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Disk Drives

Disk Drives. Storage vs. Memory. Because ROM is read only and RAM is volatile, we need something else to save our work This is storage Floppy disk Hard disk Optical disk (e.g. CD) Tape. Floppy Disk. Developed by IBM in early 1970’s 8 inch Early 1980’s 5.25 inch

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Disk Drives

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  1. Disk Drives

  2. Storage vs. Memory • Because ROM is read only and RAM is volatile, we need something else to save our work • This is storage • Floppy disk • Hard disk • Optical disk (e.g. CD) • Tape

  3. Floppy Disk • Developed by IBM in early 1970’s • 8 inch • Early 1980’s • 5.25 inch • 160 KB (single-sided, single density) – 1.2 MB (double-sided, high density) • These were genuinely “floppy” • Today’s Standard • 3.5 inch • Double sided, high density • 1.44 MB

  4. Density • The amount of information per area in a storage device • The higher the density, the larger the amount of information • single-density • double-density • high-density • extra-high-density. • If your drive doesn’t support a certain density, you cannot use it

  5. Hard disk • A magnetic disk on which you can store data. • “Hard” distinguishes it from a soft or floppy disk. • A hard disk can hold more and is faster than a floppy disk. • A hard disk can store up to several gigabytes (GB), while a standard floppy holds 1.44 megabytes (MB).

  6. Platters • A hard disk consists of several (a dozen or so) platters, round plates covered with magnetic material, upon which data is written

  7. Head • The reading of and writing to the platters is done by the heads • Most platters have two read/write heads, one for each side. • The heads are all attached to one access arm; they do not move independently. • Floppy disk drives have two heads, one for each side of the disk. • If the head becomes dirty, it will not work properly. Check this if your disk drive has problems.

  8. Head crash • In hard drives, the head normally does not actually touch the disk; it sits a few millionths of an inch from the disk. • Misalignment or dust particles can cause the head to touch the disk – this is a “head crash” • In floppy drives, the head touches the disk during normal operation.

  9. After a crash • In the event of a crash, you typically lose data and need to replace both the head and the disk. (If you’re lucky you will just end up with a few bad sectors and can continue using the disk.) • So keep your computer clean • Even smoke particles can cause a head crash.

  10. In both hard and floppy disks, the data is written in concentric circular paths known as tracks A typical floppy disk has 80 (double-density) or 160 (high-density) tracks. Tracks track

  11. Tracks (cont.) • The density of tracks is measured in units of tracks per inch (TPI). • Each track is further divided into sectors. • The location of information is remembered by noting its track and sector numbers.

  12. Sectors • Radial lines break the tracks up sectors Sector

  13. Cylinders • Each platter has tracks with the same radii. • All the tracks on different platters but with the same radius make up a cylinder. • For example, if a hard drive has • Four platters • And each platter has 600 tracks • Then • There will be 600 cylinders • Each cylinder will have 8 tracks (assuming that each platter has tracks on both sides).

  14. Clusters • The Operating System/BIOS can address a limited number of sectors • One way to beat this limit was to introduce clusters which are groups of sectors, then one addresses clusters instead of sectors • The operating system assigns a unique number to each cluster and then keeps track of files according to which clusters they use.

  15. Lost clusters • The bookkeeping is done in the File Allocation Table (FAT). The size of each cluster depends on the disk's partition size. • Sometimes the operating system indicates that a cluster is being used when it is not. This is called a lost cluster. • You can free up disk space by reassigning lost clusters with the ScanDisk utility.

  16. File Allocation Table • A bookkeeping table that the operating system uses to locate files on a disk. • Due to fragmentation, a file may be divided into many sections scattered all over the disk. The FAT keeps track of all these pieces. • In DOS systems, FATs are stored in hidden files, called FAT files • The FAT system for older versions of Windows 95 is called VFAT, and the one for new versions of Windows 95 and Windows 98 is called FAT32.

  17. Scandisk • DOS/Windows utility that locates errors on hard disks and is able to correct some of them. • In DOS, you run ScanDisk by typing scandisk at the prompt and hitting Enter. • In Windows 95/98, you can run ScanDisk by selecting Start/Programs/Accessories/System Tools/ScanDisk. • Among other things, ScanDisk looks for platter defects and for lost clusters that are sometimes created when a program aborts.

  18. format • Formatting prepares a disk for reading and writing. • When you format a disk, the operating system • erases all disk’s bookkeeping information • tests the reliability of the sectors • marks bad sectors (e.g. ones with scratches) • creates a new table for bookkeeping information. • You must format a disk before you can use it.

  19. Format (high level) • Reformatting a disk does not erase the disk’s data only the address tables (the pointers to it) • If you accidentally reformat a disk containing useful data, one should be able to recover most or even all of the information

  20. Format (low level) • The previous slide talks about high-level formatting • There is also low-level formatting, which sets certain properties of the disk such as the interleave factor. • The low-level format also determines what type of disk controller can access the disk

  21. Format (cont.) • Most hard disks that you buy have been low-level formatted by the manufacturer. • Thus you don’t have to perform a low-level format unless you want to change the interleave factor or make the disk accessible by a different type of controller. • Performing a low-level format erases all data on the disk.

  22. Interleaving • When storing a file larger than a sector, it is sometimes faster to store it on non-consecutive sectors • In one-to-one interleaving, the sectors are placed consecutively around a track. • In two-to-one interleaving, every other sector is written to

  23. Interleaving (cont.)

  24. Interleaving (cont.) • The purpose of interleaving is to make the disk drive more efficient. The disk drive can access only one sector at a time, and the disk is constantly spinning beneath the head. • This means that by the time the drive is ready to access the next sector, the disk may have already spun beyond it.

  25. Partition • To divide a storage device into separate sections. • In DOS systems, you can partition a disk, and each partition will behave like a separate disk drive. • Partitioning is particularly useful if you run more than one operating system. For example, you might reserve one partition for Windows and another for UNIX.

  26. Partition • In addition, partitioning on DOS and Windows machines can improve disk efficiency. This is because the FAT system used by these operating systems automatically assigns cluster size based on the disk size: the larger the disk, the larger the cluster. • Unfortunately, large clusters can result in a wasted disk space, called slack space. There is an entire sector of the software industry devoted to building utilities that let you partition your hard disk. • FAT32 addresses more clusters

  27. Optical disk • A type of storage disk in which data is written and read using laser light. • A CD is a portable optical disk, the optical analog of a floppy • CD’s can hold a lot more data (approx. 6 GB) than floppies (approx. 1.44 MB)

  28. Three types • CD-ROM: comes with data on it, cannot be changed (written to) • WORM: “write-once, read–many,”comes without data, can be written to once and only once • ERASABLE (REWRITABLE):comes without data, can be written to, then erased and rewritten, etc. Sometimes called EO (erasable optical) disks • Different disks and different drives

  29. LIMDOW • Light Intensity Modulation Direct OverWrite • Speeds up the writing of data from memory to a CD • The usual, magnetic-optico (MO) technology, requires that the disk pass under the head twice -- once for erasing and once for writing • LIMDOW changes the laser's intensity allowing for both processes to be done in a single pass

  30. Phase-change disk • A type of rewritable optical disk that employs the phase change recording method. • The disk drive writes data with a laser • The bits are determined by the state of the material in a given spot: amorphous (disordered) or crystalline (every aligned). • Data is read by reflecting light from the disk; crystalline and amorphous spots reflect light differently

  31. Tape • A strip of plastic coated with a magnetic material on which information can be stored • Similar to cassette tapes used to record music. • Cheap, but slow • Sequential access (as opposed to random access) • Good for • long-term storage • backup. • transporting large amounts of data

  32. Controller • A device that controls data transfer to and from a peripheral device. For example, disk drives, monitors, and printers all need controllers. • Controllers must be designed to communicate with the appropriate bus (be it ISA (AT), PCI or SCSI). • Thus your controller conform to the bus architecture used in your PC.

  33. IDE • Intelligent Drive Electronics or Integrated Drive Electronics • An IDE interface is an interface for storage devices, in which the controller is integrated into the disk or CD-ROM drive. • Although it really refers to a general technology, most people use the term to refer the ATA specification, which uses this technology.

  34. ATA • Short for AT Attachment, a disk drive that puts the controller on the disk drive itself. • There are several versions of ATA, all developed by the Small Form Factor (SFF) Committee

  35. EIDE • Short for Enhanced IDE, a newer version of the IDE storage device interface standard developed by Western Digital Corporation. • It supports data rates of between 4 and 16.6 MBps • Approx 3-4 times faster than the old IDE standard. • It also increased the storage capacity from 528 MB to 8.4 GB. • Because of its lower cost, EIDE has replaced SCSI in many areas.

  36. SCSI • Small Computer System Interface. Pronounced "scuzzy," • SCSI drive: a disk drive that works with the SCSI interface • SCSI is a parallel interface standard used for attaching peripheral devices to computers. • SCSI interfaces provide for faster data transmission rates (up to 80 megabytes per second) than standard serial and parallel ports. Also you can attach many devices to a single SCSI port, so that SCSI is really an I/O bus rather than simply an interface.

  37. SCSI (Cont.) • Although SCSI is an ANSI standard, there are many variations of it, so two SCSI interfaces may be incompatible. • For example, SCSI supports several types of connectors.

  38. SCSI (Cont.) • SCSI’s were standard in Macintoshes, the iMac comes with IDE. • You can attach SCSI devices to a PC by inserting a SCSI board in one of the expansion slots. • Many high-end new PCs come with SCSI built in. Note that the lack of a single SCSI standard means that some devices may not work with some SCSI boards.

  39. Varieties of SCSI • SCSI-1: 8-bit bus, supports data rates of 4 MBps • SCSI-2: Same as SCSI-1, but uses a 50-pin instead of a 25-pin connector, and supports multiple devices. • This is what most people mean when they refer to plain SCSI. • Wide SCSI: wider cable (168 cable lines to 68 pins) to support 16-bit transfers. • Fast SCSI: 8-bit bus, but doubles the clock rate to support data rates of 10 MBps.

  40. Varieties of SCSI • Fast Wide SCSI: 6-bit bus, supports data rates of 20 MBps. • Ultra SCSI: 8-bit bus, supports data rates of 20 MBps. • SCSI-3: 16-bit bus, supports data rates of 40 MBps. • Also called Ultra Wide SCSI. • Ultra2 SCSI: 8-bit bus, supports data rates of 40 MBps. • Wide Ultra2 SCSI: 16-bit bus, supports data rates of 80 MBps.

  41. MTBF • Mean time between failures. MTBF ratings are measured in hours and indicate the sturdiness of hard disk drives and printers. • Typical PC disk drives have MTBF ratings of approx. 300,000 hours. This means that of all the drives tested, one failure occurred every 300,000 hours of testing. • A more useful gauge of a device's lifetime is how long a warranty the manufacturer offers

  42. Ghost imaging • Ghost imaging, using ghosting software, is a method of converting the contents of a hard drive --including its configuration settings and applications--into an image, and then storing the image on a server or burning it onto a CD. • When contents of the hard drive are needed again, ghosting software converts the image back to original form.

  43. Ghosting (cont.) • Companies use ghost imaging when they want to have identically configured machines with the same software installed. • For example, to set up 100 laptops machines, one could manually set configurations and install applications on each machine. Ghosting software (usually contained on a floppy) will retrieve the ghost image from the server, convert it into its original form, and copy it onto the machine.

  44. HSM • Hierarchical Storage Management, a data storage system that automatically moves data between high-speed, high-cost devices (hard disk) and low-speed, low-cost devices (optical disk or tape) • HSM is a kind of caching. The HSM system guesses which data can be moved to slower devices and which data should stay on the hard disks.

  45. RAID • Redundant array of independent disks • A type of disk drive that uses two or more drives in combination for fault tolerance and performance. • More for servers than PCs.

  46. RAID levels • There are number of different RAID levels. The three most common are 0, 3, and 5: • Level 0: Provides data striping (spreading out blocks of each file across multiple disks) but no redundancy (copies). This improves performance but does not deliver fault tolerance.

  47. RAID levels (cont) • Level 1: Provides disk mirroring, in which data is written to two duplicate disks simultaneously. Disk mirroring is used commonly in on-line database systems where it's critical that the data be accessible at all times. • Level 3: Same as Level 0, but also reserves one dedicated disk for error correction data. It provides good performance and some level of fault tolerance. • Level 5: Provides data striping at the byte level and also stripe error correction information. This results in excellent performance and good fault tolerance.

  48. References • A+ Certification Training Kit • http://www.webopedia.com • http://www.whatis.com

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