Probably everyone is familiar with a popular myth about the origin of the Internet. A global network was designed in the ’60s for the military. The network required communication to be allowed in the potential situation of a nuclear war, which meant it had to be resistant to the partial destruction of its technical infrastructure. So even if some hubs were destroyed, the network had to remain operational. This idea perfectly illustrates the way the data is stored on the disk: information is intentionally distributed over the entire available space, so that in the case of local damage, the remaining good portion of the disk can quickly and efficiently replace the damaged parts. Today, I would like to explain how this process works.
“The smallest fragment into which we can “squeeze” data is a sector, an area containing limited information about our file. The essential feature of the sector is that it is always entirely recorded and read. Each sector has unique coordinates that allow it to be precisely located on the disk. Information about the physical coordinates (physical address – such as postcode, street and house number) is then translated to its logical counterpart, allowing the operating system to quickly find the sector”.
Usually, according to information on the operating system, the user assumes that one sector on the disk has 512 bytes of data. In reality, however, the sector has up to 600 bytes. Why? The additional data is used as localisation coordinates. Each sector must begin with so-called preamble, which is from 10 to 16 bytes long and is used to locate the beginning of the sector. Preamble ends with a synchronisation mark (sync mark) which indicates the end of the preamble and start of the actual relevant data. Additional bits are also responsible for the encoding process, which I described in the previous lesson. This means that, in reality, the physical size of your disk is much larger than you think.[bctt tweet=”#Didyouknow your hard drive stores more information than the manufacturer states?”]
During the recovery process, it is important to recover all the data, including that responsible for coding and operating the read/write system, not only the user’s data files.
It’s important to note that not all sectors on the disk are used to store the data. Why? The drive must have a certain number of spare sectors, which are used to store data from bad sectors. The disk contains information about all the bad sectors, regardless of whether they are the result of:of unavoidable defects (
- unavoidable defects – P-list: mapped out during the factory production tests, or
- subsequent bad sectors – G-list: mapped throughout the lifecycle of the drive.
What else can be found on the disk?
In order to maximise the amount of data that can be stored on the disk, the disk is divided into zones in which the density increases from the centre towards the edges of the disk (the tracks are getting longer as the distance from the centre of the platter increases). The disk surface is also divided by unique magnetic tracks called the Servo Wedge – see image above. The reading/writing head is using this to get localisation information about its current position, which allows it to navigate over the surface of the platter.
Throughout the process of coding and addressing, our files are written on the disk into specific sectors. However, the sector can only accommodate 512 bytes of data (not including the additional data, about which I wrote earlier) which means it will not fit even a very small text file. Sectors must be therefore grouped into higher structures responsible for keeping the whole file. While a sector is the smallest physical location on the drive, the smallest logical location is a cluster. A typical cluster size ranges from 1 to 128 sectors long (one cluster can be assigned to only one file, but one file can be stored on several clusters). That’s where you will find your videos, photos, text documents or programmes.
At this point, it is worth noting that the issues that we discussed so far relate to the physical structure of the data on the disk. Segments related to higher order structures (clusters) create logical data structures. We’ll be taking a look at this in the next blog post. Meanwhile, as always, I look forward to your comments and questions below.