Presentation on theme: "TECH CH06 External Memory Magnetic Disk RAID Optical Memory"— Presentation transcript:
1 TECH CH06 External Memory Magnetic Disk RAID Optical Memory Magnetic TapeTECHComputer ScienceCH05
2 Types of External Memory Magnetic DiskRAIDRemovableOpticalCD-ROMCD-Writable (WORM)CD-R/WDVDMagnetic Tape
3 Magnetic DiskMetal or plastic disk coated with magnetizable material (iron oxide…rust)Range of packagingFloppyWinchester hard diskRemovable hard disk
4 Data Organization and Formatting Concentric rings or tracksGaps between tracksReduce gap to increase capacitySame number of bits per track (variable packing density)Constant angular velocityTracks divided into sectorsMinimum block size is one sectorMay have more than one sector per block
8 Removable or Not // Removable disk Nonremovable disk Can be removed from drive and replaced with another diskProvides unlimited storage capacityEasy data transfer between systemsNonremovable diskPermanently mounted in the drive
9 Floppy Disk 8”, 5.25”, 3.5” Small capacity Up to 1.44Mbyte (2.88M never popular)Slow (disk rotate at 300 and 600 rpm, average delay 100/2 and 200/2 ms.)UniversalCheap
10 Winchester Hard Disk (1) Developed by IBM in Winchester (USA)Sealed unitOne or more platters (disks)Heads fly on boundary layer of air as disk spins (crash into disk!)Very small head to disk gapGetting more robust
11 Winchester Hard Disk (2) UniversalCheapFastest external storage (typically rotate 3600 rpm, newer faster, average rotational delay 8.3 ms.)Getting larger all the timeMultiple Gigabyte now usual
12 Removable Hard Disk ZIP JAZ L-120 (a: drive) Cheap Very common Only 100MJAZNot cheap1GL-120 (a: drive)Also reads 3.5” floppyBecoming more popular?
13 Finding Sectors Must be able to identify start of track and sector Format diskAdditional information not available to userMarks tracks and sectors
14 ST506 format (old!) Foreground reading Gap1 Gap1 Id Gap2 Data Gap3 Id SyncByteTrackSyncByteHeadSectorCRCDataCRCForeground readingFind others
15 Characteristics Fixed (rare) or movable head Removable or fixed Single or double (usually) sidedSingle or multiple platterHead mechanismContact (Floppy)Fixed gapFlying (Winchester)
16 Multiple Platter One head per side Heads are joined and aligned Aligned tracks on each platter form cylindersData is striped by cylinderreduces head movementIncreases speed (transfer rate)
17 Speed Seek time (Rotational) latency Access time = Seek + Latency Moving head to correct track(Rotational) latencyWaiting for data to rotate under headAccess time = Seek + LatencyTransfer rate T = (number of bytes to be transferred)/(rotation speed)/(number of bytes on a track) = b/(rN)total access time Ta = Ts + 1/(2r) + b/(rN)
18 Sequential organization vs. random access e.g. e.g. a hard disk has average seek time of 20 ms, a transfer rate of 1 M byte/s, and 512 byte sectors with 32 sectors per track. Need to read a file consisting 256 sectors for a total of 128 K bytes. What is the total time for the transfer?Case 1: Sequential Organization (256 sectors on 8 tracks x 32 sectors/tracks)Average seek time = 20.0 msRotational delay = 8.3 msRead 32 sections (one track) = 16.7 mstotal time to read first track = 45 msTotal time = 45 ms + 7*( ) ms = 0.22 s
19 Time required for random access on highly fragmented organization Case 2: random access rather than sequential accessAverage seek time = 20.0 msRotational delay = 8.3 msRead 1 sector = 16.7/32 = 0.5 mstime to read one sector = 28.8 msTotal time = 256 * 28.8 ms = 7.37 sDe-fragment you hard disk!
20 Optical Storage CD-ROM // Originally for audio650Mbytes giving over 70 minutes audioPolycarbonate coated with highly reflective coat, usually aluminumData stored as pitsRead by reflecting laserConstant packing densityConstant linear velocity
21 Constant Angular Velocity vs. Constant Linear Velocity
22 CD-ROM Drive Speeds Audio is single speed Constant linear velocity1.2 ms-1Track (spiral) is 5.27km longGives 4391 seconds = 73.2 minutesDate K bytes/s total capacity M BytesOther speeds are quoted as multiplese.g. 24x ~= 4 M Bytes/s (data transfer rate)The quoted figure is the maximum the drive can achieve
23 CD-ROM Format Mode 0=blank data field 00FFx 1000LayeredECCMinSecSectorModeData12 byteSync4 byteId2048 byte288 byte2352 byteMode 0=blank data fieldMode 1=2048 byte data+error correctionMode 2=2336 byte data
24 Random Access on CD-ROM DifficultMove head to rough positionSet correct speedRead addressAdjust to required location(Yawn!)
25 CD-ROM for & against Large capacity (?) Easy to mass produce Removable RobustExpensive for small runsSlowRead only
26 Other Optical Storage CD-Writable CD-RW WORM Now affordable Compatible with CD-ROM drivesCD-RWErasableGetting cheaperMostly CD-ROM drive compatible
27 DVD - what’s in a name? Digital Video Disk Digital Versatile Disk Used to indicate a player for moviesOnly plays video disksDigital Versatile DiskUsed to indicate a computer driveWill read computer disks and play video disksDogs Veritable DinnerOfficially - nothing!!!
28 DVD - technology Multi-layer Very high capacity (4.7G per layer) dual-layer (single-sided ?) hold 8.5 Gbytes ~> 4hr movieFull length movie on single diskUsing MPEG compressionFinally standardized (honest!)Movies carry regional codingPlayers only play correct region films
29 DVD - Writable Loads of trouble with standards First generation DVD drives may not read first generation DVD-W disksFirst generation DVD drives may not read CD-RW disksWait for it to settle down before buying!
30 Foreground Reading Check out optical disk storage options Check out Mini Disk
31 Magnetic TapeSerial accessSlowVery cheapBackup and archive
32 Digital Audio Tape (DAT) Uses rotating head (like video)High capacity on small tape4Gbyte uncompressed8Gbyte compressedBackup of PC/network servers
33 RAID Redundant Array of Independent Disks Redundant Array of Inexpensive Disks6 levels in common useNot a hierarchySet of physical disks viewed as single logical drive by O/SData distributed across physical drivesCan use redundant capacity to store parity information
37 RAID 0 No redundancy Data striped across all disks Round Robin stripingIncrease speedMultiple data requests probably not on same diskDisks seek in parallelA set of data is likely to be striped across multiple disks
38 RAID 1 Mirrored Disks Data is striped across disks 2 copies of each stripe on separate disksRead from eitherWrite to bothRecovery is simpleSwap faulty disk & re-mirrorNo down timeExpensive
39 RAID 2 Disks are synchronized Very small stripes Often single byte/wordError correction calculated across corresponding bits on disksMultiple parity disks store Hamming code error correction in corresponding positionsLots of redundancyExpensiveNot used
40 RAID 3Similar to RAID 2Only one redundant disk, no matter how large the arraySimple parity bit for each set of corresponding bitsData on failed drive can be reconstructed from surviving data and parity infoVery high transfer rates
41 RAID 4 Each disk operates independently Good for high I/O request rate Large stripesBit by bit parity calculated across stripes on each diskParity stored on parity disk
42 RAID 5 Like RAID 4 Parity striped across all disks Round robin allocation for parity stripeAvoids RAID 4 bottleneck at parity diskCommonly used in network servers
43 RAID 6 Two different parity calculations are carried out and stored in separate blocks on different disks.Able to regenerate data even if two disks containing user data fail
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