Class Notes (811,659)
Canada (494,847)
York University (33,727)
ITEC 1000 (36)
all (19)


13 Pages
Unlock Document

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
More Less

Related notes for ITEC 1000

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.