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What Is A Solid State Drive?
A solid state drive is defined to be one that uses solid state memory. That probably didn't help. Basically, a SSD uses no moving parts, compared to the electro-mechanical disks used by traditional hard drives. As a result, they start up more quickly, not requiring any spin-up, use less power, are quieter and more rugged, and—most importantly—are faster.
SSDs can utilize either DRAM volatile or NAND non-volatile memory, that is, memory that either requires or does not require power to retain data. For example, a popular non-volatile SSD drive is simply a flash or jump drive, which can be connected and disconnected from power—i.e., a computer—without losing its data.
So what's the catch? Unlike hard drives, SSDs require that the old data be erased before new data is written over it, instead of just writing over the old information. This doubling of wear leads to some serious issues:
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The Great Slowdown
Strictly speaking, it's not that SSDs slow down over time: it's that they can only write so many times to the drive. Because they have a two-part write/erase cycle, unlike the single write cycle of mechanical hard drives, they undergo twice as much use. How much
There's a fair bit of variation within SSDs as to how many writes they can last for, anywhere from a mere thousand for multi-level cell (MLC) upwards to five million for high-endurance cells. As the SSD approaches its limit, more fragmentation and write errors occur, and the device slows down. All SSDs, no matter the design, will suffer this problem a one point or another - it's just a question of when.
This isn't helped by the architecture of most SSDs. Usually, data is laid down within a block of available memory, meaning that it might not take up all the available space—yet will still write to all of it. This means that many portions of the drive will undergo more write/erase cycles than are really required, an issue known as “write amplification". (This is an issue also paralleled in traditional hard drive architecture, of course.)
Of course, how fast an SSD slows down depends on the amount of use that it gets. A user that uses their SSD to transfer lots of data on a daily basis will wear their drive out faster than someone who just uses it for backup. This may not even be an issue that your average user may notice during their use of the SSD, but it is a consideration nonetheless.
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However, just because your SSD isn't performing to its advertised speed doesn't mean that it's suffering from the problem already. Manufacturers have the bad habit of not quite putting the whole truth on the packaging, usually advertising the “burst rate" or “burst speed" of the drive, as opposed to its average read or write speed. Be careful when reading those I/O stats!
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Slowing The Slowdown
While no real progress has been made on fixing the actual problem, there's plenty being done to mitigate the issue. Developers have come up with a variety of techniques to lengthen the life of SSDs:
The most common is known as “wear leveling," where all writes are spread out over the entire capacity of the disk, not just sectioned off. This ensures that it wears out evenly, across the drive somewhat reducing the problem.
Some SSD devices will have special software or firmware which can optimize the write/erase cycles for maximum efficiency by combining the writes, known simply as “write combining."
Another technique is to actually include a small DRAM cache within the SSD device—which may in itself just be another SSD. What this will do is buffer the drive, thus minimizing the number of write/ erase cycles required.
Defragmenting or "defragging" a SSD takes up many write/erase cycles... which shortens the lifetime of an SSD, even if it's also cleaning up the drive. Many, usually older, OSs will do this automatically, even when it's not necessary. However, defragging here may be useful not for the traditional purpose of reducing latency, as it is used for traditional hard drives, but for purposes of alleviating the write amplification effect described above. While it's a delicate balance, how often you should defrag your SSD for optimum performance and lifetime, it's one that more people are becoming aware of when attending to the health of their SSD. Only defrag when necessary!
Larger SSD devices sometimes even contain more memory than they advertise, so that once parts of the drive start fragmenting, those previously unknown and unused portions of the drive will start being used.
However, for all the techniques that are developed and used to mitigate the problem of the slowdown, that's still all that they do - lessen the extent of the problem. It's still a problem that needs to be fixed, and there's no complete solution immediately in sight on the horizon. Despite the slowdown, however, SSDs still have a bright future, and many powerful uses in everyday life.
(Edit: I received some comments, viewable below, that had included claims that defragging is unnecessary in SSDs. While researching for this article the commentary I found elsewhere often seemed mixed, for instance, this story from System Management News. There do appear to be at least plausible reasons to defrag an SSD, which is why I brought it up as a possibility within this article with the caveat that you shouldn't do it too often. However, for emphasis, it's an ongoing debate, and I'm just one author! By all means research other sources as well. This article is not intended as a technical justification for defragging SSDs, it's a broad overview intended for laymen. I did some tweaks throughout the article based on the commentary to attempt to be more clear.)