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RAID Arrays

RAID Arrays. Redundant Array of Inexpensive Discs. What is RAID Arrays?. RAID is an acronym for Redundant Array of Independent Drives (or Disks), also known as Redundant Array of Inexpensive Drives (or Disks)

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RAID Arrays

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  1. RAID Arrays Redundant Array of Inexpensive Discs

  2. What is RAID Arrays? RAID is an acronym for Redundant Array of Independent Drives (or Disks), also known as Redundant Array of Inexpensive Drives (or Disks) The various types of RAID are data storage schemes that divide and/or replicate data among multiple hard drives

  3. Why Use RAID? Improved Reliability Improved Performance Fault Tolerance Improved Availability Higher Data Security

  4. Key Terms • Mirroring - the copying of data to more than one disk • Striping - the splitting of data across more than one disk • Parity - a redundancy check that ensures that the data is protected without having to have a full set of duplicate drives. • Duplexing - an extension of mirroring that is based on the same principle as that technique expect it goes one step further in that it also duplicates the hardware that controls the two hard drives (or sets of hard drives). RAID Arrays

  5. RAID - Redundant Array of Independent Disks RAIDController RAID Arrays Host RAID Array

  6. RAID Components Physical Array Logical Array RAIDController RAID Arrays Logical Array Host RAID Array

  7. Data Organization: Strips and Stripes Stripe 1 RAID Arrays Stripe 2 Stripe 3 Strips

  8. RAID Levels • 0 Striped array with no fault tolerance • 1 Disk mirroring • 3 Parallel access array with dedicated parity disk • 4 Striped array with independent disks and a dedicated parity disk • 5 Striped array with independent disks and distributed parity • 6 Striped array with independent disks and dual distributed parity • Combinations of levels (I.e., 1 + 0, 0 + 1, etc.) RAID Arrays

  9. RAID 0 A striped set of at least two disks without parity The data is broken down into blocks and each block is written to a separate disk drive Best performance is achieved when data is striped across multiple controllers with only one drive per controller

  10. Block 4 Block 4 Block 3 Block 3 Block 2 Block 2 Block 1 Block 1 Block 0 Block 0 RAID 0 – Striped Array with no Fault Tolerance RAIDController Host RAID Arrays

  11. Advantages of RAID 0 I/O performance is greatly improved by spreading the I/O load across many channels and drives No parity calculation overhead is involved Very simple design Easy to implement

  12. Disadvantages of RAID 0 Not a "True" RAID because it is NOT fault-tolerant The failure of just one drive will result in all data in an array being lost Should never be used in mission critical environments

  13. Block 1 Block 1 Block 1 Block 0 Block 0 Block 0 RAID 1 – Disk Mirroring RAIDController Host RAID Arrays

  14. RAID 1 Advantages High data availability and high I/O rate (small block size). Improves read performance - twice the read transaction rate of single disks, same write transaction rate as single disks 100% redundancy of data means no rebuild is necessary in case of a disk failure, just a copy to the replacement disk Simplest RAID storage subsystem design – easy to maintain

  15. RAID 1 Disadvantages Expensive due to the extra capacity required to duplicate data. Overhead cost equals 100%, while usable storage capacity is 50%. May not support hot swap of failed disk when implemented with software. Use hardware implementation.

  16. RAIDController Block 3 Block 3 Block 3 Block 2 Block 2 Block 2 Block 1 Block 1 Block 1 Block 0 Block 0 Block 0 RAID 0+1 – Striping and Mirroring RAID Arrays Host

  17. RAIDController Block 3 Block 3 Block 3 Block 2 Block 2 Block 2 Block 1 Block 1 Block 1 Block 0 Block 0 Block 0 RAID 1+0 – Mirroring and Striping RAID Arrays Host

  18. RAID 0+1 vs. RAID 1+0 • Benefits are identical under normal operations • Rebuild operations are very different • RAID 1+0 uses a mirrored pair – only 1 disk is rebuilt if a disk fails • RAID 0+1 if a single drive fails, the entire stripe is faulted • RAID is 0+1 is a poorer solution and is less common RAID Arrays

  19. 0 4 8 RAID Redundancy: Parity 1 5 9 2 RAIDController 6 RAID Arrays 10 3 7 Host 11 0 1 2 3 4 5 6 7 8 9 10 11 Parity Disk

  20. The middle drive fails: Parity Calculation Data 5 5 + 3 + 4 + 2 = 14 Data 3 Data 4 RAID Arrays 5 + 3 + ? + 2 = 14 ? = 14 – 5 – 3 – 2 ? = 4 Data 2 Parity 14 RAID Array

  21. RAIDController ParityGenerated Block 1 Block 2 Block 3 Block 0 Block 3 Block 2 Block 1 Block 0 P 0 1 2 3 RAID 3 – Parallel Transfer with Dedicated Parity Disk RAID Arrays Host

  22. P 0 1 2 3 P 0 1 2 3 Block 1 Block 0 Block 3 Block 0 Block 2 P 4 5 6 7 Block 4 Block 5 Block 6 Block 7 RAIDController Block 0 Block 0 ParityGenerated P 0 1 2 3 RAID 4 – Striping with Dedicated Parity Disk RAID Arrays Host

  23. P 0 1 2 3 P 0 1 2 3 Block 2 Block 0 Block 0 Block 3 Block 1 ParityGenerated P 4 5 6 7 P 4 5 6 7 Block 4 Block 5 Block 4 Block 6 Block 7 RAIDController Block 0 Block 4 Block 0 Block 4 P 4 5 6 7 ParityGenerated P 0 1 2 3 RAID 5 – Independent Disks with Distributed Parity RAID Arrays Host

  24. RAID 6 – Dual Parity RAID • Two disk failures in a RAID set leads to data unavailability and data loss in single-parity schemes, such as RAID-3, 4, and 5 • Increasing number of drives in an array and increasing drive capacity leads to a higher probability of two disks failing in a RAID set • RAID-6 protects against two disk failures by maintaining two parities • Horizontal parity which is the same as RAID-5 parity • Diagonal parity is calculated by taking diagonal sets of data blocks from the RAID set members • Even-Odd, and Reed-Solomon are two commonly used algorithms for calculating parity in RAID-6 RAID Arrays

  25. RAID Implementations • Hardware (usually a specialized disk controller card) • Controls all drives attached to it • Performs all RAID-related functions, including volume management • Array(s) appear to the host operating system as a regular disk drive • Dedicated cache to improve performance • Generally provides some type of administrative software • Software • Generally runs as part of the operating system • Volume management performed by the server • Provides more flexibility for hardware, which can reduce the cost • Performance is dependent on CPU load • Has limited functionality RAID Arrays

  26. Hot Spares RAIDController RAID Arrays

  27. RAIDController Hot Swap RAIDController RAID Arrays RAIDController

  28. Check Your Knowledge  • What is a RAID array? • What benefits do RAID arrays provide? • What methods can be used to provide higher data availability in a RAID array? • What is the primary difference between RAID 3 and RAID 5? • What is a hot spare? RAID Arrays

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