Not all SSDs are created equal. If you’re choosing which drive to buy, there are several key points to consider.
1. SATA or M.2?
The first question is whether you should choose a SATA drive or a more modern M.2 model. SATA SSDs use the same data and power connectors as a regular hard disk, so they’re pretty much guaranteed to work with any system made in the past ten years. The catch is that the SATA interface is limited to around 550MB/sec – and on older systems, some or all ports may only support half that data rate. You’ll still feel the benefit of an SSD versus a mechanical drive, but it means you can’t realise the full performance potential of modern memory chips.
That’s why the industry is moving to the compact M.2 connector, which is effectively a miniature PCI-Express x4 slot designed specifically for SSDs. This supports the NVMe (Non-Volatile Memory Express) standard, which offers over five times the bandwidth of SATA. Allowing for technical overheads, the theoretical maximum transfer speed for an NVMe drive is around 3,940MB/sec – more than enough headroom for even today’s very fastest drives.
If you’re building a new PC, it’s likely your chosen motherboard will have a built-in M.2 slot. If it doesn’t – or if you’re upgrading an existing desktop system – it may be possible to add one via a cheap PCI-E expansion card. But bear in mind that choosing M.2 doesn’t automatically guarantee stellar performance: drive speeds can vary considerably.
A final format worth mentioning is mSATA, as found on some laptops and all-in-one PCs from the past five years. This is a compact design similar to M.2, but it connects to the SATA bus rather than PCI-E. A few manufacturers still offer mSATA variants of their SSDs; effectively these are the same drives as their regular SATA models, just in a different shape, so you can expect identical performance.
2. Physical form factors
So, you’ve picked your SSD and, since your PC has the right connector, it should just fit, right? Possibly not. Modern SATA SSDs come exclusively in the 2.5in format, and don’t fit securely into the 3.5in drive bays in a typical desktop PC. You may need to invest in a 3.5in bracket to make it fit.
If you’re upgrading a laptop, meanwhile, it might have been designed for a 9.5mm-thick mechanical disk. Since most SSDs use the slimline 7mm format, your new drive might rattle around rather than fitting neatly into the drive cavity. Some SATA drives come with a 2.5mm spacer that you can place on top of the drive to make sure everything fits snugly into place; if yours doesn’t then you can buy one very cheaply online, or bodge it with Blu-Tack.
With M.2, things are simpler: consumer drives invariably use the standard “2280” format, which means they’re 22mm wide by 80mm long, and are secured to the motherboard by a single screw. Technically, though, the M.2 specification does allow for drives to be different sizes; to be on the safe side, check that your chosen drive is in the 2280 format before buying.
3. SLC, MLC and TLC
In the early days of SSDs, a lot of fuss was made over single-level cell (SLC) versus multi-level cell (MLC) flash memory. The difference is simple: SLC memory stores a single bit of data in each physical memory cell, while MLC uses multiple charge levels to store two bits of data in each cell. This lets MLC deliver twice the capacity of SLC for roughly the same price, but it’s a more complex design that’s slower to write to, and wears out more quickly. Some predicted that MLC would die out as prices fell and SLC became viable for everyone.
In fact, the opposite has happened. MLC technology has become faster and more reliable, to the point where every consumer SSD uses it. Indeed, many drives use newer triple-level cell (TLC) technology that stores three bits of information in every cell. This is often partnered with a buffer of SLC memory or DRAM, to give write operations an extra boost.
If you’re buying an SSD for an enterprise-grade server, it’s worth weighing up the benefits of SLC versus MLC, but for a personal PC, this is one issue you don’t need to worry about.
4. How many gigabytes?
You can now buy SSDs in sizes up to 4TB. That’s largely thanks to the advent of 3D NAND technology, which stacks memory cells on top of each other, making it possible to squeeze huge capacities into small chips.
Even so, SSDs are still much more expensive than their mechanical counterparts. If you’re replacing a 1TB hard drive, you can save money by downsizing to a 512GB SSD, or even a 256GB one. Avoid 128GB if you can.
You will notice, incidentally, that SSD capacities don’t always come in neat powers of two. For example, if you’re looking for a half-terabyte drive, you might see drives advertising only 480GB or 500GB of space. Internally, these drives normally do contain 512GB of flash memory, but some cells are set aside for so-called “overprovisioning”. They can then be cycled into service as older cells wear out. Some drives come with software that lets you adjust how much space is set aside for overprovisioning.
5. Write tolerance and MTTF
SSDs don’t need defragmenting. Indeed, disk optimisation tools can shorten the life of a solid-state drive, by subjecting it to large numbers of unnecessary write operations.
That cells wear out is an inherent limitation of the technology, but it isn’t as disastrous as it sounds. The drive’s on-board controller hardware keeps track of the state of the drive, and will warn you when its demise is imminent so you can copy off your data before it’s too late.
The good news is unless you’re using it continuously for heavy database operations, your SSD will almost certainly outlast the computer it’s installed in. The physical properties of flash memory cells are well understood, so manufacturers can estimate the amount of data that can be written to a drive before it’s likely to fail – its “write tolerance”. A typical drive may offer a tolerance of 200TBW (terabytes written), meaning you could write 50GB to the drive every day for ten years without hitting the limit.
The other measure of endurance is MTTF, or “mean time to failure” – an engineering prediction of how many hours the drive will work for before giving up the ghost. Most drives promise at least 1.5 million hours, which is equivalent to 171 years. So it’s not something you need to worry about much.
Some SSDs come with built-in hardware encryption. This is a great feature to have – since the encryption is handled by the drive’s controller, it ensures that absolutely everything you write to the drive is protected, and there’s no performance hit.
However, encryption isn’t enabled by default. Or, to be accurate, by default files are encrypted as they’re written to disk, then automatically decrypted again as you access them – so it’s as if you had no protection at all. If someone hops onto your PC or steals your laptop, your files will be open.
To make your drive’s encryption feature useful, you need to set up authentication on your PC. There are various ways to do this: if your drive is compliant with the Opal 2 standard then your BIOS may allow you to create a password for it. Once you’ve done this, you’ll need to provide the password to boot from the disk; without it, the drive is completely inaccessible, even if it’s removed and hooked up to another PC.
If you’re running a “Professional” edition of Windows, you can use the built-in BitLocker encryption system: as long as the SSD supports Microsoft’s eDrive standard, BitLocker will use its built-in encryption for maximum security and performance.
Like the SLC versus MLC debate, TRIM used to be a hot topic in the world of SSDs. To understand why, it’s necessary to know a bit about how SSDs write to their flash memory cells. For technical reasons, it’s not possible to update a single cell in isolation; the controller has to rewrite the entire virtual data block containing the cell. The more data that happens to be stored in that block, the more there is to rewrite: in an extreme case, you could end up writing 512KB of data every time you wanted to update a single bit. As you can imagine, this has a pretty serious effect on performance.
TRIM is an operating system feature that tells the SSD which bits of data are no longer needed, so it doesn’t bother rewriting them and thus keeps slowdown to a minimum. TRIM is incorporated into all recent releases of Windows, but it’s not supported on Vista or older systems. To upgrade an ancient PC, you might need to use the manufacturer’s software to manually run TRIM from time to time, to ensure your SSD isn’t being slowed down by continually rewriting junk.