Hard Disks

HARD DISKS

The PC's upgradeability has led software companies to believe that it doesn't matter how large their applications are. As a result, the average size of the hard disk rose from 100MB to 1.2GB in just a few years and by the start of the new millennium a typical desktop hard drive stored 18GB across three 3.5in platters. Thankfully, as capacity has gone up prices have come down, improved areal density levels being the dominant reason for the reduction in price per megabyte.

It's not just the size of hard disks that has increased. The performance of fixed disk media has also evolved considerably. When the Intel Triton chipset arrived, EIDEPIO mode 4 was born and hard disk performance soared to new heights, allowing users to experience high-performance and high-capacity data storage without having to pay a premium for a SCSI-based system.

Construction
Hard disks are rigid platters, composed of a substrate and a magnetic medium. The substrate - the platter's base material - must be non-magnetic and capable of being machined to a smooth finish. It is made either of aluminium alloy or a mixture of glass and ceramic. To allow data storage, both sides of each platter are coated with a magnetic medium - formerly magnetic oxide, but now, almost exclusively, a layer of metal called a thin-film medium. This stores data in magnetic patterns, with each platter capable of storing a billion or so bits per square inch (bpsi) of platter surface.

Platters vary in size and hard disk drives come in two form factors, 5.25in or 3.5in. The trend is towards glass technology since this has the better heat resistance properties and allows platters to be made thinner than aluminium ones. The inside of a hard disk drive must be kept as dust-free as the factory where it was built. To eliminate internal contamination, air pressure is equalised via special filters and the platters are hermetically sealed in a case with the interior kept in a partial vacuum. This sealed chamber is often referred to as the head disk assembly (HDA).

Typically two or three or more platters are stacked on top of each other with a common spindle that turns the whole assembly at several thousand revolutions per minute. There's a gap between the platters, making room for magnetic read/write head, mounted on the end of an actuator arm. This is so close to the platters that it's only the rush of air pulled round by the rotation of the platters that keeps the head away from the surface of the disk - it flies a fraction of a millimetre above the disk. On early hard disk drives this distance was around 0.2mm. In modern-day drives this has been reduced to 0.07mm or less. A small particle of dirt could cause a head to "crash", touching the disk and scraping off the magnetic coating. On IDE and SCSI drives the disk controller is part of the drive itself.

There's a read/write head for each side of each platter, mounted on arms which can move them towards the central spindle or towards the edge. The arms are moved by the head actuator, which contains a voice-coil - an electromagnetic coil that can move a magnet very rapidly. Loudspeaker cones are vibrated using a similar mechanism.

The heads are designed to touch the platters when the disk stops spinning - that is, when the drive is powered off. During the spin-down period, the airflow diminishes until it stops completely, when the head lands gently on the platter surface - to a dedicated spot called the landing zone (LZ). The LZ is dedicated to providing a parking spot for the read/write heads, and never contains data.

When a disk undergoes a low-level format, it is divided it into tracks and sectors. The tracks are concentric circles around the central spindle on either side of each platter. Tracks physically above each other on the platters are grouped together into cylinders which are then further subdivided into sectors of 512 bytes apiece. The concept of cylinders is important, since cross-platter information in the same cylinder can be accessed without having to move the heads. The sector is a disk's smallest accessible unit. Drives use a technique called zoned-bit recording in which tracks on the outside of the disk contain more sectors than those on the inside.

Operation

Data is recorded onto the magnetic surface of the disk in exactly the same way as it is on floppies or digital tapes. Essentially, the surface is treated as an array of dot positions, with each "domain' of magnetic polarisation being set to a binary "1" or "0". The position of each array element is not identifiable in an "absolute" sense, and so a scheme of guidance marks helps the read/write head find positions on the disk. The need for these guidance markings explains why disks must be formatted before they can be used.

When it comes to accessing data already stored, the disk spins round very fast so that any part of its circumference can be quickly identified. The drive translates a read request from the computer into reality. There was a time when the cylinder/head/sector location that the computer worked out really was the data's location, but today's drives are more complicated than the BIOS can handle, and they translate BIOS requests by using their own mapping.

In the past it was also the case that a disk's controller did not have sufficient processing capacity to be able to read physically adjacent sectors quickly enough, thus requiring that the platter complete another full revolution before the next logical sector could be read. To combat this problem, older drives would stagger the way in which sectors were physically arranged, so as to reduce this waiting time. With an interleave factor of 3, for instance, two sectors would be skipped after each sector read. An interleave factor was expressed as a ratio, "N:1", where "N" represented the distance between one logical sector and the next. The speed of a modern hard disk drive with an integrated controller and its own data buffer renders the technique obsolete.

The rate at which hard disk capacities have increased over the years has given rise to a situation in which allocating and tracking individual data sectors on even a typical drive would require a huge amount of overhead, causing file handling efficiency to plummet. Therefore, to improve performance, data sectors have for some time been allocated in groups called clusters. The number of sectors in a cluster depends on the cluster size, which in turn depends on the partition size.

When the computer wants to read data, the operating system works out where the data is on the disk. To do this it first reads the FAT (File Allocation Table) at the beginning of the partition. This tells the operating system in which sector on which track to find the data. With this information, the head can then read the requested data. The disk controller controls the drive's servo-motors and translates the fluctuating voltages from the head into digital data for the CPU.

More often than not, the next set of data to be read is sequentially located on the disk. For this reason, hard drives contain between 256KB and 8MB of cache buffer in which to store all the information in a sector or cylinder in case it's needed. This is very effective in speeding up both throughput and access times. A hard drive also requires servo information, which provides a continuous update on the location of the heads. This can be stored on a separate platter, or it can be intermingled with the actual data on all the platters. A separate servo platter is more expensive, but it speeds up access times, since the data heads won't need to waste any time sending servo information.

However, the servo and data platters can get out of alignment due to changes in temperature. To prevent this, the drive constantly rechecks itself in a process called thermal recalibration. During multimedia playback this can cause sudden pauses in data transfer, resulting in stuttered audio and dropped video frames. Where the servo information is stored on the data platters, thermal recalibration isn't required. For this reason the majority of drives embed the servo information with the data.

Source :- www.pctechguide.com