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Lecture 24 Disk IO and RAID

CS 15-447: Computer Architecture. Lecture 24 Disk IO and RAID. November 28, 2007 Karem A. Sakallah. RAID 0. RAID 1. RAID 3. RAID 5. Interfacing Processors and Peripherals. Disk Access. Seek : position head over the proper track (3 to 14 ms. avg.)

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Lecture 24 Disk IO and RAID

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  1. CS 15-447: Computer Architecture Lecture 24Disk IO and RAID November 28, 2007 Karem A. Sakallah

  2. RAID 0

  3. RAID 1

  4. RAID 3

  5. RAID 5

  6. Interfacing Processors and Peripherals

  7. Disk Access • Seek: position head over the proper track(3 to 14 ms. avg.) • Rotate: wait for desired sector(.5 / RPM) • Transfer: get the data(one or more sectors @ 30 to 80 MB/sec

  8. Manufacturing Advantages of Disk Arrays Disk Product Families Conventional: 4 disk designs 3.5” 5.25” 10” 14” High End Low End Disk Array: 1 disk design 3.5”

  9. RAID: Redundant Array of Inexpensive Disks • RAID 0: Striping (misnomer: non-redundant) • RAID 1: Mirroring • RAID 2: Striping + Error Correction • RAID 3: Bit striping + Parity Disk • RAID 4: Block striping + Parity Disk • RAID 5: Block striping + Distributed Parity

  10. Non-Redundant Array • Striped: write sequential blocks across disk array • High performance • Poor reliability:MTTFArray = MTTFDisk / NMTTFDisk = 50,000 hours (6 years)N = 70 DisksMTTFArray= 700 hours (1 month) Odd Blocks Even Blocks

  11. Redundant Arrays of Disks • Files are "striped" across multiple spindles • Redundancy yields high data availability • When disks fail, contents are reconstructed from data redundantly stored in the array • High reliability comes at a cost: • Reduced storage capacity • Lower performance

  12. RAID 1: Mirroring • Each disk is fully duplicated onto its “shadow”  very high availability • Bandwidth sacrifice on writes:Logical write = two physical writes • Reads may be optimized • Most expensive solution: 100% capacity overhead Used in high I/O rate , high availability environments

  13. RAID 3: bit striping + parity

  14. Redundant Arrays of Disks RAID 3: Parity Disk 10010011 11001101 10010011 . . . P logical record 1 0 0 1 0 0 1 1 1 1 0 0 1 1 0 1 1 0 0 1 0 0 1 1 0 0 1 1 0 0 0 0 Striped physical records • Parity computed across recovery group to protect against hard disk failures 33% capacity cost for parity in this configuration wider arrays reduce capacity costs, decrease expected availability, increase reconstruction time • Arms logically synchronized, spindles rotationally synchronized logically a single high capacity, high transfer rate disk Targeted for high bandwidth applications: Scientific, Image Processing

  15. Redundant Arrays of Disks RAID 5+: High I/O Rate Parity Increasing Logical Disk Addresses D0 D1 D2 D3 P A logical write becomes four physical I/Os Independent writes possible because of interleaved parity Reed-Solomon Codes ("Q") for protection during reconstruction D4 D5 D6 P D7 D8 D9 P D10 D11 D12 P D13 D14 D15 Stripe P D16 D17 D18 D19 Targeted for mixed applications Stripe Unit D20 D21 D22 D23 P . . . . . . . . . . . . . . . Disk Columns

  16. Redundant Arrays of Disks (RAID) 1 0 0 1 0 0 1 1 1 0 0 1 0 0 1 1 • Disk Mirroring, Shadowing (RAID 1) Each disk is fully duplicated onto its "shadow" Logical write = two physical writes 100% capacity overhead 1 0 0 1 0 0 1 1 0 0 1 1 0 0 1 0 1 1 0 0 1 1 0 1 1 0 0 1 0 0 1 1 • Parity Data Bandwidth Array (RAID 3) Parity computed horizontally Logically a single high data bw disk • High I/O Rate Parity Array (RAID 5) Interleaved parity blocks Independent reads and writes Logical write = 2 reads + 2 writes Parity + Reed-Solomon codes

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