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Raid

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RAID (originally redundant array of inexpensive disks, now commonly redundant array of independent disks) is a data storage virtualization technology that combines multiple physical disk drive components into a single logical unit for the purposes of data redundancy, performance improvement, or both.

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Raid

  1. 1. RAIDRAID Redundant Array ofRedundant Array of Independent DisksIndependent Disks
  2. 2. Outline Outline What is RAID? What is RAID?RAID configurations used. RAID configurations used. Performance of each Performance of each configuration. configuration.Implementations. Implementations.Way forward with RAID. Way forward with RAID.
  3. 3. WhatisRAID? WhatisRAID? Stands for Redundant Array of Stands for Redundant Array of Independent Disks. Independent Disks.It’s a technology that enables It’s a technology that enables greater levels of performance, greater levels of performance, reliability and/or large volumes reliability and/or large volumes when dealing with data. when dealing with data.How?? How?? By concurrent use of two By concurrent use of two or more ‘ or more ‘hard disk drives’. hard disk drives’. How Exactly?? How Exactly?? Mirroring, Mirroring, Stripping (of data) and Error Stripping (of data) and Error correction techniques combined correction techniques combined with multiple disk arrays give you with multiple disk arrays give you the reliability and performance. the reliability and performance.
  4. 4. Mean Time Between FailuresMean Time Between Failures (MTBF)(MTBF)
  5. 5. RAIDflavors RAIDflavors Commonly used ones: Commonly used ones: 1.1.RAID 0 RAID 02.2.RAID 1 RAID 13.3.RAID 5 RAID 54.4.RAID 10 RAID 10Other types used…but Other types used…but rarely: RAID 2,3,4,6,50…… rarely: RAID 2,3,4,6,50……
  6. 6. RAID 0RAID 0 a.a. It splits data among two or more disks.It splits data among two or more disks. b.b.Provides good performance.Provides good performance. c.c. Lack of data redundancy means there isLack of data redundancy means there is no fail over support with thisno fail over support with this configuration.configuration. d.d.In the diagram to the right, the oddIn the diagram to the right, the odd blocks are written to disk 0 and the evenblocks are written to disk 0 and the even blocks to disk 1 such that A1, A2, A3,blocks to disk 1 such that A1, A2, A3, A4, … would be the order of blocks readA4, … would be the order of blocks read if read sequentially from the beginning.if read sequentially from the beginning. e.e. Used in read only NFS systems andUsed in read only NFS systems and gaming systems.gaming systems.
  7. 7. RAID 0 analysisRAID 0 analysis Failure Rate:Failure Rate:  MTBF of RAID0 is roughly proportional to the numberMTBF of RAID0 is roughly proportional to the number of disks in the array.of disks in the array.  Pr(disk fail) = 5%, thenPr(disk fail) = 5%, then Pr(atleastonefails) = 1 – Pr(nonefails) = 1 – [1-0.05]Pr(atleastonefails) = 1 – Pr(nonefails) = 1 – [1-0.05]22 == 9.75%9.75% Performance:Performance:  The fragments are written to their respective disksThe fragments are written to their respective disks simultaneously on the same sector.simultaneously on the same sector.  This allows smaller sections of the entire chunk of data toThis allows smaller sections of the entire chunk of data to be read off the drive in parallel, hence good performance.be read off the drive in parallel, hence good performance.
  8. 8. RAID 1RAID 1 •RAID1 is ‘data mirroring’.RAID1 is ‘data mirroring’. •Two copies of the data are held onTwo copies of the data are held on two physical disks, and the data istwo physical disks, and the data is always identical.always identical. • Twice as many disks are requiredTwice as many disks are required to store the same data whento store the same data when compared to RAID 0.compared to RAID 0. •Array continues to operate so longArray continues to operate so long as at least one drive is functioning.as at least one drive is functioning.
  9. 9. RAID1analysis RAID1analysis Failure Rate: Failure Rate:If Pr(disk fail) = 5%, then If Pr(disk fail) = 5%, then the probability of both the the probability of both the drives failing in a 2 disk drives failing in a 2 disk array is P(both fail) = (0.05) array is P(both fail) = (0.05)22 = 0.25%. = 0.25%. Performance: Performance:If we use independent disk If we use independent disk controllers for each disk, controllers for each disk, then we can increase the then we can increase the read or write speeds by read or write speeds by doing operations in parallel. doing operations in parallel.
  10. 10. RAID 5RAID 5 • RAID 5 is an ideal combinationRAID 5 is an ideal combination of good performance, good faultof good performance, good fault tolerance and high capacity andtolerance and high capacity and storage efficiency.storage efficiency. • An arrangement of parity andAn arrangement of parity and CRC to help rebuilding drive dataCRC to help rebuilding drive data in case of disk failures.in case of disk failures. • ““Distributed Parity” is the keyDistributed Parity” is the key word here.word here.
  11. 11. RAID5analysis RAID5analysis because ecauseof one disk is not of one disk is not quite a harm. We need more quite a harm. We need more time if 2 or more disks fail. time if 2 or more disks fail. Performance is also as good Performance is also as good as RAID 0, if not better. We as RAID 0, if not better. We can read and write parallel can read and write parallel blocks of data. blocks of data.One of the drawbacks is that One of the drawbacks is that the write involves heavy the write involves heavy parity calculations by the parity calculations by the RAID controller. Write RAID controller. Write operations are slower operations are slower compared to RAID 0. compared to RAID 0. Pretty useful for general Pretty useful for general purpose uses where ‘rea purpose uses where ‘rea are more fre are more freq‘‘w
  12. 12. RAID 10RAID 10 a.a. Combines RAID 1 and RAIDCombines RAID 1 and RAID 0.0. b.b. Which means having theWhich means having the pleasure of both - goodpleasure of both - good performance and good failoverperformance and good failover handling.handling. c.c. Also called ‘Nested RAID’.Also called ‘Nested RAID’.
  13. 13. Implementations Implementations tware based RAID: tware based RAID: Software implementations Software implementations are provided by many are provided by many Operating Systems. Operating Systems. A software layer sits above A software layer sits above the disk device drivers and the disk device drivers and provides an abstraction layer provides an abstraction layer between the logical between the logical drives(RAIDs) and physical drives(RAIDs) and physical drives. drives. Server's processor is used to Server's processor is used to run the RAID software. run the RAID software. Used for simpler Used for simplerconfigurations like RAID configurations like RAID and RAID1. and RAID1.
  14. 14. Implementations (contd…)Implementations (contd…)  of RAID req of RAID re special-purpose RAID special-purpose RAID controller. controller. On a desktop system this On a desktop system this may be built into the may be built into the motherboard. motherboard. Processor is not used for Processor is not used for RAID calculations as a RAID calculations as a separate controller present. separate controller present. A PCI-bus-based, IDE/ATA hard disk RAID controller, supporting levels 0, 1, and 01.
  15. 15. What’s happening present day?What’s happening present day? RAID 6:RAID 6: a.a.It is seen as the best way toIt is seen as the best way to guarantee data integrity as it usesguarantee data integrity as it uses double parity.double parity. b.b.Lesser MTBF compared toLesser MTBF compared to RAID5.RAID5. c.c.It has a drawback though ofIt has a drawback though of longer write time.longer write time.
  16. 16. Roadahead… Roadahead… a virtual virtual certainty. ertainty. RAID vendors to support RAID vendors to support "fast rebuild" features that "fast rebuild" features that can restore hundreds of can restore hundreds of gigabytes in just an hour or gigabytes in just an hour or so!!so!! Striping(of data) would Striping(of data) would extend across RAID groups extend across RAID groups -- not just across drives -- not just across drives within a group. within a group.Improved disk diagnostic Improved disk diagnostic features should offer more features should offer more reliable predictions of reliable predictions of impending drive failures, impending drive failures, allowing the rebui allowing the rebui to begi to begi
  17. 17. References References hstorage.tec hstorage.tec arget.com/generic/0,2 rget.com/generic/0,2 95582,sid5_gci1196310,0 95582,sid5_gci1196310,0 0.html 0.html http://www.lascon.co.uk http://www.lascon.co.uk /d008005.htm /d008005.htmhttp://en.wikipedia.org/ http://en.wikipedia.org/ wiki/RAID_0#RAID_0 wiki/RAID_0#RAID_0 http://en.wikipedia.org/ http://en.wikipedia.org/ wiki/RAID wiki/RAIDPaper - A Case for Paper - A Case for Redundant Arrays of Redundant Arrays of Inexpensive Disks Inexpensive Disks (RAID) - David A (RAID) - David A Patterson, Garth Gibson Patterson, Garth Gibson and Randy H K and Randy H K

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