Hard disk drives (HDDs) are still one of the most common forms of data storage, despite the growing popularity of SSDs (solid-state drives). One of the key metrics that determine the performance and capacity of an HDD is its areal density. This refers to the amount of data that can be stored in a given amount of physical space on the disk.
As the demand for greater storage capacity and faster read and write speeds grows, areal density has become a major area of research and innovation in the development of HDDs.
In this article, we will explore what areal density is, how it is calculated, and recent advancements and technologies involved in increasing the storage density of an HDD.
What is Areal Density?
The areal density of an HDD is the amount of data that can be stored per unit area on a hard drive platter. In simple terms, it describes how tightly packed the data is within the disk, which directly affects the total capacity of the HDD. Areal density is a combination of two fundamental factors: TPI (Tracks Per Inch) and BPI (Bits Per Inch).
1. TPI (Tracks Per Inch) refers to the number of tracks (or concentric circles) that can be positioned in one inch of disk diameter. Each track can store a specific amount of data.
2. BPI (Bits Per Inch) ) is the number of bits that can be stored along a single track. In other words, it is the density of data within each track on the disk.
Areal density is therefore the product of TPI and BPI:
Areal Density = TPI × BPI
In a HDD, tracks are recorded on circular disks called platters, and each platter has a magnetic surface where data is recorded. The amount of data stored on each platter depends on both the number of tracks that can be stacked and the number of bits that can be stored along each track.
TPI and BPI - Image Credit: Disclosure
How is Areal Density Calculated?
Calculating areal density is quite simple, but it involves understanding the relationship between the number of tracks and the number of bits per track. Let's explain in more detail how it works:
1. Tracks per Inch (TPI): This is a measure of how close the tracks are to each other. On modern hard drives, the number of tracks per inch can be in the tens of thousands, and on high-density drives, this value can exceed 100,000 TPI. The more tracks that fit into an inch, the greater the storage capacity.
2. Bits per Inch (BPI): This measures the data density along each track. BPI can vary depending on the recording technology used and can reach hundreds of thousands or even millions of bits per inch on high-capacity disks.
To calculate areal density, you need to multiply the TPI by the BPI:
For example, if a hard drive has 100,000 tracks per inch and 1,000,000 bits per inch, the areal density would be:
Areal Density = 100,000 × 1,000,000 = 10^11 bits per square inch.
This value tells you how many bits can be stored in a 1-square-inch area on the hard drive.
Recent Advances in HDD Areal Density
In recent years, there have been significant advances in the areal density of hard drives. These advances are driven by increasingly sophisticated technologies that enable more data to be packed into a smaller area. Historically, HDDs had a very low areal density, but as the demand for more storage increased, engineers sought ways to increase that density.
1. PMR
One of the first major innovations was Perpendicular Magnetic Recording (PMR). Before PMR, magnetic recording on HDDs was done lengthwise, which limited the areal density. With perpendicular recording, the magnetic particles are vertically oriented with respect to the disk, allowing the tracks to be closer together and increasing storage density.
2. HAMR
In recent years, one of the most promising technologies for increasing areal density has been HAMR (Heat Assisted Magnetic Recording). This technology uses a laser to momentarily heat the area of the disk where data is being written, allowing bits to be written more densely and stably, aiming to optimize storage capacity.
3. MAMR
Another innovative technology is MAMR (Microwave-Assisted Magnetic Recording), which uses microwaves to provide the energy needed to record data at higher densities.
4. TDMR
Two-Dimensional Magnetic Recording (TDMR) is a technology that increases the accuracy of reading data at high densities. By using multiple read heads to capture information from the same track, TDMR HDDs can reduce signal noise caused by interference between tracks. By rereading the track with an additional sensor placed very close by, the effects of interference can be significantly reduced.
5. Nanotechnology Layered Storage
With nanotechnology, HDD manufacturers are exploring the possibility of creating extremely thin and highly controllable magnetic layers. This would allow for the creation of even thinner and more tightly packed tracks, further increasing areal density.
Recent research points to Graphene as a promising material to replace carbon-based coatings (COCs) for ultra-high density storage on magnetic media.
TPI and BPI: Paths to Increasing Areal Density
As mentioned earlier, there are two main ways to increase the areal density of a hard drive platter: increasing the number of tracks per inch (TPI) and increasing the number of bits per inch (BPI).
Increasing the TPI
Increasing the TPI means that more tracks are stacked in a smaller area. This is a major technical challenge because the more tracks are placed on a disk, the greater the possibility of magnetic interference between them. However, advances in recording materials and technologies, such as perpendicular recording and magnetic flux control techniques, have enabled this increase in density.
Increasing BPI
Increasing BPI involves packing bits more tightly along a track. This requires not only more precise read/write heads but also the use of advanced technologies such as microwave-assisted or heat-assisted recording, which allow for more precise manipulation of bits.
Conclusion
Areal density in HDDs is one of the crucial factors that determines the capacity and performance of hard drives. As technology evolves, manufacturers are able to increase storage density, resulting in higher-capacity hard drives at lower costs per gigabyte. Technologies that are being explored, such as HAMR and MAMR, have the potential to transform the HDD market by providing more storage space on 3.5-inch drives, even for applications that require very high data capacity.
In addition, innovations in TPI and BPI will continue to play vital roles as engineers seek to balance the demands of higher capacity and higher read/write reliability. The future of HDDs looks promising, with areal densities increasing exponentially, which could meet ever-growing storage needs in a variety of industries, from server to personal computing.
