N-Tier Client/Server Architectures Chapter 4 Server - RAID Copyright 2002, Dr. Ken Hoganson All rights reserved. OS Kernel Concept RAID – Redundant Array of Inexpensive Disks: Server Storage System Increase reliability through redundancy (duplicate copies) Increase performance through multiple hard drives – parallel I/O Increase reliability with error detection and correction (same sort of algorithms you should have seen in CSIS 2520 – cyclical error checking, Hamming Codes. Combinations of above Multiple RAID “Levels” Pg 153+ Goldman

N-Tier Client/Server Architectures Chapter 4 Server - RAID Copyright 2002, Dr. Ken Hoganson All rights reserved. Foundation Techniques Disk Mirroring –Two disks on the same disk controller –Duplicate data on each disk – parallel writes –Does not speed up I/O –Weak point is single disk controller card –Slow if consecutive writes through software Disk Duplexing –Adds second controller card –Still duplicate storage done concurrently –No slowdown if all logic in hardware

N-Tier Client/Server Architectures Chapter 4 Server - RAID Copyright 2002, Dr. Ken Hoganson All rights reserved. RAID Level 0 RAID Level 0: Disk Striping Stripes data across multiple disks without redundancy Increases I/O speed – parallel disk controllers working concurrently Not really RAID (no redundancy) Increased probability of failure (more drives and controllers), failure of any one compromises all

N-Tier Client/Server Architectures Chapter 4 Server - RAID Copyright 2002, Dr. Ken Hoganson All rights reserved. Level 1 Disk Mirroring or Disk Shadowing Duplicate data on multiple disks Redundancy increases reliability Software implementation can reduce performance Used for critical systems and/or critical data Variation I used on a system requiring high availability: Delayed Mirroring Backup process duplicates the entire drive on a second drive Very fast backup Very fast recovery (switch over to backup drive) No more than one day’s lose if drive fails

N-Tier Client/Server Architectures Chapter 4 Server - RAID Copyright 2002, Dr. Ken Hoganson All rights reserved. Level 2 Stripped Array Plus Hamming Code Not available at present – customized drive controller Stripes data across multiple disks for high performance Adds Hamming code for error detection and correction

N-Tier Client/Server Architectures Chapter 4 Server - RAID Copyright 2002, Dr. Ken Hoganson All rights reserved. Level 3 Striped Array plus Parity Disk Striped by bytes Each byte gets a parity bit Parity bits on a separate drive Stripes yield high performance Parity bit adds error detection

N-Tier Client/Server Architectures Chapter 4 Server - RAID Copyright 2002, Dr. Ken Hoganson All rights reserved. Level 4 Independent Stripped Array Plus Parity Disk Variation on Level 3 Data written in sectors, rather than bytes Disks can work independently on different I/O requests Write penalty– multi-sector writes write to multiple disks, blocking reads on those disks Parity for error detection and correction Not widely available

N-Tier Client/Server Architectures Chapter 4 Server - RAID Copyright 2002, Dr. Ken Hoganson All rights reserved. Level 5 Independent Striped Array plus Striped Parity Parity is striped as well as data Parity is interleaved, so disk doesn’t have data and associated parity Poor write performance Good read performance and error detection and correction

N-Tier Client/Server Architectures Chapter 4 Server - RAID Copyright 2002, Dr. Ken Hoganson All rights reserved. Level 6 Independent Striped Array plus Striped Double Parity Parity is striped as well as data Parity is interleaved, so disk doesn’t have data and associated parity Poor write performance Worse write performance than Level 5, because of consecutive parity calculations and writes (parity of (parity of data)) Good read performance and better error detection and correction

N-Tier Client/Server Architectures Chapter 4 Server - RAID Copyright 2002, Dr. Ken Hoganson All rights reserved. Level 10 Raid Levels 1 plus 0 Disk striping for performance Disk mirroring for reliabilty Good reliability and performance, but with some cost Disks are relatively cheap

N-Tier Client/Server Architectures Chapter 4 Server - RAID Copyright 2002, Dr. Ken Hoganson All rights reserved. Redundancy Redundancy is used at multiple levels for Performance Fault Tolerance Multiprocessor Server Redundancy Multiple CPUs (symmetric multiprocessing) Multiple cache at each CPU Multiple cache levels Multiple levels of buses (divides and localizes BW across levels) Multiple buses at some levels Multiple memory banks (on some multiprocessors) Multiple controllers and drives Multiple network cards With the Virtual Machine OS design level, multiple copies of the OS for testing, upgrades, development, and compatibility. Diagram of clustered multiprocessor