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ITEC 1011 Introduction to Information Technologies 9. Computer Peripherals – Part I Chapt. 10.

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Presentation on theme: "ITEC 1011 Introduction to Information Technologies 9. Computer Peripherals – Part I Chapt. 10."— Presentation transcript:

1 ITEC 1011 Introduction to Information Technologies 9. Computer Peripherals – Part I Chapt. 10

2 ITEC 1011 Introduction to Information Technologies Plan Storage (hierarchy and terminology) Magnetic disks

3 ITEC 1011 Introduction to Information Technologies Storage Terminology Medium The technology or product type that holds the data Access time The time to get to the data Specified as an average in seconds (e.g., s, ms, µs, ns, etc.) Throughput The rate of transfer for consecutive bytes of data Specified in bytes/s (e.g., Kbytes/s, Mbytes/s)

4 ITEC 1011 Introduction to Information Technologies Offline storage Secondary storage Primary storage Storage Hierarchy Medium CPU registers Cache memory Conventional memory Expanded memory Hard disk Floppy disk CD-ROM Tape Access Time ns ns ns ms 95 ms ms 0.5+ s Throughput Kbytes/s Kbytes/s Kbytes/s 5-20 Kbytes/s (cartridge) Kbytes/s (reel-to-reel)

5 ITEC 1011 Introduction to Information Technologies Terminology Online storage Memory that is accessible to programs without human intervention Primary storage and secondary storage are online Primary storage Semiconductor technology (e.g., RAM) Volatile (contents are loss when powered off) Secondary storage Magnetic technology (e.g., disk drives) Non-volatile (contents are retained in the absence of power)

6 ITEC 1011 Introduction to Information Technologies Terminology Offline storage Memory that requires human intervention in order for it to be accessed by a program (e.g., loading a tape) Sometimes called archival storage Direct Access Storage Device (DASD) Pronounced dazz-dee Term coinded by IBM Distinguishes disks (disk head moves directly to the data) from tapes (see below) Sequential access storage devices Tape drives Tape reel must wind forward or backward to the data

7 ITEC 1011 Introduction to Information Technologies Plan Storage (hierarchy and terminology) Magnetic disks

8 ITEC 1011 Introduction to Information Technologies Magnetic Disks A magnetic substance is coated on a round surface The 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)

9 ITEC 1011 Introduction to Information Technologies Floppy Disks Also called flexible disks or diskettes The platter is floppy, or flexible (e.g., mylar) Most floppy disk drives can hold one diskette (two surfaces) The diskette is removable Typical rpm: 300, 360 Capacities: 700 KB to 1.4 MB (& up to 100 MB zip disks)

10 ITEC 1011 Introduction to Information Technologies Floppy Disk Example Write protect tab Spindle Shutter Access window Cutaway showing disk Case

11 ITEC 1011 Introduction to Information Technologies Hard Disks The platter is hard (e.g., aluminum) Most hard disk drives contain more than one platter On 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: 500 MB to 1+ TB (terabyte = 2 40 bytes)

12 ITEC 1011 Introduction to Information Technologies Hard Disk Example

13 ITEC 1011 Introduction to Information Technologies Winchester Disks Invented by IBM A type of hard disk drive The disk is contained within a sealed unit No dust particles When powered off, the head is parked at the outer edge of the platter and rests on the platter surface When powered on, the aerodynamics of the head and enclosure create a cushion of air between the head and the disk surface The head floats above the surface (very close!) and does not touch the surface Thus, head crash (the head touches the surface, with damage resulting)

14 ITEC 1011 Introduction to Information Technologies Hard Disk Layout Platter Track Cylinder Drive motor Head motor Head, on moving arm Block Sector Track Head Head assembly

15 ITEC 1011 Introduction to Information Technologies Terminology Platter A round surface – the disk – containing a magnetic coating Track A circle on the disk surface on which data are contained Head A transducer attached to an arm for writing/reading data to/from the disk surface Head assembly A mechanical unit holding the heads and arms All the head/arm units move together, via the head assembly Cylinder A set of tracks simultaneously accessible from the heads on the head assembly

16 ITEC 1011 Introduction to Information Technologies Terminology Drive motor The motor that rotates the platters Typically a DC motor (DC = direct current) The disk rotates at a fixed speed (e.g., 3600 rpm, revolutions per minute) Head motion A mechanism is required to move the head assembly in/out Two possibilities: A stepper motor (digital, head moves in steps, no feedback) A servo motor (analogue, very precision positioning, but requires feedback)

17 ITEC 1011 Introduction to Information Technologies Terminology Sector That portion of a track falling along a predefined pie-shaped portion of the disk surface The number of bytes stored in a sector is the same, regardless of where the sector is located; thus, the density of bits is greater for sectors near the centre of the disk The rotational speed is constant; i.e., constant angular velocity Thus, the transfer rate is the same for inner sectors and outer sectors Block The smallest unit of data that can be written or read to/from the disk (typically 512 bytes)

18 ITEC 1011 Introduction to Information Technologies Locating a Block of Data Seek TimeLatency TimeTransfer Rate Desired track Seek Head Transfer Latency Note: Access time = seek time + latency

19 ITEC 1011 Introduction to Information Technologies Terminology Seek time The time for the head to move to the correct track Specified as an average for all tracks on the disk surface Latency time The time for the correct block to arrive at the head once the head is positioned at the correct track Specified as an average, in other words, ½ the period of rotation Also called rotational delay Access time is the time to get to the data (remember!) Access time = seek time + latency Transfer rate Same as throughput

20 ITEC 1011 Introduction to Information Technologies Latency Example A hard disk rotates at 3600 rpm What is the average latency? Period of rotation= (1 / 3600) minutes = (1 / 3600) 60 seconds = s = ms Average latency= / 2 ms = 8.33 ms

21 ITEC 1011 Introduction to Information Technologies Factors Determining Transfer Rate Transfer rate can be determined, given… Rotational speed of the disk platters Number of sectors per track Number of bytes per sector

22 ITEC 1011 Introduction to Information Technologies Transfer Rate Example Q: Determine the transfer rate, in Mbytes/s, for a hard disk drive, given Rotational speed = 7200 rpm Sectors per track = 30 Data per sector = 512 bytes = 0.5 Kbytes A: 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 / 2 10 = 1.76 Mbytes/s

23 ITEC 1011 Introduction to Information Technologies Exercise - Transfer Rate Q: Determine the transfer rate, in Mbytes/s, for a hard disk drive, given Rotational speed = 7000 rpm Sectors per track = 32 Data per sector = 1024 bytes Skip answer Answer

24 ITEC 1011 Introduction to Information Technologies Exercise - Transfer Rate Q: Determine the transfer rate, in Mbytes/s, for a hard disk drive, given Rotational speed = 7000 rpm Sectors per track = 32 Data per sector = 1024 bytes A: 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 / 2 10 = 3.65 Mbytes/s Answer

25 ITEC 1011 Introduction to Information Technologies Typical Specs Specification3 ½ Floppy2 GB Hard Disk Platters/heads1 / 25 / 9 Cylinders Sectors/track18Varies Block size512 Capacity1.44 MB2.1 GB Rotation speed360 rpm7200 rpm Avg. seek time95 ms8.5 ms Latency83 ms4.2 ms Transfer rate54 Kbyte/s10 Mbyte/s

26 ITEC 1011 Introduction to Information Technologies Track Format Format of each track: dataheader gap CRC Sector Previous sector Next sector Inter-block gap Inter-block gap Note: CRC stands for cyclic redundancy code. Its the footer at the end of each sector. CRC is a sophisticated form of parity for checking that the data read are accurate.

27 ITEC 1011 Introduction to Information Technologies Disk Formatting The track positions, blocks, headers, and gaps must be established before a disk can be used The process for doing this is called formatting The header, at the beginning of each sector, uniquely identifies the sector, e.g., by track number and sector number

28 ITEC 1011 Introduction to Information Technologies Disk Controller Interface between the disk drive and the system is known as a disk controller A primary function is to ensure data read/write operations are from/to the correct sector Since data rate to/from the disk is different than data rate to/from system memory, buffering is needed

29 ITEC 1011 Introduction to Information Technologies Buffering SystemDisk controller Disk RAM Buffer (RAM) 1. Read data from disk into a buffer in the disk controller 2. Transfer data from buffer to system RAM (Note: this is a DMA operation) Example: Reading data from a disk

30 ITEC 1011 Introduction to Information Technologies Multi-block Transfers (1 of 2) The smallest transfer is one block (e.g., 512 bytes) However, often multi-block transfers are required The inter-block gap provides time for the controller electronics to adjust from the end of one sector to the beginning of the next time may be needed for a few reasons: Compute and/or verify the CRC bytes Switch circuits from read mode to write mode During a write operation the header is read but the data are written (Remember, the header is only written during formatting.) Perform a DMA operation

31 ITEC 1011 Introduction to Information Technologies Multi-block Transfers (2 of 2) Sometimes, sectors simply cannot be read or written consecutively There is not enough time (see preceding slide) The result is lost performance because the disk must undergo a full revolution to read the next sector The solution: interleaving

32 ITEC 1011 Introduction to Information Technologies Interleaving Rather than numbering blocks consecutively, the system skips one or more blocks in its numbering This allows multi-block transfers to occur as fast as possible Interleaving minimizes lost time due to latency Interleaving factor (see next slide) is established when the disk is formatted Can have a major impact on system performance

33 ITEC 1011 Introduction to Information Technologies Interleaving Examples :1 etc. 2:1 3:1 Factor

34 ITEC 1011 Introduction to Information Technologies 2:1 Interleaving

35 ITEC 1011 Introduction to Information Technologies File System Considerations There is no direct relationship between the size and physical layout of blocks on a disk drive and the size and organization of files on a system File system Determines the organization of information on a computer Performs logical-to-physical mapping of information A file system is part of each and every operating system Logical mapping The way information is perceived to be stored Physical mapping The way information is actually stored

36 ITEC 1011 Introduction to Information Technologies Disks Capacity in Windows To determine the capacity of the C: hard disk on Windows From the Desktop, double click on My Computer Right click on C: and select Properties Next slide

37 ITEC 1011 Introduction to Information Technologies Note: Varies on different systems Demo

38 ITEC 1011 Introduction to Information Technologies Alternative Technologies (1 of 3) Removable hard disks Also called disk packs A stack of hard disks enclosed in a metal or plastic removable cartridge Advantages High capacity and fast, like hard disk drives Portable, like floppy disks Disadvantage Expensive

39 ITEC 1011 Introduction to Information Technologies Alternative Technologies (2 of 3) Fixed heads Fewer tracks but eliminates seek time DiskSpindle Moving head Fixed heads

40 ITEC 1011 Introduction to Information Technologies Alternative Technologies (3 of 3) 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 performance Frequently used on servers, but not generally used on PCs There are a number of different R.A.I.D. levels (next slide)

41 ITEC 1011 Introduction to Information Technologies R.A.I.D. Levels (1 of 2) Level 0 Provides data striping (spreading out blocks of each file across multiple disks) No redundancy Improves performance, but does not deliver fault tolerance Level 1 Provides data mirroring Data are written to two duplicate disks simultaneously If one drive fails, the system can switch to the other without loss of data or service Delivers fault tolerance

42 ITEC 1011 Introduction to Information Technologies R.A.I.D. Levels (2 of 2) Level 3 Same as level 0, but also reserves one dedicated disk for error correction data Good performance, and some level of fault tolerance Level 5 Data striping at the byte level and stripe error correction information Excellent performance, good fault tolerance

43 ITEC 1011 Introduction to Information Technologies Terminology Fault tolerance The ability of a computer system to respond gracefully to unexpected hardware or software failure Many levels of fault tolerance E.g., the ability to continue operating in the event of a power failure Some systems mirror all operations Every operation is performed on two or more duplicate systems, so if one fails, another can take over

44 ITEC 1011 Introduction to Information Technologies Terminology Data mirroring A technique in which data are written to two duplicate disks simultaneously If one disk fails, the system can instantly switch to the other disk without loss of data or service Used commonly in on-line database systems where it is critical that data are accessible at all times

45 ITEC 1011 Introduction to Information Technologies Terminology Data striping A technique for spreading data over multiple disks Speeds operations that retrieve data from disk storage Data are broken into units (blocks) and these are spread across the available disks Implementations allow selection of data units size, or stripe width

46 ITEC 1011 Introduction to Information Technologies Thank you


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