CS 61C L27 RAID and Performance (1)A Carle, Summer 2005 © UCBinst.eecs.berkeley.edu/~cs61c/su05CS61C : Machine StructuresLecture #27: RAID & Performance2005-08-08Andy CarleCS 61C L27 RAID and Performance (2)A Carle, Summer 2005 © UCBOutline•RAID•PerformanceCS 61C L27 RAID and Performance (3)A Carle, Summer 2005 © UCBUse 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?CS 61C L27 RAID and Performance (4)A Carle, Summer 2005 © UCBReplace 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 potentially high performance, high MB per cu. ft., high MB per KW, but what about reliability?9X3X8X6XCS 61C L27 RAID and Performance (5)A Carle, Summer 2005 © UCBArray Reliability•Reliability- whether or not a component has failed• measured as Mean Time To Failure (MTTF)• Reliability of N disks = Reliability of 1 Disk ÷ N(assuming failures independent)- 50,000 Hours ÷ 70 disks = 700 hour• Disk system MTTF: Drops from 6 years to 1 month!• Disk arrays (JBOD) too unreliable to be useful!CS 61C L27 RAID and Performance (6)A Carle, Summer 2005 © UCBRedundant 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 infoCS 61C L27 RAID and Performance (7)A Carle, Summer 2005 © UCBBerkeley History, RAID-I•RAID-I (1989) • Consisted of a Sun 4/280 workstation with 128 MB of DRAM, four dual-string SCSI controllers, 28 5.25-inch SCSI disks and specialized disk striping software•Today RAID is $27 billion dollar industry, 80% nonPC disks sold in RAIDsCS 61C L27 RAID and Performance (8)A Carle, Summer 2005 © UCB“RAID 0”: Striping•Assume have 4 disks of data for this example, organized in blocks•Large accesses faster since transfer from several disks at onceThis and next 5 slides from RAID.edu, http://www.acnc.com/04_01_00.htmlCS 61C L27 RAID and Performance (9)A Carle, Summer 2005 © UCBRAID 1: Mirror• Each disk is fully duplicated onto its “mirror”• Very high availability can be achieved• Bandwidth reduced on write:• 1 Logical write = 2 physical writes• Most expensive solution: 100% capacity overheadCS 61C L27 RAID and Performance (10)A Carle, Summer 2005 © UCBRAID 3: Parity • Parity computed across group to protect against hard disk failures, stored in P disk• Logically, a single high capacity, high transfer rate disk• 25% capacity cost for parity in this example vs. 100% for RAID 1 (5 disks vs. 8 disks)CS 61C L27 RAID and Performance (11)A Carle, Summer 2005 © UCBRAID 4: parity plus small sized accesses• RAID 3 relies on parity disk to discover errors on Read• But every sector has an error detection field• Rely on error detection field to catch errors on read, not on the parity disk• Allows small independent reads to different disks simultaneouslyCS 61C L27 RAID and Performance (12)A Carle, Summer 2005 © UCBInspiration for RAID 5•Small writes (write to one disk): • Option 1: read other data disks, create new sum and write to Parity Disk (access all disks)• Option 2: since P has old sum, compare old data to new data, add the difference to P: 1 logical write = 2 physical reads + 2 physical writes to 2 disks•Parity Disk is bottleneck for Small writes: Write to A0, B1 => both write to P disk A0 B0 C0D0PA1 B1 C1PD1CS 61C L27 RAID and Performance (13)A Carle, Summer 2005 © UCBRAID 5: Rotated Parity, faster small writes• Independent writes possible because of interleaved parity• Example: write to A0, B1 uses disks 0, 1, 4, 5, so can proceed in parallel• Still 1 small write = 4 physical disk accessesCS 61C L27 RAID and Performance (14)A Carle, Summer 2005 © UCBOutline•RAID•PerformanceCS 61C L27 RAID and Performance (15)A Carle, Summer 2005 © UCBPerformance•Purchasing Perspective: given a collection of machines (or upgrade options), which has the - best performance ?- least cost ?- best performance / cost ?•Computer Designer Perspective: faced with design options, which has the- best performance improvement ?- least cost ?- best performance / cost ?•All require basis for comparison and metric for evaluation•Solid metrics lead to solid progress!CS 61C L27 RAID and Performance (16)A Carle, Summer 2005 © UCBTwo Notions of “Performance”PlaneBoeing 747BAD/SudConcordeTopSpeedDC to ParisPassen-gersThroughput (pmph)610 mph6.5 hours470 286,7001350 mph3 hours132 178,200•Which has higher performance?•Time to deliver 1 passenger?•Time to deliver 400 passengers?•In a computer, time for 1 job calledResponse Time or Execution Time•In a computer, jobs per day calledThroughput or BandwidthCS 61C L27 RAID and Performance (17)A Carle, Summer 2005 © UCBDefinitions•Performance is in units of things per sec• bigger is better•If we are primarily concerned with response time• performance(x) = 1 execution_time(x)" F(ast) is n times faster than S(low) " means…performance(F) execution_time(S)n ==performance(S) execution_time(F)CS 61C L27 RAID and Performance (18)A Carle, Summer 2005 © UCBExample of Response Time v. Throughput• Time of Concorde vs. Boeing 747?• Concord is 6.5 hours / 3 hours = 2.2 times faster• Throughput of Boeing vs. Concorde?• Boeing 747: 286,700 pmph / 178,200 pmph= 1.6 times faster• Boeing is 1.6 times (“60%”) faster in terms of throughput• Concord is 2.2 times (“120%”) faster in terms of flying time (response time)We will focus primarily on execution time for a single jobCS 61C L27 RAID and Performance (19)A Carle, Summer 2005 © UCBAdministrivia•Final Exam:• Friday, August 12, 11:00 – 2:00• 306 Soda (Same as Midterm 2)• Same rules as Midterms, except you can now have a two-sided cheat sheet•Project 4: Due Friday•HW8: Due Friday, but…• It is
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