9. Computer Peripherals – Part I

1. 9. Computer Peripherals – Part I Chapt. 10

2. Plan • Storage (hierarchy and terminology) • Magnetic disks

3. 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. Primarystorage Secondarystorage Offlinestorage Storage Hierarchy Medium CPU registers Cache memory Conventional memory Expanded memory Hard disk Floppy disk CD-ROM Tape Access Time - 15-30 ns 50-100 ns 75-500 ns 10-50 ms 95 ms 100-600 ms 0.5+ s Throughput - - - - 600-6000 Kbytes/s 100-200 Kbytes/s 150-1000 Kbytes/s 5-20 Kbytes/s (cartridge) 200-3000 Kbytes/s (reel-to-reel)

5. 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. 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. Plan • Storage (hierarchy and terminology) • Magnetic disks

8. 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. 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. Floppy Disk Example Shutter Access window Cutawayshowing disk Case Spindle Writeprotect tab

11. 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 = 240 bytes)

12. Hard Disk Example

13. 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. Hard Disk Layout Head Block Headmotor Platter Sector Track Cylinder Track Drivemotor Head, onmoving arm Head assembly

15. 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. 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. 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. Locating a Block of Data Seek Time Latency Time Transfer Rate Latency Transfer Head Seek Desiredtrack Note: Access time = seek time + latency

19. 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. Latency Example • A hard disk rotates at 3600 rpm • What 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

21. 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. 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 / 210 = 1.76 Mbytes/s

23. 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. Exercise - Transfer Rate Answer • 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 / 210 = 3.65 Mbytes/s

25. Typical Spec’s

26. Sector Next sector Previous sector Inter-block gap Inter-block gap Note: CRC stands for “cyclic redundancy code”. It’s the “footer” at the end of each sector. CRC is a sophisticated form of parity for checking that the data read are accurate. Track Format • Format of each track: gap header data CRC gap

27. 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. 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. 2. Transfer data from buffer to system RAM (Note: this is a DMA operation) 1. Read data from disk into a buffer in the disk controller Buffering Example: Reading data from a disk System Diskcontroller Disk RAM Buffer (RAM)

30. 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. 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. 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. Interleaving Examples Factor 1:1 1 2 3 4 5 6 7 8 9 etc. 1 2 3 4 5 2:1 etc. 1 2 3 3:1 etc.

34. 2:1 Interleaving 2 6 1 7 5 3 9 8 4

35. 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. 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. Note:Varies on different systems Demo

38. 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. Alternative Technologies (2 of 3) • Fixed heads • Fewer tracks but eliminates seek time Moving head Disk Spindle Fixed heads

40. 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. 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. 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. 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. 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. 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. Thank you