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Storage Devices

Storage Devices. Objectives. In this chapter, you will: -Understand how hard drives read and write data -Know the difference between tracks, sectors and cylinders -Understand the difference between high-level and low-level formatting

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Storage Devices

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  1. Storage Devices

  2. Objectives • In this chapter, you will: -Understand how hard drives read and write data -Know the difference between tracks, sectors and cylinders -Understand the difference between high-level and low-level formatting -Identify the major internal components and understand the workings of a typical hard drive -Identify cables and connectors used with hard drives

  3. Definition • A hard disk drive is a sealed unit that a PC uses for nonvolatile data storage. Nonvolatile, or semi-permanent, storage means that the storage device retains the data even when no power is supplied to the computer. • A hard disk drive contains rigid, disk-shaped platters, usually constructed of aluminum or glass.

  4. Drive Operation • The basic physical construction of a hard disk drive consists of spinning disks with heads that move over the disks and store data in tracks and sectors. The heads read and write data in concentric rings called tracks, which are divided up into segments called sectors, which normally store 512 bytes each.

  5. Drive Operation • One side of a platter is called a “head”. • Hard drives can have different numbers of platters, depending on their design and storage capacity. • On the heads, you will see concentric rings (tracks) and pieces of rings (sectors) just like on the floppy disks. Sector Cluster Track

  6. Drive Operation • Many hard drives today use a technology called “zone bit recording” which enables the hard drive to have more sectors on the outer tracks, where there is more room than on the inner tracks. • This allows more room for storage than using same number of sectors on each track. Original Zone Bit

  7. Drive Operation • Another logical unit of a hard drive is the cylinder. • Let’s say we have a disk drive with three platters. Imagine passing a cylinder down through both sides of each platter (6 tracks). • These 6 tracks make up a logical cylinder on the disk.

  8. Computer Data Storage • The read/write head on the drive moves very close to the spinning disk as it goes by and writes to specific concentric rings on the disk, called tracks. • The electrical particles on the disk are arranged according to the charge given to them by the head. • The head can read the magnetic data as it passes by, or write to it by using an electrical charge. • Preparing a disk drive for data storage involves three steps: • Low-Level formatting (LLF) • Partitioning • High-level formatting (HLF)

  9. Low Level Formatting Sector Cluster • Before you can write data to a disk, you must first low level format the disk. In this process, the drive manufacturer marks the tracks and sectors of the disk. • A sector is a small section of a track. Sectors are the smallest units of accessible data on a drive, and one or more sectors compose a cluster. Track

  10. High Level Formatting • During the high level formatting, the operating system writes the structures necessary for managing the files and data on the disk. • FAT partitions have a Volume Boot Sector, two copies of a file allocation table, and a root directory on each formatted logical drive. • These data structure enable the operating system to manage the space on the disk, keep track of files, and even manage defective areas. • High level formatting divides the disk into two specific area: A system area for DOS system files and a files area for data and programs.

  11. High Level Formatting • A part of the system area is called the “Master boot sector”. • The Master boot sector is: • Always the first sector (sector 1) of the first track (track 0) of the first cylinder (cylinder 0) disk. • 512 bytes long, just like any other sector • Contains information on all logical drives, regardless of whether they are bootable. • Formatting also creates the root directory. • If the disk is to be made bootable, COMMAND.COM and the two system files must be in the root directory of the bootable drive.

  12. Partitioning • Partitioning a disk is the act of defining areas of the disk for an operating system to use. • Partitioning is required because a hard disk is designed to be used with more than one operating system. • Partitioning enables a single hard disk drive to run more than one type of operating system (dual boot), or it can enable a single operating system to use the disk as several volumes or logical drives. • A “physical component” refers to an actual physical unit of hardware. We can take groups of these physical components, or small parts of these physical components, and group them into “logical components”.

  13. Partitioning • A “logical component” is either a defined grouping of physical objects or parts of physical objects that act as one unit. • Each file system can then use its own method to allocate file space in logical units called clusters or allocation units. • Every disk must have at least one partition on it and can have up to four partitions, each of which can support the same or different type file systems. • For example if you are partitioning a hard disk with a capacity of 10GB and you would like to logically divide this space into three drives with different drive letters.

  14. Partitioning • You decide you want to break the 10GB space into three logical partitions: one with 5GB of space, one with 3GB, and one with 2GB. • The operating systems will logically view these three partitions as three separate drives and gives them separate drive letters C:, D:, and E:. • Physically all you have is one hard drive with three logical drives. • Hard drive partitions must always begin at C:; because the A: and B: drives are reserved for floppies.

  15. File Allocation Table (FAT) • Partitioning creates the Master Boot Sector and a file allocation table (fat) that keeps track of which clusters the operating system assigns to which specific files. • The file allocation table is an important concept, not only for floppy drives but also for disk drives in general. • The FAT consists of information that keeps track of the clusters on a disk that belong to specific files • The common file systems used by PC operating systems today are: • Fat16 (file allocation table, 16 bit); Fat32 (32bit) • NTFS (Windows NT file system) • Ext2FS (Linux’s Second Extended file system)

  16. Disk Structure Disk Structure Master boot sector FAT FAT • The Master Boot Sector is the first sector. • The FAT comes directly after the boot sector. There are two copies of the FAT to protect against one being corrupted. • The root directory comes after the FAT. • The general file storage area comes after the space set aside for the root directory. Root Directory General File Storage

  17. Disk Structure with One partition Master boot sector Volume boot sector • The Master Boot Sector is the first sector. • The Volume Boot Sector is the first sector or sectors in the partition. • The FAT comes directly after the boot sector. There are two copies of the FAT to protect against one being corrupted. • The root directory comes after the FAT. • The general file storage area comes after the space set aside for the root directory. FAT FAT Root Directory General File Storage

  18. Volume boot sector Volume boot sector FAT FAT FAT FAT Root Directory Root Directory General File Storage General File Storage Disk Structure with Multiple partition Master boot sector • The Master Boot Sector is the first sector. • The Volume Boot Sector is the first sector or sectors in the partition. • The FAT comes directly after the boot sector. There are two copies of the FAT to protect against one being corrupted. • The root directory comes after the FAT. • The general file storage area comes after the space set aside for the root directory.

  19. Components • The basic components of a typical hard disk drive are as follows: -Disk platters -Read/write heads -Head actuator mechanism -Spindle motor (inside platter hub) -Logic board (controller or Printed Circuit Board) -Cables and connectors -Configuration items (such as jumpers or switches) The platters, spindle motor, heads, and head actuator mechanisms usually are contained in a sealed chamber called the Head Disk Assembly (HDA).

  20. Platters • A hard disk drive has one or more platters, or disks. • Most hard disk drives have two or more platters, although some of the smaller drives used in portable systems have only one. The number of platters a drive can have is limited by the drive's vertical physical size. • Platters have traditionally been made from an aluminum/magnesium alloy, which provides both strength and light weight. However, manufacturers' desire for higher and higher densities and smaller drives has led to the use of platters made of glass.

  21. Recording Media • No matter which substrate is used, the platters are covered with a thin layer of a magnetically retentive substance, called the medium, on which magnetic information is stored. Two popular types of magnetic media are used on hard disk platters: -Oxide medium -Thin-film medium

  22. Read Write Heads • A hard disk drive usually has one read/write head for each platter surface (meaning that each platter has two sets of read/write heads - one for the top side and one for the bottom side). These heads are connected, or ganged, on a single movement mechanism.

  23. Head Actuator Mechanism • Possibly more important than the heads themselves is the mechanical system that moves them: the head actuator. This mechanism moves the heads across the disk and positions them accurately above the desired cylinder. Many variations on head actuator mechanisms are in use, but all fall into one of two basic categories: -Stepper motor actuators -Voice coil actuators • The head actuator is the single most important specification in the drive, and the type of head actuator mechanism in a drive tells you a great deal about the drive's performance and reliability characteristics.

  24. Air Filters • Nearly all hard disk drives have two air filters. One filter is called the recirculating filter, and the other is called either a barometric or breather filter. These filters are permanently sealed inside the drive and are designed never to be changed for the life of the drive.

  25. Spindle Motors • The motor that spins the platters is called the spindle motor because it is connected to the spindle around which the platters revolve. Spindle motors in hard disk drives are always connected directly; no belts or gears are involved. • The spindle motor also must be precisely controlled for speed. The platters in hard disk drives revolve at speeds ranging from 3,600rpm to 15,000rpm or more, and the motor has a control circuit with a feedback loop to monitor and control this speed precisely.

  26. Logic Boards • All hard disk drives have one or more logic boards mounted on them. The logic boards contain the electronics that control the drive's spindle and head actuator systems and present data to the controller in some agreed-upon form. On ATA drives, the boards include the controller itself, whereas SCSI drives include the controller and the SCSI bus adapter circuit.

  27. Cables and Connectors • Hard disk drives typically have several connectors for interfacing to the computer, receiving power, and sometimes grounding to the system chassis. Most drives have at least these three types of connectors: -Interface connector(s) -Power connector -Optional ground connector (tab) • The interface connectors are the most important because they carry the data and command signals between the system and the drive. • The power connector is usually the same four-pin type that is used in floppy disk drives, and the same power-supply connector plugs into it.

  28. Configurations Items • To configure a hard disk drive for installation in a system, you usually must set several jumpers (and, possibly, terminating resistors) properly.

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