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

2. 15-447 Computer ArchitectureFall 2007 © RAID 0

3. 15-447 Computer ArchitectureFall 2007 © RAID 1

4. 15-447 Computer ArchitectureFall 2007 © RAID 3

5. 15-447 Computer ArchitectureFall 2007 © RAID 5

6. 15-447 Computer ArchitectureFall 2007 © Interfacing Processors and Peripherals

7. 15-447 Computer ArchitectureFall 2007 © 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. 15-447 Computer ArchitectureFall 2007 © Manufacturing Advantages of Disk Arrays 14”10”5.25”3.5” Disk Array: 1 disk design Conventional: 4 disk designs Low End High End Disk Product Families

9. 15-447 Computer ArchitectureFall 2007 © 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. 15-447 Computer ArchitectureFall 2007 © Non-Redundant Array Striped: write sequential blocks across disk array High performance Poor reliability: MTTF Array = MTTF Disk / N MTTF Disk = 50,000 hours (6 years) N = 70 Disks MTTF Array = 700 hours (1 month) Odd Blocks Even Blocks

11. 15-447 Computer ArchitectureFall 2007 © 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. 15-447 Computer ArchitectureFall 2007 © 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. 15-447 Computer ArchitectureFall 2007 © RAID 3: bit striping + parity

14. 15-447 Computer ArchitectureFall 2007 © Redundant Arrays of Disks RAID 3: Parity Disk P 10010011 11001101 10010011... logical record 1001001110010011 1100110111001101 1001001110010011 0011000000110000 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. 15-447 Computer ArchitectureFall 2007 © Redundant Arrays of Disks RAID 5+: High I/O Rate Parity A logical write becomes four physical I/Os Independent writes possible because of interleaved parity Reed-Solomon Codes ("Q") for protection during reconstruction A logical write becomes four physical I/Os Independent writes possible because of interleaved parity Reed-Solomon Codes ("Q") for protection during reconstruction D0D1D2 D3 P D4D5D6 P D7 D8D9P D10 D11 D12PD13 D14 D15 PD16D17 D18 D19 D20D21D22 D23 P.............................. Disk Columns Increasing Logical Disk Addresses Stripe Unit Targeted for mixed applications

16. 15-447 Computer ArchitectureFall 2007 © Redundant Arrays of Disks (RAID) Disk Mirroring, Shadowing (RAID 1) Each disk is fully duplicated onto its "shadow" Logical write = two physical writes 100% capacity overhead 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 1001001110010011 1100110111001101 1001001110010011 0011001000110010 1001001110010011 1001001110010011