What a solid state we're in!

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The SSD (solid state drive) is the high performance golden child of the storage world, but is it all good news? Let's bust some myths about solid state drives and figure out how suitable they are for different applications.


First, let's consider what we need from our persistent storage - that is, the data that must not be lost after the computer is turned off. We obviously care that the data will remain readable for a 'reasonable' period of time, so that we can access our old files with high confidence that they are 100% intact. We care about access speed - how quickly we can read our data from the drive, and how quickly we can write new files or change existing ones. We have some idea of how much data we need to store - although that may change significantly over time. We may have expectations of the overall lifetime of the drive, so it's expected to run continuously for a minimum length of time with certain amounts of data reading and writing. We may care about data deletion, so that we understand what has become of the physical data when we've instructed a file to be permanently removed.

All of these requirements expose the differences between SSDs and the incumbent 'spinning platter' hard drives (HDDs). Let's look at them in turn.

Access speed

The huge advantage of SSDs over HDDs is access speed. We can read files almost as quickly as accessing data from the computer's RAM (its non-persistent Random Access Memory), and writing files is very fast too - massively faster than with HDDs.

Storage size

Whilst SSDs are fast, they have traditionally been very small. It's only since 2011 that the largest SSD capacities have started to approach the smallest HDD capacities, and even in late 2012 it's not possible to buy a single SSD with the same capacity as the largest single HDD. However, the smaller capacity of the SSD may be quite adequate for our needs; 60GB drives are now quite cheap, and even 240GB drives are quite affordable.

Data persistence and readability

In the right conditions, both SSDs and HDDs are very good at just storing data. Data access is the only significant factor affecting persistence in both types of drive; the more you read from and write to the drive, the shorter its long-term integrity will be. You may have specific data integrity requirements, but for the most part you should be mitigating data degradation with a pragmatic data backup policy.

Lifetime

HDDs have an excellent reputation for the number of times they can read and write the same piece of disc over and over. SSDs, on the other hand, do not take kindly to being written to. There are very low limits on this type of operation before the storage cells inside an SSD cannot be written to any more, and it is because of this that the SSD controller (the hardware on the SSD that acts on the computer's requests) has to be quite complex and capable of deciding how it will process a particular write operation. The technique of 'wear levelling' is designed to prevent hot-spots of wear on the drive, but the success of this comes down to the intelligence in the controller. The controller may also be smart enough to delay writes on frequently changing data, so it isn't writing data that may be changed again a few seconds later. The more you pay, the better your controller should be at keeping writes to a minimum and spreading wear evenly over all of the drive. Unfortunately, higher capacity SSDs use smaller storage technology and consequently have worse endurance at data writing - so the situation isn't getting better.

Data deletion

When a file is deleted from a drive, chances are that the physical data is left intact on the drive itself; only the record of accessing the file is destroyed. Additionally, when a file has grown it may be re-written to another part of the drive. Both of these scenarios leave fragments or complete versions of files all over the drive, creating headaches for companies and individuals who need to know that certain files are now inaccessible. Both SSDs and HDDs cause these problems, but the SSD will do a lot more of the re-writing technique as its wear-levelling controller tries to avoid excessive wear on specific parts of the drive.

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