Chico CSCI 640 - Chapter 12.4 Mass­Storage Systems

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Chapter 12.4 Mass-Storage SystemsChapter 12-3 Mass-Storage SystemsRAID – ContinuedRAID – different source – modified here and there.RAID - (continued) - FeaturesRAID - (continued) – Important LevelsRAID Levels 0, 1, and 2RAID – (continued) – Computations InvolvedRAID (continued) – Rebuilding SystemsRAID (continued) Backup?RAID Level3RAID Levels 4More RAID LevelsMore RAID LevelsStable Storage ImplementationStable-Storage Implementation (two slides)Stable Storage – ImplementationTertiary Storage IssuesTertiary Storage DevicesTapesOperating System IssuesSlide 22Application InterfaceTape DrivesFile NamingPerformance IssuesPerformance Issues for Tertiary StorageSpeedSpeed (Cont.)ReliabilityCostPrice per Megabyte of DRAM, From 1981 to 2004Price per Megabyte of Magnetic Hard Disk, From 1981 to 2004Price per Megabyte of a Tapes, From 1984-2000Price of StorageEnd of Chapter 12.4Chapter 12.4 Mass-Storage SystemsChapter 12.4 Mass-Storage Systems12.2Silberschatz, Galvin and Gagne ©2005Operating System ConceptsChapter 12-3 Mass-Storage SystemsChapter 12-3 Mass-Storage SystemsChapter 12-1:Overview of Mass Storage StructureChapter 12-2:Disk AttachmentDisk SchedulingChapter 12-3: Disk Management Swap-Space Management RAID StructureChapter 12-4RAID StructureStable-Storage ImplementationTertiary Storage DevicesOperating System IssuesPerformance Issues12.3Silberschatz, Galvin and Gagne ©2005Operating System ConceptsRAID – Continued12.4Silberschatz, Galvin and Gagne ©2005Operating System ConceptsRAID – different source – modified here and there.RAID is a technology that employs the simultaneous use of two or more hard disk drives to achieve greater levels of performance, reliability, and/or larger data volume sizes.The phrase "RAID" is an umbrella term for computer data storage schemes that can divide and replicate data among multiple hard disk drives. RAID's various designs all involve two key design goals: increased data reliability and increased input/output performance. When several physical disks are set up to use RAID technology, they are said to be in a RAID array. This array distributes data across several disks, but the array is seen by the computer user and operating system as one single disk.12.5Silberschatz, Galvin and Gagne ©2005Operating System ConceptsRAID - (continued) - FeaturesPrinciple: Some arrays are "redundant" in a way such that write operations result in extra data derived from original data captured across an array of disks organized so that the failure of one (sometimes more) disks in the array will not result in loss of data; a bad disk is replaced by a new one, and the data on it can be reconstructed from the remaining data and the extra data.Other RAID arrays are arranged so that they are faster to write to and read from than a single disk.It is important to note that a redundant array allows less data to be stored. For instance, a 2-disk RAID 1 array loses half of its capacity, (because data is stored twice) and a RAID 5 array with several disks loses the capacity of one full disk.There are various combinations of these approaches giving different trade offs of protection against data loss, capacity, and speed.  RAID levels 0, 1, and 5 are the most commonly found, and cover most requirements.12.6Silberschatz, Galvin and Gagne ©2005Operating System ConceptsRAID - (continued) – Important LevelsRAID 0 (striped disks) distributes data across several disks in a way which gives improved speed and full capacity, but all data on all disks will be lost if any one disk fails. (figures ahead)RAID 1 (mirrored disks) (could be described as a backup solution) uses two (possibly more) disks which each store the same data (hence the term ‘mirrored’), so that data is not lost as long as one disk survives. Total capacity of the array is just the capacity of a single disk. The failure of one drive, in the event of a hardware or software malfunction, does not increase the chance of a failure nor decrease the reliability of the remaining drives (second, third, etc). RAID 5 (striped disks with parity) combines three or more disks in a way that protects data against loss of any one disk.Here, capacity of the array is reduced by one disk.RAID 6 (less common) can recover from the loss of two disks.12.7Silberschatz, Galvin and Gagne ©2005Operating System ConceptsRAID Levels 0, 1, and 2In RAID Level 0: we’ve got no redundancy.Our array of disks has striping at the block level.In RAID Level 1: we have disk mirroring.For RAID Level 2, I will not discuss, because it is rarely used.12.8Silberschatz, Galvin and Gagne ©2005Operating System ConceptsRAID – (continued) – Computations InvolvedRAID involves significant computation when reading and writing information. With true RAID hardware the disk controller does all of this computation work. In other cases the operating system or simpler and often less expensive controllers require the host computer's processor to do the computing, (which reduces the computer's performance on processor-intensive tasks). Simpler RAID controllers may provide only levels 0 and 1, which require less processing.12.9Silberschatz, Galvin and Gagne ©2005Operating System ConceptsRAID (continued) – Rebuilding SystemsA very nice feature is that RAID systems with redundancy continue working without interruption when one, or sometimes more, disks of the array fail, although they are vulnerable to further failures. When the bad disk is replaced by a new one the array is rebuilt while the system continues to operate normally. Some systems have to be shut down when removing or adding a drive; others support this kind of hot swapping, allowing drives to be replaced without powering down. RAID with hot-swap drives is often used in high availability systems, where it is essential that the system keeps running as much of the time as possible.12.10Silberschatz, Galvin and Gagne ©2005Operating System ConceptsRAID (continued) Backup?RAID is not, in general, a good alternative to backing up data. In general, data may become damaged or destroyed without harm to the drive(s) on which it is stored. For example, Part of the data may be overwritten by a system malfunction; A file may be damaged or deleted by user error or malice and not noticed for days or weeks;Of course the


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