CS 15 447 Computer Architecture Lecture 24 Disk

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

RAID 0 15 -447 Computer Architecture Fall 2007 ©

RAID 1 15 -447 Computer Architecture Fall 2007 ©

RAID 3 15 -447 Computer Architecture Fall 2007 ©

RAID 5 15 -447 Computer Architecture Fall 2007 ©

Interfacing Processors and Peripherals 15 -447 Computer Architecture Fall 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 15 -447 Computer Architecture Fall 2007 ©

Manufacturing Advantages of Disk Arrays Disk Product Families Conventional: 4 disk designs 3. 5” 5. 25” 10” Low End 14” High End Disk Array: 1 disk design 3. 5” 15 -447 Computer Architecture Fall 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 15 -447 Computer Architecture Fall 2007 ©

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

RAID 3: bit striping + parity 15 -447 Computer Architecture Fall 2007 ©

Redundant Arrays of Disks RAID 3: Parity Disk 10010011 11001101 10010011. . . logical record Striped physical records P 1 0 0 1 1 0 0 1 1 0 0 • 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 -447 Computer Architecture Fall 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 Targeted for mixed applications D 0 D 1 D 2 D 3 P D 4 D 5 D 6 P D 7 D 8 D 9 P D 10 D 11 D 12 P D 13 D 14 D 15 P D 16 D 17 D 18 D 19 D 20 D 21 D 22 D 23 P . . Disk Columns. . 15 -447 Computer Architecture Increasing Logical Disk Addresses Stripe Unit . . . Fall 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) 1 0 0 1 1 1 0 0 1 1 0 0 1 1 Interleaved parity blocks Independent reads and writes Logical write = 2 reads + 2 writes Parity + Reed-Solomon codes 15 -447 Computer Architecture Fall 2007 © 0 0 1 1 0 0 1 0