The Hard Disk DriveStarting back near the beginning, when the "Hard Disk" was invented, the hard disk drive consisted of a hard metal platter coated with magnetic particles. (Fun fact: a Hard Disk is a kind of persistent storage - when the power is turned off, the data stored on it persists until the next power-on.) This spun around on its axis, and an arm moved back and forth with strong earth electro-magnets. Electrical pulses then altered the magnetic substrate on the platter's surface to store data - by using the magnetic polarity it could set one of two states ("north" or "south" polarity). Without a current through the magnet, the magnet passing over the surface "read" the surface by magnetic induction - presence of a North vs. South-facing magnetic field in that portion translated to a "1" or "0" in computer memory, and the data was transferred into powered (also called volatile) memory, or RAM. The computer then conducted its computations on RAM.
Computers still work exactly the same today as all those decades ago, but let's think about the limitations of a hard disk as I describe above. First, how small can you make a magnetic particle, so that you can cram more data in a smaller space? There is a limit. Second, how long does that stay charged to the polarity, and how susceptible is it to changing, thus producing a difference from what was stored? Over time, that likelihood increases, and computers use various techniques to introduce redundancy. That takes some overhead of using some of the storage for that redundancy. If the slightest bit of dust gets onto the platter, it ruins the ability to read/write on it, so it is sealed in a vacuum chamber.
Solid State DrivesNow, Solid State drives store differently. They are similar to RAM, in that they use transistors to store 0/1 values, but they are non-volatile (i.e. they don't need a constant supply of electricity to hold their values). In this respect, they are the same as your Compact Flash, SD, USB sticks, or other flash memory, but with SSD the response speed is much faster than even hard drives. With no moving parts, they last a lot longer, are more reliable, and of course are much more expensive to manufacture. (Think about this: the arm holding the magnet used to read/write the data doesn't exist, so it doesn't have to move into place mechanically on a SSD drive before it can read/write the data. Data is accessed directly via electrical routing through transistors and gates.) SSD's are interchangeable with hard drives - that is, they have the same plugs and can hook into the same cables, in the same sizes and screw mounts as conventional hard drives.
Another benefit of SSD technology, is because it is electronic instead of electromechanical, it uses a lot less electricity and generates a lot less heat. But the cost is a lot more, around $0.50 or more per Gigabyte, as compared to $0.05 per Gigabyte. For some, the speed (massively faster) and energy savings is worth it.
Hybrid DrivesEnter 2 new categories. The first, is a hybrid drive. Many manufacturers make, in a single drive enclosure, a compound of both technologies. They have special circuitry that determines files you use most often, and puts those on the SSD portion of the drive, while files you access less often are stored on the platters. In theory, this will give you the best of both worlds. Bigger capacity at a more reasonable price, with better performance than a Hard Disk Drive.
For relatively small files, this is great. Think about what happens when a hybrid drive "changes its mind" about where it wants to store a file - it has to move it. A very large file, such as a virtual disk file for a Virtual Machine, may not get accessed for a while - say for a week when you don't use the VM. But then when you do, the access rate alters, and the drive "decides" to move it. Then, the whole system slows down tremendously while it waits for the drive to respond - and it can't until it's done moving the large multi-gigabyte file. In these scenarios, Hybrid Drives are not very good.
The nice thing here, is you are getting a single piece of hardware - a physical drive, but inside it is 2 disks, one SSD, and one a set of platters with read/write head on a physical arm.
Fusion DriveJust like a Hybrid Drive, Apple has, since OS X 10.8 Mountain Lion, supported a software-hybrid drive they call a Fusion Drive. At the OS level, you create what looks to you like a single drive, but behind the scenes it is composed of 2 physical drives (like a storage array). I don't know how well this performs in similar circumstances where large files change usage frequencies, and thus have to be moved from one disk to another. However, I would venture to guess it is not any better, because now the file has to move across a bus external to the physical drives.
RecommendationsIf you have a computer with a Hard Drive, and don't care too much about its performance (it is just fine with you), then stick where you are at. But, if you want it to go faster, I would suggest this order of priorities:
- If money is no object, get an SSD with the same size or bigger than the one you have. Go for broke, you will NOT be disappointed.
- If you can squeeze by with a smaller capacity to save some money, stay with a true SSD drive. For performance, you will NOT be disappointed.
- If you are buying a new computer, and have a choice between SSD and Hard Disk, go for SSD. The savings in time and energy are tremendous - you will get significantly better boot time, things will run massively faster, and your battery will last a lot longer.
- If you know you can't afford a full SSD with the capacity you need, and all you deal with are small files, then a Hybrid (or Mac Fusion) drive solution may work for you. If you deal with large files that sometimes don't get accessed, but when they do, get accessed a lot, stay away from Hybrid. A conventional Hard Disk will be faster.
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