Zebra is a network file system that increases throughput by striping file data across multiple servers. Rather than striping each file separately, Zebra forms all the new data from each client into a single stream, which it then stripes using an approach similar to a log-structured file system. This provides high performance for writes of small files as well as for reads and writes of large files. Zebra also writes parity information in each stripe in the style of RAID disk arrays; this increases storage costs slightly but allows the system to continue operation even while a single storage server is unavailable. A prototype implementation of Zebra, built in the Sprite operating system, provides 4-5 times the throughput of the standard Sprite file system or NFS for large files and a 15 % to 300 % improvement for writing small files. 1

The principal trend in the design of computer systems is the expectation of much greater computational power in future generations of microprocessors. This trend applies to embedded systems as well as host processors. As a result, devices such as storage controllers have excess capacity and growing computational capabilities. Storage system designers are exploiting this trend with higher-level interfaces to storage and increased intelligence inside storage devices. One development in this direction is Network-Attached Secure Disks (NASD) which attaches storage devices directly to the network and raises the storage interface above the simple (fixed-size block) memory abstraction of SCSI. This allows devices more freedom to provide efficient operations; promises more scalable subsystems by offloading file system and storage management functionality from dedicated servers; and reduces latency by executing common case requests directly at storage devices. In this paper, we push this increa...

and the United States Postal Service. The views and conclusions in this document are my own and should not be interpreted as representing the official policies, either expressed or implied, of any supporting organization or the U.S. Government.

To meet the bandwidth needs of modern computer systems, parallel storage systems are evolving beyond RAID levels 1 through 5. The Parallel Data Lab at Carnegie Mellon University has constructed three Scotch parallel storage testbeds to explore and evaluate five directions in RAID evolution: first, the development of new RAID architectures to reduce the cost/performance penalty of maintaining redundant data; second, an extensible software framework for rapid prototyping of new architectures; third, mechanisms to reduce the complexity of and automate error-handling in RAID subsystems; fourth, a file system extension that allows serial programs to exploit parallel storage; and lastly, a parallel file system that extends the RAID advantages to distributed, parallel computing environments. This paper describes these five RAID evolutions and the testbeds in which they are being implemented and evaluated.

A new RAID-x (redundant array of inexpensive disks at level x) architecture is presented for distributed I/O processing on a serverless cluster of computers. The RAID-x architecture is based on a new concept of orthogonal striping and mirroring (OSM) across all distributed disks in the cluster. The primary advantages of this OSM approach lie in: (1) a significant improvement in parallel I/O bandwidth, (2) hiding disk mirroring overhead in the background, and (3) greatly enhanced scalability and reliability in cluster computing applications. All claimed advantages are substantiated with benchmark performance results on the Trojans cluster built at USC in 1999. Throughout the paper, we discuss the issues of scalable I/O performance, enhanced system reliability, and striped checkpointing on distributed RAID-x in a serverless cluster environment. 1.

AbstractÐThis paper presents a new distributed disk-array architecture for achieving high I/O performance in scalable cluster computing. In a serverless cluster of computers,all distributed local disks can be integrated as a distributed-software redundant array of independent disks �ds-RAID) with a single I/O space. We report the new RAID-x design and its benchmark performance results. The advantage of RAID-x comes mainly from its orthogonal striping and mirroring �OSM) architecture. The bandwidth is enhanced with distributed striping across local and remote disks,while the reliability comes from orthogonal mirroring on local disks at the background. Our RAID-x design is experimentally compared with the RAID-5,RAID-10,and chained-declustering RAID through benchmarking on a research Linux cluster at USC. Andrew and Bonnie benchmark results are reported on all four disk-array architectures. Cooperative disk drivers and Linux extensions are developed to enable not only the single I/O space,but also the shared virtual memory and global file hierarchy. We reveal the effects of traffic rate and stripe unit size on I/O performance. Through scalability and overhead analysis,we find the strength of RAID-x in three areas: 1) improved aggregate I/O bandwidth especially for parallel writes,2) orthogonal mirroring with low software overhead,and 3) enhanced scalability in cluster I/O processing. Architectural strengths and weakness of all four ds-RAID architectures are evaluated comparatively. The optimal choice among them depends on parallel read/write performance desired,the level of fault tolerance required,and the cost-effectiveness in specific I/O processing applications. Index TermsÐDistributed computing,parallel I/O,software RAID,single I/O space,Linux clusters,fault tolerance,Andrew and Bonnie benchmarks,network file servers,scalability and overhead analysis. 1

Abstract: In a serverless cluster of computers, all local disks can be integrated as a distributed software RAID (ds-RAID) with a single I/O space. This paper presents the architecture and performance of a new RAID-x for building ds-RAID. Through experimentation, we evaluate the RAID-x along with RAID-5, chained-declustering, and RAID-10 architectures, all embedded in a Linux cluster environment. All four ds-RAID architectures aim to scale in aggregate I/O bandwidth. The RAID-x is unique with its orthogonal striping and mirroring (OSM) architecture. The reliability comes from orthogonal mirroring, while the bandwidth is enhanced from distributed striping. To support single I/O space, we have developed cooperative disk drivers (CDD) at the Linux kernel to enable fast remote disk accesses without using a central file server such as the NFS. The performance of the RAID-x is experimentally proven superior in three areas: (1) significant improvement in I/O bandwidth especially in parallel write operations, (2) pipelined mirroring in the background with low overhead, and (3) enhanced scalability and reliability in cluster computing with a single I/O space. These claims are supported by Andrew and Bonnie benchmark results, obtained on the USC cluster of 16 Linux PCs. Reliable cluster middleware and Linux extensions are developed to enable not only single I/O space, but also shared virtual memory and global file hierarchy. Toward this end, we