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. These tracks are divided into segments called sectors, which normally store 512 bytes each.
The tracks and sectors on a disk.
Hard disk drives usually have multiple disks, called platters, that are stacked on top of each other and spin in unison, each with two sides on which the drive stores data. Most drives have one, two, or three platters, resulting in two, four, or six sides. The identically aligned tracks on each side of every platter together make up a cylinder. A hard disk drive normally has one head per platter side, with all the heads mounted on a common carrier device or rack. The heads move radially across the disk in unison; they cannot move independently because they are mounted on the same carrier or rack, called an actuator.
Hard disk cylinders.
Originally, most hard disks spun at 3,600rpmapproximately 10 times faster than a floppy disk drive. For many years, 3,600rpm was pretty much a constant among hard drives. Now, however, most drives spin even faster. Although speeds can vary, modern drives normally spin the platters at either 4,200, 5,400, 7,200, 10,000, or 15,000rpm. Most standard-issue drives found in portable systems spin at the slower 4,200 or 5,400rpm speeds, with a few high performance models now available that spin at 7,200rpm. The 10,000 or 15,000rpm drives are normally found only in very high performance desktop-based workstations or servers, where their higher prices, heat generation, and noise can be more easily dealt with. High rotational speeds combined with a fast head-positioning mechanism and more sectors per track are what make one hard disk overall faster than another.
The heads in most hard disk drives do not (and should not!) touch the platters during normal operation. However, on most drives, the heads do rest on the platters when the drive is powered off. In most drives, when the drive is powered off, the heads move to the innermost cylinder, where they land on the platter surface. This is referred to as contact start stop (CSS) design. When the drive is powered on, the heads slide on the platter surface as they spin up, until a very thin cushion of air builds up between the heads and platter surface, which causes the heads to lift off and remain suspended a short distance above or below the platter. If the air cushion is disturbed by a particle of dust or a shock, the head can come into contact with the platter while it is spinning at full speed. When contact with the spinning platters is forceful enough to do damage, the event is called a head crash. The result of a head crash can be anything from a few lost bytes of data to a completely ruined drive. Most drives have special lubricants on the platters and hardened surfaces that can withstand the daily “takeoffs” and “landings” as well as more severe abuse.
Some newer drives do not use CSS design and instead use a load/unload mechanism, which does not allow the heads to contact the platters, even when the drive is powered off. First used in the 2.5-inch form factor notearticle or laptop drives, where resistance to mechanical shock is more important, load/unload mechanisms use a ramp positioned just off the outer part of the platter surface. When the drive is powered off or in a power-saving mode, the heads ride up on the ramp. When powered on, the platters are allowed to come up to full speed before the heads are released down the ramp, allowing the airflow (air bearing) to prevent any head/platter contact.
Because the platter assemblies are sealed and nonremovable, the track densities on the disk can be very high. Hard disks today have up to 96,000 or more tracks per inch (tpi) recorded on the media (for example, Hitachi Travelstar 80GN). Head disk assemblies (HDAs), which contain the platters, are assembled and sealed in clean rooms under absolutely sanitary conditions. Because few companies repair HDAs, repair or replacement of the parts inside a sealed HDA can be expensive. Every hard disk ever made eventually fails. The only questions are when the failure will occur and whether your data is backed up.
Tracks and Sectors
A track is a single ring of data on one side of a disk. A disk track is too large to manage data effectively as a single storage unit. Many disk tracks can store 100,000 or more bytes of data, which would be very inefficient for storing small files. For that reason, tracks are divided into several numbered divisions known as sectors. These sectors represent arc-shaped pieces of the track.
Various types of disk drives split their disk tracks into different numbers of sectors, depending on the density of the tracks. For example, floppy disk formats use 836 sectors per track, although hard disks usually store data at a higher density and today can have 900 or more sectors per track physically. The sectors created by the standard formatting procedure have a capacity of 512 bytes, which has been one constant throughout the history of the PC. In order to be compatible with most older BIOS and drivers, drives will usually perform an internal translation so that they pretend to have 63 sectors per track when addressed in CHS (cylinder, head, sector) mode.
The sectors on a track are numbered starting with 1, unlike the heads or cylinders that are numbered starting with 0. For example, a 1.44MB floppy disk contains 80 cylinders, numbered 079, and two heads, numbered 0 and 1, whereas each track on each cylinder has 18 sectors numbered 118.
When a disk is formatted, the formatting program creates ID areas before and after each sector’s data that the disk controller uses for sector numbering and for identifying the start and end of each sector. These areas precede and follow each sector’s data area and consume some of the disk’s total storage capacity. This accounts for the difference between a disk’s unformatted and formatted capacities. Note that most modern hard disks are sold preformatted and advertise only the formatted capacity. The unformatted capacity is usually not mentioned anymore. Another interesting development is that many new drives use what is called No-ID sector formatting, which means that the sectors are recorded without ID marks before and after each sector. This means that more of the disk can be used for actual data.
Each sector on a disk normally has a prefix portion, or header, that identifies the start of the sector and contains the sector number, as well as a suffix portion, or trailer, that contains a checksum (which helps ensure the integrity of the data contents). Many newer drives omit this header and have what is called a No-ID recording, allowing more space for actual data. With a No-ID recording, the start and end of each sector are located via predetermined clock timing.
Each sector contains 512 bytes of data. The low-level formatting process normally fills the data bytes with some specific value, such as F6h (hex) or some other repeating test pattern used by the drive manufacturer. Some patterns are more difficult for the electronics on the drive to encode/decode, so these patterns normally are used when the manufacturer is testing the drive during initial formatting. A special test pattern might cause errors to surface that a normal data pattern would not show. This way, the manufacturer can more accurately identify marginal sectors during testing.
The sector headers and trailers are independent of the operating system, the file system, and the files stored on the drive. In addition to the headers and trailers, gaps exist within the sectors, between the sectors on each track, and also between tracks, but none of these gaps contain usable data space. The gaps are created during the low-level format process when the recording is turned off momentarily. They serve the same function as having gaps of silence between the songs recorded on a cassette tape. The prefix, suffix, and gaps account for the lost space between the unformatted capacity of a disk and the formatted capacity. For example, a 2MB (unformatted) floppy has a formatted capacity of 1.44MB, and an older 20GB unformatted capacity hard disk (for instance, a Quantum Fireball LCT20) has a capacity of only 18.3GB when it is formatted. Because the ATA and SCSI hard disks you purchase today are low-level formatted at the factory, the manufacturers now advertise only the formatted capacity. Even so, nearly all drives use some reserved space for managing the data that will be stored on the drive.
PEOPLE FIND THIS PAGE BY THIS WORDS:
harddisk work;
