Disk sector

Abstract: The disk drive memory of the present invention uses a large plurality of small form factor disk drives to implement an inexpensive, high performance, high reliability disk drive memory that emulates the format and capability of large form factor disk drives. The plurality of disk drives are switchably interconnectable to form parity groups of N+1 parallel connected disk drives to store data thereon. The N+1 disk drives are used to store the N segments of each data word plus a parity segment. In addition, a pool of backup disk drives is maintained to automatically substitute a replacement disk drive for a disk drive in a parity group that fails during operation.

Abstract: Disk access patterns are becoming ever more important to understand as the gap between processor and disk performance increases. The study presented here is a detailed characterization of every lowlevel disk access generated by three quite different systems over a two month period. The contributions of this paper are the detailed information we provide about the disk accesses on these systems (many of our results are significantly different from those reported in the literature, which provide summary data only for file-level access on small-memory systems); and the analysis of a set of optimizations that could be applied at the disk level to improve performance. Our traces show that the majority of all operations are writes; disk accesses are rarely sequential; 25‐ 50% of all accesses are asynchronous; only 13‐41% of accesses are to user data (the rest result from swapping, metadata, and program execution); and I/O activity is very bursty: mean request queue lengths seen by an incoming request range from 1.7 to 8.9 (1.2‐1.9 for reads, 2.0‐14.8 for writes), while we saw 95th percentile queue lengths as large as 89 entries, and maxima of over 1000. Using a simulator to analyze the effect of write caching at the disk level, we found that using a small non-volatile cache at each disk allowed writes to be serviced considerably faster than with a regular disk. In particular, short bursts of writes go much faster ‐ and such bursts are common: writes rarely come singly. Adding even 8 KB of non-volatile memory per disk could reduce disk traffic by 10‐ 18%, and 90% of metadata write traffic can be absorbed with as little as 0.2 MB per disk of nonvolatile RAM. Even 128KB of NVRAM cache in each disk can improve write performance by as much as a factor of three. FCFS scheduling for the cached writes gave better performance than a more advanced technique at small cache sizes. Our results provide quantitative input to people investigating improved file system designs (such as log-based ones), as well as to I/O subsystem and disk controller designers.

Abstract: The reliability measures in today's disk drive-based storage systems focus predominantly on protecting against complete disk failures. Previous disk reliability studies have analyzed empirical data in an attempt to better understand and predict disk failure rates. Yet, very little is known about the incidence of latent sector errors i.e., errors that go undetected until the corresponding disk sectors are accessed.Our study analyzes data collected from production storage systems over 32 months across 1.53 million disks (both nearline and enterprise class). We analyze factors that impact latent sector errors, observe trends, and explore their implications on the design of reliability mechanisms in storage systems. To the best of our knowledge, this is the first study of such large scale our sample size is at least anorder of magnitude larger than previously published studies and the first one to focus specifically on latent sector errors and their implications on the design and reliability of storage systems.

Abstract: A data processing system with a RAID cache disk subsystem utilizes three RAID cache disk controllers to provide increased performance along with increased reliability, especially in the event of a failure of one of the disk controllers. Disk writes are mirrored in two disk controllers in order to guarantee integrity in the event of a disk controller or interface failure. Typically this write caching must be terminated when one of the controllers fails in order to maintain integrity. In the present invention, write caching continues utilizing the two remaining disk controllers.