Storage anatomy

Every desktop PC and notebook has a non-volatile storage device to store data. Here, WINDOWS explains exactly what makes hard drives and newer solid state drives tick.

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By  Jason Saundalkar Published  January 11, 2009

Every desktop PC and notebook has a non-volatile storage device to store data. Here, WINDOWS explains exactly what makes hard drives and newer solid state drives tick.

Storage devices, whether they are the tried and tested hard disk drive (HDD) or newer solid state drives (SSDs), have long been regarded as rather uninteresting, compared to the other components sitting within a machine.

However, without a storage drive your PC or notebook would be of absolutely no use because the rest of the components would have no data to work with.

Non-volatile storage devices (these do not lose data even when the PC is switched off) not only store the operating system (OS) that you need to use the PC but they also act as a repository for documents, digital images, music files, games and more.

Today, as consumers, we have more choice in terms of what storage device we want to use in our machine than we did two years ago. Rather than just deciding on the capacity, spindle speed and interface the drive uses, we now have a choice of a different drive type (or technology) altogether.

SSD technology is based on high-speed flash memory and although it is new to the consumer market segment, it has actually been present in the workstation and server markets for a lot longer.

The reason this technology remained in these markets for so long and has only now crossed over to the consumer segment is because of its extremely high price. Now though, thanks to advancements in technology, the drives are cheaper to manufacture and now fall into a price bracket that mid- or high-performance desktop or notebook users will agree with.

Before we look at SSDs however, lets take a look at the old workhorse of the computer world first, the standard hard disk drive.

Driving hard since the 50s

Hard disk drives were invented in the 1950s and in their first form they were fairly large and measured 20-inches in diameter (today's desktop-aimed drives are 3.5-inch). One of the earliest disk drives developed by IBM offered 30Mbytes of fixed storage and 30Mbytes of removable storage and thus this drive was named ‘Winchester' after the 30-30 variant of that rifle.

Though this Winchester drive offers an insignificant amount of space compared to today's HDDs, the Winchester name is still used to describe hard drives as the underlying technology is virtually identical.

Of course, although the basic underlying technology has remained the same, the individual components that comprise a hard disk drive have been improved tremendously. They are not only faster, more robust and denser but are also cheaper to manufacture and thus the price of hard disk drives has been steadily dropping over the last few years. Today, you can get a 7200rpm, 1.5Tbyte (1500Gbytes) HDD for well under US $500. The same money 10 years ago would have, most likely, got you a 2.1Gbyte 5400rpm disk.

A hard drive is made up of two distinct pieces both of which will not function without the other being present. The first piece is visible to the naked eye and sits on the underside of a hard drive.

This is a printed circuit board (PCB), which contains the circuitry for the interface connecting the drive to the computer as well as the cache memory, which is used to improve a drive's reading and writing performance. This circuit board also has an ID chip that allows the hard drive to identify itself to the computer's BIOS.

Perhaps most importantly, the PCB is responsible for assembling the magnetic domains on the drive's platter into bytes (known as a read operation) and, on the other hand, can turn data bytes into magnetic domains (known as a write operation). Beyond this, the components that actually perform the physical read/write operations sit on the inside of the drive, out of plain view.

What lies beneath?

The inside of a HDD comprises the components that are actually used to perform reading and writing operations. (Note that a hard drive will fail if it is ever opened regardless of whether it is on or off at the time.)

The components visible are the platters (the shiny discs) and the arm which sits over them. Although the arm looks like it is making physical contact with the platters, it is actually sitting a few millimeters off the surface of the disc. Should these two components ever make contact, it would lead to disk failure. The arm holds the read/write heads that are responsible for transferring data to and from the drive's platters.

It is able to move the heads from the hub of the drive (at the centre) to the outer edge, meaning it is able to cover the entire surface of the platter.(When reading or writing data to the drive's platters, the arm can move as many as 100 times per second in a modern 15,000rpm HDD.)

The platters themselves spin at a certain speed between 5400rpm to 15,000rpm (depending on the drive) and as they are moving, the read/write heads must pick out the spaces on the platters to either read or write data to.

Today, data is generally stored on both sides of the storage platter and thus a separate read/write head is required for each platter surface. So, if you take the case of a hard drive with two platters (four storage surfaces) for example, it would need four read/write heads in all.

A drive's capacity is determined by how dense its storage platters are. This density is generally referred to as ‘areal density'. This is one aspect of the hard drive that has evolved tremendously since IBM introduced the RAMAC hard disk in 1956. This particular drive featured platters that offered a density of two thousand bits per square inch. Today however, drives offer densities greater than 100 gigabits per square inch.

Dense storage platters enable manufacturers to build large hard drives as more data can be packed into the same amount of physical space. At the same time, denser platters also help a drive perform better in terms of reading and writing because the read/write heads don't have to travel as far to perform their operations.

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