Logical disk

Abstract: A computer system having a virtual drive array controller incorporated therein The virtual drive array controller enables plural physical devices, which may be of a single type of device or a combination of multiple types of devices, to be represented to the computer system as a single logical drive The virtual drive array controller includes a front end coupled to a secondary storage bus and a back end to which plural physical devices are coupled Additional virtual drive array controllers may be arranged in a cascading configuration by coupling the additional virtual drive array controllers to the back end of the virtual drive array controller

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: During the past decade, advances in processor and memory technology have given rise to increases in computational performance that far outstrip increases in the performance of secondary storage technology. Coupled with emerging small-disk technology, disk arrays provide the cost, volume, and capacity of current disk subsystems but, by leveraging parallelism, many times their performance. Unfortunately, arrays of small disks may have much higher failure rates than the single large disks they replace. Redundant Arrays of Inexpensive Disks (RAID) use simple redundancy schemes to provide high data reliability. This dissertation investigates the data encoding, performance, and reliability of redundant disk arrays. Organizing redundant data into a disk array is treated as a coding problem in this dissertation. Among alternatives examined, codes as simple as parity are shown to effectively correct single, self-identifying disk failures. The performance advantages of striping data across multiple disks are reviewed in this dissertation. For large transfers this parallelism reduces response time. Striping data also automatically distributes independent, small accesses across disks to increase throughput. This dissertation evaluates the performance lost to the maintenance of redundant data. This loss is negligible for large transfers but can be significant for small writes because of increases in aggregate disk service time. Because disk arrays include redundancy to protect against the high failure rates caused by large numbers of disk components, it is crucial that disk failures be characterized. This dissertation provides evidence that disk lifetimes can be modeled as exponential random variables. Building on an exponential model for disk lifetimes, this dissertation presents analytic models for disk-array lifetime, evaluates these against event-driven simulation, and applies them to an example redundant disk array. These models incorporate the effects of independent and dependent disk failures (shared support hardware) as well as the effects of on-line spare disks. For the example redundant disk array, these models show that a 10% overhead for an N + 1-parity encoding plus a 10% overhead for on-line spares can provide higher reliability than the 100% overhead of conventional mirrored disks.