How Hard Disk Drives Work

Written: 2009/08/22

Hard Disk Drive Structure

The two most important parts in a hard disk drive (HDD) are the platters and the read/write head. The platters are where the data is stored, and the read/write head moves across the platters to either read or write data.


The platters are typically made up of 3 layers. The center layer which everything else is stuck onto is usually made of aluminum or a glass and ceramic compound. The layers on both sides of the platter are a complex metallic compound with magnetic particles that can be affected by the read/write head in order to store the data. On top of these layers is a very thin carbon-based protective layer.


Most HDDs have several platters, with a read/write head for each side of each platter. This allows for a much larger data storage capacity. For example 4 platters capable of holding 250 GB each would make a 1 TB drive.


The read write heads move to and from the center of the platter, while the platters spin, the platters generally spin at speeds of 7500 RPM, 10000 RPM or 15000 RPM. This allows the heads to access data anywhere on the platters.

The HDD stores data in binary form, a bunch of 1's and 0's. This is achieved by the layers of magnetizable material on the platters. The read/write head determines the 1's a 0's by the direction of the current which is induced in the head by the magnetized particles on the platter.

The platter is broken down into parts, called tracks, sectors and cylinders.
A track is a ring on the surface of the platter, many of these tracks expand outwards from the center of the platter and grow larger and larger. The cylinders are the combination of all tracks, on both sides of each platter, that are the same distance away from the center.


The sectors are the division of these tracks into 512 byte (4096 bit) pieces. There are two types of sectoring. The original sectoring which basically cuts the platter into slices like a pie, and zoned sectoring which makes more efficient use of the space on the platter. In zoned sectoring, the tracks gain more and more sectors as their radius increases. In normal sectors, each track has the same number of sectors regardless of it's radius, thus wasting space since regardless of its size, a sector is 512 bytes.


The sectors, tracks and cylinders are known as the "geography" of the drive. This is decided by the manufacturer of the HDD and is set onto the drive by a low level formatting. These days low level formatting is almost exclusively done by the manufacturer. It is not recommended to try doing one unless the utility is provided by the manufacturer and is intended for your specific model of hard drive. Doing this improperly can lead to your owning an expensive paperweight.

High Level Format & Logical Disc Structure

A high level format is what most people still do today with a new hard drive. A high level sets up the file system, such as NTFS or FAT32, and the boot sector. This will not physically destroy data on the disk, but will mark the space the data is taking up as being available. This means sensitive data can still be recovered if people are not careful when throwing away old hard drives.

Partitioning and RAID are the logical structure of disks. This means the way in which you see the available drives through your operating system might not really be how they are set up physically.

You can have partitions, which is the most common type of logical structure. One physical disk can appear to be several on your system with partitioning. This is useful for avoiding fragmentation on a whole drive if you know you will be doing a lot of deleting/rewriting of files, then make a partition of the appropriate size and do that work in there.

Using RAID also changes the logical structure of disks, for example RAID0/Striped reads/writes to 2 or more disks at once, improving performance and storage, but is only seen as one disk by the user.

Fragmentation

Fragmentation, as the name implies, is when your files get broken up into pieces physically scattered across the hard drive. This is unavoidable in the long run, however on a freshly formatted drive it is easy to delay it by knowing exactly what you want to place on the drive and doing so with as few file deletions/overwrites as possible. Fragmentation occurs because of how your hard disk works. When you write files to a clean drive, they fill up the space neatly one after the other. When you delete files, the space they took up is marked as being available. So now the next file you write to the drive will start up by filling up these spaces, then moving onto the end of the drive where all the free space is (if it needs to). Now consider how many hundreds, if not thousands of files you write onto a drive, all of varying sizes. All of which get deleted, overwritten and so on over time.


The problem with this is that in order to read the file, the read/write head of the drive will now read the start of the file, then physically move somewhere else on the platter (or maybe even on another platter) to continue reading it, on heavily fragmented drives a large file might be in several dozen different physical locations. This can lead to long seek times for files and overall poor performance. Defragmentation tools are available to solve this problem. What they will do is move the data around on the drive so that the files are as unfragmented as possible.

To learn how to defrag a hard drive click here.

Types of HDDs

There are many types of hard drives available, with ever increasing storage capacity and decreasing costs. There are hard drives that can be used externally, most commonly via USB or Firewire, and internal drives that can be connected by many different interfaces. The most common interfaces for internal drives are IDE, SATA, SCSI and Fiber Channel, the last two more commonly found on servers.

There are also Solid State Disks starting to make their appearance in the market. The technology behind them dates back to the mid to late 90's. These disks have no moving parts and are exponentially faster at read/write processes than conventional hard disks. They will not be replacing hard disks any time soon due to their price, however in the years to come they should be gaining much more popularity.