude faster on a virtual log...

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

This article describes soft updates, their incorporation into the 4.4BSD fast file system, and the resulting effects on the system. We show that a disk-based file system using soft updates achieves memory-based file system performance while providing stronger integrity and security guarantees than most disk-based file systems. For workloads that frequently perform updates on metadata (such as creating and deleting files), this improves performance by more than a factor of two and up to a factor of 20 when compared to the conventional synchronous write approach and by 4-19% when compared to an aggressive write-ahead logging approach. In addition, soft updates can improve file system availability by relegating crash-recovery assistance (e.g., the }sck utility) to an optional and background role, reducing file system recovery time to less than one second