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© 2007 A.W. KringsRAIDRAID: Redundant Arrays of Inexpensive Disks–this discussion is based on the paper: »A Case for Redundant Arrays of Inexpensive Disks (RAID), »David A Patterson, Garth Gibson, and Randy H Katz, »In Proceedings of the ACM SIGMOD International Conference on Management of Data (Chicago, IL), pp.109--116, 1988.1© 2007 A.W. KringsRAIDMotivation–single chip computers improved in performance by 40% per year–RAM capacity quadrupled capacity every 2-3 years–Disks (magnetic technology)»capacity doubled every 3 years»price cut in half every 3 years»raw seek time improved 7% every year–Note: values presented in Pattersons’ paper are dated!–Note: paper discusses “pure” RAID, not smarter implementations, e.g. caching.2© 2007 A.W. KringsRAID–Amdahl’s Law: Effective Speedup»f = fraction of work in fast mode»k = speedup while in fast mode Example:»assume 10% I/O operation»if CPU 10x => effective speedup is 5 »if CPU 100x => effective speedup is 1090 % of potential speedup is wasted3© 2007 A.W. KringsRAIDMotivation–compare “mainframe mentality” with “todays” possibilities, e.g. cost, configurationCPUMemory ChannelControllerSCSICPUMemory DMAMainframeSmall Computer4© 2007 A.W. KringsRAID–Reliability–e.g. MTTFdisk = 30,000 h MTTF100 = 300 h ( < 2 weeks) MTTF1000 = 30 h–Note, that these numbers are very dated. Today’s drives are much better. MTBF > 300,000 to 800,000 hours.–even if we assume higher MTTF of individual disks, the problem stays.Bad news!5© 2007 A.W. KringsRAIDRAID Reliability–partition disks into reliability groups and check disks»D = total number of data disks»G = # data disks in group»C = # check disks in group6© 2007 A.W. KringsRAIDTarget Systems–Different RAID solutions will benefit different target system configurations.–Supercomputers»larger blocks of data, i.e. high data rate–Transaction processing»small blocks of data»high I/O rate»read-modify-write sequences7© 2007 A.W. KringsRAID5 RAID levels–RAID 1: mirrored disks–RAID 2: hamming code for ECC–RAID 3: single check disk per group–RAID 4: independent read/writes–RAID 5: no single check disk8© 2007 A.W. KringsRAIDRAID level 1: Mirrored Disks–Most expensive option–Tandem doubles controllers too–Write to both disks–Read from one disk–Characteristics:»S = slowdown. In synchronous disks spindles are synchronized so that the corresponding sectors of a group of disks can be accessed simultaneously. For synchr. disks S = 1.»Reads = 2D/S, i.e. concurrent read possible »Write = D/S, i.e. no overhead for concurrent write of same data»R-Modify-Write = 4D/(3S)»Pat88 Table II (pg. 112)9© 2007 A.W. KringsRAID10© 2007 A.W. KringsRAIDRAID level 2: Hamming Code–DRAM => problem with α-particles Solution, e.g. parity for SED, Hamming code for SEC–Recall Hamming Code–Same idea using one disk drive per bit–Smallest accessible unit per disk is one sector»access G sectors, where G = # data disks in a group–If operation on a portion of a group is needed:1) read all data2) modify desired position3) write full group including check info11© 2007 A.W. KringsRecall Hamming Codem = data bitsk = parity bits12© 2007 A.W. KringsCompute Check13© 2007 A.W. KringsRAID–Allows soft errors to be corrected “on the fly”.–Useful for supercomputers, not useful for transaction processing e.g. used in Thinking Machine (Connection Machine) “Data Vault” with G = 32, C = 8.–Characteristics:»Pat88 Table III (pg 112)14© 2007 A.W. Krings15© 2007 A.W. KringsRAIDRAID level 3: Single Check Disk per Group–Parity is SED not SEC!–However, often controller can detect if a disk has failed»information of failed disk can be reconstructed»extra redundancy on disk, i.e. extra info on sectors etc.–If check disk fails »read data disks to restore replacement–If data disk fails»compute parity and compare with check disk»if parity bits are equal => data bit = 0»otherwise => data bit = 116© 2007 A.W. KringsRAID–Since less overhead, i.e. one check disk only => Effective performance increases–Reduction in disks over L2 decreases maintenance–Performance same as L2, however, effective performance per disk increases due to smaller number of check disks–Better for supercomputers, not good for transaction proc.–Maxtor, Micropolis introduced first RAID-3 in 1988–Characteristics:»Pat88 Table IV (pg 113)17© 2007 A.W. Krings18© 2007 A.W. KringsRAIDRAID level 4: Independent Reads/Writes–Pat88 fig 3 pg. 113 compares data locations –Disk interleaving has advantages and disadvantages–Advantage of previous levels:»large transfer bandwidth–Disadvantages of previous levels:»all disks in a group are accessed on each operation (R,W) »spindle synchronizationif none => probably close to worse case average seek times, access times (tracking + rotation)–Interleave data on disks at sector level–Uses one parity disk 19© 2007 A.W. Krings20© 2007 A.W. KringsRAID–for small accesses »need only access to 2 disks, i.e. 1 data & parity»new parity can be computed from old parity + old/new data»compute: Pnew = dataold XOR datanew XOR Pold–e.g. small write1) read old data + parity 2) write new data + parity–Bottleneck is parity disk–e.g. small read»only read one drive (data)–Characteristics:»Pat88 Table V (pg 114)in parallel21© 2007 A.W. Krings22© 2007 A.W. KringsRAIDRAID level 5: No Single Check Disk–Distributes data and check info across all disks, i.e. there are no dedicated check disks.–Supports multiple individual writes per group–Best of 2 worlds»small Read-Modify-Write»large transfer performance»1 more disk in group => increases read performance–Characteristics:»Pat88 Table VI (pg 114)23© 2007 A.W. Krings24© 2007 A.W. KringsRAIDPatterson Paper–discusses all levels on pure hardware problem–refers to software solutions and alternatives, e.g. disk buffering–with transfer buffer the size of a track, spindle synchronization of groups not necessary–improving MTTR by using spares–low power consumption allows use of UPS–relative performance shown in Pat88 fig. 5 pg. 11525© 2007 A.W. Krings26© 2007 A.W. KringsRAIDSummary–Data Striping for improved performance»distributes data transparently over multiple disks to make them appear as a


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UI CS 449 - RAID

Course: Cs 449-
Pages: 37
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