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York University
Information Technology
ITEC 1000

Introduction to Information Technology ITEC 1000 – Winter 2010 – Peter Khaiter Lecture 9/10 – Computer Peripherals – Mar 8/15 Peripherals - Devices that are external to the main processing function of the computer o Not the CPU, memory, power supply - Classified as input, output, and storage - Connected via o Ports  Parallel, USB, serial o Interface to systems bus  SCSI, IDE, PCMCIA Storage Devices: Terminology - Medium o The technology or product type that holds the data - Access time o The time to locate data and read it o Specified as an average in seconds (e.g., s, ms, µs, ns, etc.) - Throughput/Transfer rate o Amount of data (in consecutive bytes) moved per second o Specified in bytes/s (e.g., Kbytes/s, Mbytes/s) - Online storage o Memory that is accessible to programs without human intervention o Primary storage and secondary storage are “online” - Primary storage o Semiconductor technology (e.g., RAM) o Volatile (contents might be lost when powered off ) - Secondary storage o Magnetic technology (e.g., disk drives) o Non-volatile (contents are retained in the absence of power) - Offline storage o Memory that requires human intervention in order for it to be accessed by a program (e.g., loading a tape) o Sometimes called “archival storage” - Direct Access Storage Device (DASD) o Pronounced “dazz-dee” o Term coined by IBM o Distinguishes disks (disk head moves “directly” to the data) from tapes (tape reel must wind forward or backward to the data: sequential access) Storage Devices - Primary memory (cache, conventional memory) – immediate access by CPU - Expanded storage (e.g., RAM) – a buffer between conventional memory and secondary memory) - Secondary storage o Data and programs must be copied to primary memory for CPU access o Permanence of data o Mechanical devices o Direct access storage devices (DASDs) o Online storage o Offline storage – loaded when needed Secondary Storage Devices - Hard drives, floppy drives - CD-ROM and DVD-ROM drives - CD-R, CD-RW, DVD-RAM, DVD-RW - Tape drives - Network drives - Direct access vs. Sequential access - Rotation vs. Linear Magnetic Disks - Track – circle - Cylinder – same track on all platters - Block – small arc of a track - Sector – pie-shaped part of a platter - Head – reads data off the disk - Head crash - Parked heads - Number of bits on each track is the same! Denser towards the center. - CAV – constant angular velocity o Spins the same speed for every track o Hard drives – 3600 rpm – 7200 rpm o Floppy drives – 360 rpm Floppy Disks - Also called “flexible disks” or “diskettes” - The platter is “floppy”, or flexible (e.g., mylar) (typical: 5.25”, 3.5”) - Most floppy disk drives can hold one diskette (two surfaces) - The diskette is removable - Typical rpm: 300, 360 - Capacities: 180 KB to 1.4 MB (& up to 100 MB “zip” disks, more) 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: 5 MB to 1+ TB (terabyte = 2 bytes) 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) Hard Disk: Terminology - Platter o A round surface – the disk – containing a magnetic coating - Track o A circle on the disk surface on which data are contained - Head o A transducer attached to an arm for writing/reading data to/from the disk surface - Head assembly o A mechanical unit holding the heads and arms o All the head/arm units move together, via the head assembly - Cylinder o A set of tracks simultaneously accessible from the heads on the head assembly - Drive motor o The motor that rotates the platters o Typically a DC motor (DC = direct current) o The disk rotates at a fixed speed (e.g., 3600 rpm, revolutions per minute) - Head motion o A mechanism is required to move the head assembly in/out o Two possibilities:  A stepper motor (digital, head moves in steps, no feedback)  A servo motor (analogue, very precision positioning, but requires feedback) - Sector o That portion of a track falling along a predefined pie-shaped portion of the disk surface o 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 o The rotational speed is constant; i.e., constant angular velocity o Thus, the transfer rate is the same for inner sectors and outer sectors - Block o The smallest unit of data that can be written or read to/from the disk (typically 512 bytes) - Seek time o The time for the head to move to the correct track o Specified as an average for all tracks on the disk surface - Latency time o The time for the correct block to arrive at the head once the head is positioned at the correct track o Specified as an average, in other words, ½ the period of rotation o Also called “rotational delay” - Access time is the time “to get to” the data (remember!) o Access time = seek time + latency - Transfer rate o Same as throughput Disk Access Times - Avg. Seek time o Average time to move from one track to another - Avg. Latency time o Average time to rotate to the beginning of the sector o Avg. Latency time = ½ * 1/rotational speed - Transfer time o 1/(# of sectors * rotational speed) - Total Time to access a disk block o Avg. Seek time + avg. Latency time + avg. Transfer time Factors Determining Transfer Rate - Transfer rate can be determined, given… o Rotational speed of the disk platters o Number of sectors per track o Number of bytes per sector 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 Multi-Block Transfers - 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: o Compute and/or verify the CRC bytes o 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.) o Perform a DMA operation - 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 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 Alternate Disk Technologies - Removable hard drives o Disk pack – disk platters are stored in a plastic case that is removable o Another version includes the disk head and arm assembly in the case - Fixed-head disk drives o One head per track o Eliminates the seek time - Bernoulli Disk Drives o Hybrid approach that incorporates both floppy and hard disk technology o Zip drives Removable Hard Disks - Also called “disk packs” - A stack of hard disks enclosed in a metal or plastic removable cartridge - Advantages o High capacity and fast, like hard disk drives o Portable, like floppy disks - Disadvantage o Expensive Fixed Heads - Fewer tracks but eliminates seek time 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” R.A.I.D. Levels - Level 0 o Provides “data striping” (spreading out blocks of each file across multiple disks) o No redundancy o Improves performance, but does not deliver “fault tolerance” - Level 1 o Provides “data mirroring”: (a.k.a.: “shadowing”) o Data are written to two duplicate disks simultaneously o If one drive fails, the system can switch to the other without loss of data or service o Delivers fault tolerance - Level 3 o Same as level 0, but also reserves one dedicated disk for error correction data o Good performance, and some level of fault tolerance - Level 5 o Data striping at the byte level and stripe error correction information o Excellent performance, good fault tolerance Fault Tolerance - The ability of a computer system to respond gracefully to unexpected hardware or software failure - Many levels of fault tolerance o E.g., the ability to continue operating in the event of a power failure - Some systems “mirror” all operations o Every operation is performed on two or more duplicate systems, so if one fails, another can take over Data Mirroring (Shadowing) - 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 Data Stripping - 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 t
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