Download Lecture Slides on Storage Systems | CS 6810 and more Study notes Computer Architecture and Organization in PDF only on Docsity! 1 Lecture 26: Storage Systems • Topics: Storage Systems (Chapter 6), other innovations • Final exam stats: Highest: 95 Mean: 70, Median: 73 Toughest questions: TM, SC 2 Role of I/O • Activities external to the CPU are typically orders of magnitude slower • Example: while CPU performance has improved by 50% per year, disk latencies have improved by 10% every year • Typical strategy on I/O: switch contexts and work on something else • Other metrics, such as bandwidth, reliability, availability, and capacity, often receive more attention than performance 5 RAID • Reliability and availability are important metrics for disks • RAID: redundant array of inexpensive (independent) disks • Redundancy can deal with one or more failures • Each sector of a disk records check information that allows it to determine if the disk has an error or not (in other words, redundancy already exists within a disk) • When the disk read flags an error, we turn elsewhere for correct data 6 RAID 0 and RAID 1 • RAID 0 has no additional redundancy (misnomer) – it uses an array of disks and stripes (interleaves) data across the arrays to improve parallelism and throughput • RAID 1 mirrors or shadows every disk – every write happens to two disks • Reads to the mirror may happen only when the primary disk fails – or, you may try to read both together and the quicker response is accepted • Expensive solution: high reliability at twice the cost 7 RAID 3 • Data is bit-interleaved across several disks and a separate disk maintains parity information for a set of bits • For example: with 8 disks, bit 0 is in disk-0, bit 1 is in disk-1, …, bit 7 is in disk-7; disk-8 maintains parity for all 8 bits • For any read, 8 disks must be accessed (as we usually read more than a byte at a time) and for any write, 9 disks must be accessed as parity has to be re-calculated • High throughput for a single request, low cost for redundancy (overhead: 12.5%), low task-level parallelism 10 RAID Summary • RAID 1-5 can tolerate a single fault – mirroring (RAID 1) has a 100% overhead, while parity (RAID 3, 4, 5) has modest overhead • Can tolerate multiple faults by having multiple check functions – each additional check can cost an additional disk (RAID 6) • RAID 6 and RAID 2 (memory-style ECC) are not commercially employed 11 Tiled Processors • Similar to multi-core, but a single thread can be spread across multiple cores • Need smart scheduling to reduce inter-core communication 12 Redundancy • Transient faults: a bit-flip caused by a high-energy particle • Error rates per transistor are not increasing, but number of transistors is increasing • Need some form of redundant computation to detect errors
