saphalline
08-27-2005, 04:46 AM
In the continuation of the recent vid card thread, and as requested, I've started a new thread devoted to hard drives and how they work. First of all, there's a lot of good info on the PC Guide itself, so don't forget to look there, too! In any case, I'll be going over a few quick facts about hard drives.
Basic Construction: Hard drives are composed of a motor, some platters, an actuator, and the heads. That's the short list, but those are the four basic parts to any hard drive.
The motor (aka the spindle) sits in the middle and is responsible for turning the platters. When referring to the rotational speed of a hard drive, such as 7200rpm, that's the speed of the motor and that's how fast the platters spin. The faster the better, but not too fast. There needs to be a good balance of speed (which translates into performance), reliability, heat, and in the case of laptops and other portables, energy consumption. Faster motors in general produce more heat and use more energy. Reliability has gone up considerably in recent years, so this is less of a concern for modern hard drives. Likewise, heat has been kept to a reasonable level, but it's still not a good idea to block off a hard drive from all airflow. In fact, mine always have a fan in front of them.
The platters are the CD-shaped disks that hold the actual data. They are made out of metal or glass, and have a thin magnetic dust coating over them. This coating is what can be "written to" and "read from" by magnetizing tiny areas to represent bits of data. Exactly how tightly packed these tiny areas are is called the areal density of a hard drive. Areal density is always increasing, allowing us ever larger hard drives on which to store our programs. Right now, areal density is a couple hundred Gigabits per square inch. The other way to increase storage capacity in a hard drive is have more than one platter. Desktop hard drives have 2-3 platters on average, with a few high-end & server hard drives having 4 platters. Portable hard drives usually have 1-2 platters because they're a lot thinner.
The actuator is the "arm" assembly in a hard drive. It's shaped like an elongated triangle with an axis near its base. The actuator arm is able to rotate on this axis so that it can reach different parts of the platters as they go spinning by. The back end of the arm has a counterweight of sorts that has an electromagnet built into it. Above and below this electromagnet are strong earth magnets. By adjusting the current in the electromagnet, the actuator arm can move to precise locations over the platters. The front of the actuator arm is the pointy end of the triangle that holds the read/write heads.
The heads themselves are nothing but very very small electromagnets. When data needs to be written to the hard drive platters, the heads are charged with the proper current needed to alter the magnetic field on the platter. When data needs to be read from the hard drive platters, the heads are kept passive and thus the magnetic field on the platter causes current to flow through the head and be interpreted by the hard drive as data. The most interesting thing about these heads is that they float above the platters. Since the platters are spinning at such high speeds, this creates enough "wind" for the heads to float effortlessly above the platters (at a precise height) which keeps them from scraping the coating off the platters. If you've ever had a hard drive die from the dreaded "click of death" you know what happens when those heads scrape the platters! :eek: It's not a good thing. When the hard drive turns off or spins down for any reason, the heads are "parked" so as not to scrape the platters. Some hard drives have a designated parking area for the heads that prevents them from moving at all (mostly portable hard drives that are bumped around a lot) while some just have a flat section in the center ring where the heads can rest without being on the platters.
Those are the basics of how a hard drive works. And that's how they have worked for years (even decades). Nothing much changes, but then again it doesn't need to. Magnetic-based platter storage has remained the cheapest and largest capacity semi-permanent storage technology since it was first invented. And until some form of solid-state storage becomes as cheap and large, this method of making hard drives will continue.
But because they have moving parts, hard drives are slow to evolve. Storage capacity has increased with the leaps and bounds that we've all come to expect from computer-paced advancements, but speed has not. Out of all the types of memory in our computers today, hard drives are only faster than optical drives. And even then, they're both standing still compared to RAM. Hard drives are about one million times slower than RAM, making it an absolute joke when you load a file or program. And because of the moving parts and head/platter construction, there is no such thing as random accesses like with RAM. If you want some data that's stored on the other side of the platter, you'll have to wait for the platter to spin around so you can read it! Most of the time that is devoted to reading/writing data is spent just waiting for the platters to spin around to the correct location. And most of the remaining time is devoted to moving the actuator so that the heads are positioned over the correct area of the platter. Now that our processors are operating well above 1 GHz, the old 7200rpm speed of hard drives is showing its age. 7 thousand can't keep up with 1 billion. This is why the hard drive is still the biggest overall bottleneck in any system.
Well, that's it for tonight...
Basic Construction: Hard drives are composed of a motor, some platters, an actuator, and the heads. That's the short list, but those are the four basic parts to any hard drive.
The motor (aka the spindle) sits in the middle and is responsible for turning the platters. When referring to the rotational speed of a hard drive, such as 7200rpm, that's the speed of the motor and that's how fast the platters spin. The faster the better, but not too fast. There needs to be a good balance of speed (which translates into performance), reliability, heat, and in the case of laptops and other portables, energy consumption. Faster motors in general produce more heat and use more energy. Reliability has gone up considerably in recent years, so this is less of a concern for modern hard drives. Likewise, heat has been kept to a reasonable level, but it's still not a good idea to block off a hard drive from all airflow. In fact, mine always have a fan in front of them.
The platters are the CD-shaped disks that hold the actual data. They are made out of metal or glass, and have a thin magnetic dust coating over them. This coating is what can be "written to" and "read from" by magnetizing tiny areas to represent bits of data. Exactly how tightly packed these tiny areas are is called the areal density of a hard drive. Areal density is always increasing, allowing us ever larger hard drives on which to store our programs. Right now, areal density is a couple hundred Gigabits per square inch. The other way to increase storage capacity in a hard drive is have more than one platter. Desktop hard drives have 2-3 platters on average, with a few high-end & server hard drives having 4 platters. Portable hard drives usually have 1-2 platters because they're a lot thinner.
The actuator is the "arm" assembly in a hard drive. It's shaped like an elongated triangle with an axis near its base. The actuator arm is able to rotate on this axis so that it can reach different parts of the platters as they go spinning by. The back end of the arm has a counterweight of sorts that has an electromagnet built into it. Above and below this electromagnet are strong earth magnets. By adjusting the current in the electromagnet, the actuator arm can move to precise locations over the platters. The front of the actuator arm is the pointy end of the triangle that holds the read/write heads.
The heads themselves are nothing but very very small electromagnets. When data needs to be written to the hard drive platters, the heads are charged with the proper current needed to alter the magnetic field on the platter. When data needs to be read from the hard drive platters, the heads are kept passive and thus the magnetic field on the platter causes current to flow through the head and be interpreted by the hard drive as data. The most interesting thing about these heads is that they float above the platters. Since the platters are spinning at such high speeds, this creates enough "wind" for the heads to float effortlessly above the platters (at a precise height) which keeps them from scraping the coating off the platters. If you've ever had a hard drive die from the dreaded "click of death" you know what happens when those heads scrape the platters! :eek: It's not a good thing. When the hard drive turns off or spins down for any reason, the heads are "parked" so as not to scrape the platters. Some hard drives have a designated parking area for the heads that prevents them from moving at all (mostly portable hard drives that are bumped around a lot) while some just have a flat section in the center ring where the heads can rest without being on the platters.
Those are the basics of how a hard drive works. And that's how they have worked for years (even decades). Nothing much changes, but then again it doesn't need to. Magnetic-based platter storage has remained the cheapest and largest capacity semi-permanent storage technology since it was first invented. And until some form of solid-state storage becomes as cheap and large, this method of making hard drives will continue.
But because they have moving parts, hard drives are slow to evolve. Storage capacity has increased with the leaps and bounds that we've all come to expect from computer-paced advancements, but speed has not. Out of all the types of memory in our computers today, hard drives are only faster than optical drives. And even then, they're both standing still compared to RAM. Hard drives are about one million times slower than RAM, making it an absolute joke when you load a file or program. And because of the moving parts and head/platter construction, there is no such thing as random accesses like with RAM. If you want some data that's stored on the other side of the platter, you'll have to wait for the platter to spin around so you can read it! Most of the time that is devoted to reading/writing data is spent just waiting for the platters to spin around to the correct location. And most of the remaining time is devoted to moving the actuator so that the heads are positioned over the correct area of the platter. Now that our processors are operating well above 1 GHz, the old 7200rpm speed of hard drives is showing its age. 7 thousand can't keep up with 1 billion. This is why the hard drive is still the biggest overall bottleneck in any system.
Well, that's it for tonight...