*Secondary Storage DevicesHard drives, floppy drivesCD-ROM and DVD-ROM drivesCD-R, CD-RW, DVD-RAM, DVD-RWTape drivesNetwork drivesDirect access vs. Sequential accessRotation vs. Linear

*Magnetic DisksA magnetic substance is coated on a round surfaceThe magnetic substance can be polarized in one of two directions with an electromagnet (writing data)The electromagnet can also sense the direction of magnetic polarization (reading data)Similar to a read/write head on a tape recorder (except the information is digital rather than analogue)

*Hard Disks: ExampleTop view of a 36 GB, 10,000 RPM, IBM SCSI server hard disk, with its top cover removed, 10 stacked platters (The IBM Ultrastar 36ZX)

*Hard Disk LayoutPlatterTrackCylinderDrive motorHead motorHead, on moving armBlockSectorTrackHeadHead assembly

Disk Data Layout

Multiple Platters

*Hard Disk: TerminologyPlatterA round surface the disk containing a magnetic coatingTrackA circle on the disk surface on which data are containedHeadA transducer attached to an arm for writing/reading data to/from the disk surfaceHead assemblyA mechanical unit holding the heads and armsAll the head/arm units move together, via the head assemblyCylinderA set of tracks simultaneously accessible from the heads on the head assembly

*Hard Disk: TerminologyDrive motorThe motor that rotates the plattersTypically a DC motor (DC = direct current)The disk rotates at a fixed speed (e.g., 3600 rpm, revolutions per minute)Head motionA mechanism is required to move the head assembly in/outTwo possibilities:A stepper motor (digital, head moves in steps, no feedback)A servo motor (analogue, very precision positioning, but requires feedback)

*Hard DisksThe platter is hard (e.g., aluminum)Most hard disk drives contain more than one platterOn most hard disk drives, the disks are fixed (i.e., not removable)On some hard disk drives, the disks are in a removable pack (hence, disk pack)Typical speed of rotation: 3600, 5400, 7200 rpm (rpm = revolutions per minute)Capacities: 5 MB to 1+ TB (terabyte = 240 bytes)

*Locating a Block of DataSeek TimeLatency TimeTransfer RateDesired trackSeekHeadTransferLatencyNote: Access time = seek time + latency

SpeedSeek timeMoving head to correct track(Rotational) latencyWaiting for data to rotate under headAccess time = Seek + LatencyTransfer rate

Timing of Disk I/O Transfer

*Hard Disk: TerminologySeek timeThe time for the head to move to the correct trackSpecified as an average for all tracks on the disk surfaceLatency timeThe time for the correct block to arrive at the head once the head is positioned at the correct trackSpecified as an average, in other words, the period of rotationAlso called rotational delay

*Disk Access TimesAvg. Seek timeaverage time to move from one track to anotherAvg. Latency timeaverage time to rotate to the beginning of the sectorAvg. Latency time = * 1/rotational speedTotal Time to retrieve a disk blockAvg. seek time + avg. latency time + avg. transfer time

*Latency ExampleA hard disk rotates at 3600 rpmWhat is the average latency?Period of rotation= (1 / 3600) minutes= (1 / 3600) 60 seconds= 0.01667 s= 16.67 ms

Average latency= 16.67 / 2 ms= 8.33 ms

*Factors Determining Transfer RateTransfer rate can be determined, givenRotational speed of the disk platters Number of sectors per trackNumber of bytes per sector

*Factors Determining Disk CapacityDisk Capacity can be determined, givenNo of SurfacesNo of Tracks per SurfaceNo of Sectors per TrackSector Size

Consider Magnetic disk with 8 surfaces, 512 tracks per surface and 64 sectors per track Sector size is 1kB. What is disk capacityAns:256MB

*Transfer Rate: ExampleQ: Determine the transfer rate, in Mbytes/s, for a hard disk drive, givenRotational speed = 7200 rpmSectors per track = 30Data per sector = 512 bytes = 0.5 KbytesA: Transfer rate= 7200 x 30 = 216,000 sectors/min= 216,000 x 0.5 = 108,000 Kbytes/min= 108,000 / 60 = 1,800 Kbytes/s= 1,800 / 210 = 1.76 Mbytes/s

*Exercise - Transfer RateQ: Determine the transfer rate, in Mbytes/s, for a hard disk drive, givenRotational speed = 7000 rpmSectors per track = 32Data per sector = 1024 bytes

*Exercise - Transfer RateQ: Determine the transfer rate, in Mbytes/s, for a hard disk drive, givenRotational speed = 7000 rpmSectors per track = 32Data per sector = 1024 bytes = 1 KbA: Transfer rate= 7000 x 32 = 224,000 sectors/min= 224,000 x 1 = 224,000 Kbytes/min= 224,000 / 60 = 3,733 Kbytes/s= 3,733 / 210= 3.65 Mbytes/sAnswer

*R.A.I.D. = Redundant array of inexpensive disks

A category of disk drive that employs two or more drives in combination for fault tolerance and performanceFrequently used on servers, but not generally used on PCsThere are a number of different R.A.I.D. levels (next slide)

RAID 0No redundancyData striped across all disksIncrease speedMultiple data requests probably not on same diskDisks seek in parallelA set of data is likely to be striped across multiple disks

RAID 1Mirrored DisksData is striped across disks2 copies of each stripe on separate disksRead from eitherWrite to bothRecovery is simpleSwap faulty disk & re-mirrorNo down timeExpensive

*R.A.I.D. LevelsLevel 3Same as level 0, but also reserves one dedicated disk for error correction dataGood performance, and some level of fault tolerance

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