EECS 252 Graduate Computer ArchitectureLec 18 – StorageDavid PattersonElectrical Engineering and Computer SciencesUniversity of California, Berkeleyhttp://www.eecs.berkeley.edu/~pattrsnhttp://vlsi.cs.berkeley.edu/cs252-s06 4/12/2006 CS252 s06 Storage 2Review• Virtual Machine Revival– Overcome security flaws of modern OSes– Processor performance no longer highest priority– Manage Software, Manage Hardware• “… VMMs give OS developers another opportunity to develop functionality no longer practical in today’s complex and ossified operating systems, where innovation moves at geologic pace .”[Rosenblum and Garfinkel, 2005]• Virtualization challenges for processor, virtual memory, I/O– Paravirtualization, ISA upgrades to cope with those difficulties• Xen as example VMM using paravirtualization– 2005 performance on non-I/O bound, I/O intensive apps: 80% of native Linux without driver VM, 34% with driver VM• Opteron memory hierarchy still critical to performance4/12/2006 CS252 s06 Storage 3Case for Storage• Shift in focus from computation to communication and storage of information – E.g., Cray Research/Thinking Machines vs. Google/Yahoo– “The Computing Revolution” (1960s to 1980s) ⇒ “The Information Age” (1990 to today)• Storage emphasizes reliability and scalability as well as cost-performance• What is “Software king” that determines which HW acually features used?– Operating System for storage– Compiler for processor • Also has own performance theory—queuing theory—balances throughput vs. response time 4/12/2006 CS252 s06 Storage 4Outline• Magnetic Disks• RAID• Administrivia• Advanced Dependability/Reliability/Availability• I/O Benchmarks, Performance and Dependability• Intro to Queueing Theory (if we have time)• Conclusion4/12/2006 CS252 s06 Storage 5Disk Figure of Merit: Areal Density• Bits recorded along a track– Metric is Bits Per Inch (BPI)• Number of tracks per surface– Metric is Tracks Per Inch (TPI)• Disk Designs Brag about bit density per unit area– Metric is Bits Per Square Inch: Areal Density = BPI x TPIYear Areal Density1973 2 1979 8 1989 63 1997 3,090 2000 17,100 2006 130,000 1101001,00010,000100,0001,000,0001970 1980 1990 2000 2010Areal Density4/12/2006 CS252 s06 Storage 6Historical Perspective• 1956 IBM Ramac — early 1970s Winchester– Developed for mainframe computers, proprietary interfaces– Steady shrink in form factor: 27 in. to 14 in.• Form factor and capacity drives market more than performance• 1970s developments– 5.25 inch floppy disk formfactor (microcode into mainframe)– Emergence of industry standard disk interfaces• Early 1980s: PCs and first generation workstations• Mid 1980s: Client/server computing – Centralized storage on file server» accelerates disk downsizing: 8 inch to 5.25– Mass market disk drives become a reality» industry standards: SCSI, IPI, IDE» 5.25 inch to 3.5 inch drives for PCs, End of proprietary interfaces• 1900s: Laptops => 2.5 inch drives• 2000s: What new devices leading to new drives?4/12/2006 CS252 s06 Storage 7Future Disk Size and Performance• Continued advance in capacity (60%/yr) and bandwidth (40%/yr)• Slow improvement in seek, rotation (8%/yr)• Time to read whole disk Year Sequentially Randomly(1 sector/seek)1990 4 minutes 6 hours2000 12 minutes 1 week(!)2006 56 minutes 3 weeks (SCSI)2006 171 minutes 7 weeks (SATA)4/12/2006 CS252 s06 Storage 8Use Arrays of Small Disks?14”10”5.25”3.5”3.5”Disk Array: 1 disk designConventional: 4 disk designsLow EndHigh End•Katz and Patterson asked in 1987: •Can smaller disks be used to close gap in performance between disks and CPUs?4/12/2006 CS252 s06 Storage 9Advantages of Small FormfactorDisk DrivesLow cost/MBHigh MB/volumeHigh MB/wattLow cost/ActuatorCost and Environmental Efficiencies4/12/2006 CS252 s06 Storage 10Replace Small Number of Large Disks with Large Number of Small Disks! (1988 Disks)Capacity Volume PowerData Rate I/O Rate MTTF CostIBM 3390K20 GBytes97 cu. ft.3 KW15 MB/s600 I/Os/s250 KHrs$250KIBM 3.5" 0061320 MBytes0.1 cu. ft.11 W1.5 MB/s55 I/Os/s50 KHrs$2Kx7023 GBytes11 cu. ft.1 KW120 MB/s3900 IOs/s??? Hrs$150KDisk Arrays have potential for large data and I/O rates, high MB per cu. ft., high MB per KW, but what about reliability?9X3X8X6X4/12/2006 CS252 s06 Storage 11Array Reliability• Reliability of N disks = Reliability of 1 Disk ÷ N50,000 Hours ÷ 70 disks = 700 hoursDisk system MTTF: Drops from 6 years to 1 month!• Arrays (without redundancy) too unreliable to be useful!Hot spares support reconstruction in parallel with access: very high media availability can be achievedHot spares support reconstruction in parallel with access: very high media availability can be achieved4/12/2006 CS252 s06 Storage 12Redundant Arrays of (Inexpensive) Disks• Files are "striped" across multiple disks• Redundancy yields high data availability– Availability: service still provided to user, even if some components failed• Disks will still fail• Contents reconstructed from data redundantly stored in the array⇒ Capacity penalty to store redundant info⇒ Bandwidth penalty to update redundant info4/12/2006 CS252 s06 Storage 13Redundant Arrays of Inexpensive DisksRAID 1: Disk Mirroring/Shadowing• Each disk is fully duplicated onto its “mirror”Very high availability can be achieved• Bandwidth sacrifice on write:Logical write = two physical writes• Reads may be optimized• Most expensive solution: 100% capacity overhead• (RAID 2 not interesting, so skip)recoverygroup4/12/2006 CS252 s06 Storage 14Redundant Array of Inexpensive Disks RAID 3: Parity DiskP100100111100110110010011. . .logical record10100011110011011010001111001101P contains sum ofother disks per stripe mod 2 (“parity”)If disk fails, subtract P from sum of other disks to find missing informationStriped physicalrecords4/12/2006 CS252 s06 Storage 15RAID 3• Sum computed across recovery group to protect against hard disk failures, stored in P disk• Logically, a single high capacity, high transfer rate disk: good for large transfers• Wider arrays reduce capacity costs, but decreases availability• 33% capacity cost for parity if 3 data disks and 1 parity disk4/12/2006 CS252 s06 Storage 16Inspiration for RAID 4• RAID 3 relies on parity disk to discover errors on Read• But every sector has an error detection field• To catch errors on
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