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Solid-state drive buying advice: what to look for when buying a SSD

Everything you need to know when buying a SSD

SSD buying advice

There's no doubt now that the solid-state drive has arrived. And we're not just talking smartphones and tablets, whose success owes much to being liberated from the spectre of spinning disks by the flood of cheap flash memory. See all Storage reviews.

Take a look at Group test: What's the best SSD?

But for the world of PCs and laptops, usefully large SSD storage has only recently been available at prices that won't make your eyes water. When we published our first round-up of solid-state drives in 2010, the largest drives available were usually 256 GB in size and priced well out of the comfort zone of most users, at more than £500 each. That was well over £2 per gigabyte. Nowadays the figure is more like 80p or less.

If you're looking to spruce up a tired PC, nothing works like a new SSD. For desktop computers you can sometimes get away with a smaller (read: cheaper) solid-state boot drive and then dump all your own files on a second slower-but-big hard disk. But Windows programs, for instance, typically expect to be installed on the boot drive, so don't expect a 60 GB drive to remain usable for long.

Most PCs sold today are actually laptops, and here there's an even greater need for solid-state storage – and a demand for a goodly amount of space, since few offer the luxury of a second internal drive. USB 3.0 has helped enormously here, enabling external storage to work at the same speed as internal hard disks at least. But they will force you to faff around with plugging and unplugging wayward devices. Not at all easy when supporting a laptop on your knees on the train.

We find a sweetspot balancing capacity, performance and price is currently 256 GB, although with 512 GB in more realistic reach we now ask vendors to supply their largest drive. These often perform and measure better too.

This group test includes an equal mix of three 256 GB and three 512 GB drives. Note that user-accessible capacity can vary, as some drives reserve more of the available storage to improve their maintenance routines and/or long-term reliability, a process known as over-provisioning.

What makes a good SSD?

Just about any solid-state drive today, even the cheapest from a known brand at least, will knock spots off the fastest hard disk money can buy. So you could just look for the cheapest available SSD of the capacity you need.

For performance-led applications like servers and media production, it pays to find a drive that is both faster and more robust than the cheapest. And even at the consumer level there are crucial factors to consider besides just price, such as after-sales maintenance support and device reliability.

Performance

The easiest test, and in many ways the most meaningless, is the sequential read test. This is the headline metric that carries over from the hard-disk days, and while it's useful to know that your drive can replay the entire contents of a 25 GB Blu-ray in under a minute, it doesn't demonstrate why an SSD makes your computer feel so startingly fast in daily use.

Where the solid-state technology excels is in its dexterity with juggling small files. Not only can it read and write small files (around 500 byte to 500 kilobyte) extremely quickly compared to mechanical disks; it can do so with more than one thread at a time. So while your spreadsheet app is rifling through many small revisions to your document, other data-shuffling tasks are also tackled concurrently by your computer. Hard disks used memory cache to help manage such tasks simultaneously, but continuous intensive tasks can defeat that. Modern SSDs, with the right controller technology and firmware programming, truly excel with 32 or more simultaneous computing threads.

We can get an idea of just how well an SSD copes with parallel data processing tasks by measuring its small-file transfer speeds, and in particular using the industry-standard 4kB random read or write test with increased queue depth. This will indicate how many input and output operations it can handle at once, giving an IOPS results (input/output operations per second). The very finest SSDs can now hit 100,000 IOPS, a nominal figure waved in marketing materials but a worthy benchmark target nonetheless.

Besides the fresh-from-the-box speed, an SSD must take care of itself after continued daily work. Different strategies are adopted behind the scenes by different brands. All SSDs will support the Trim command to pre-erase deleted data, although your OS may not support the facility automatically. Garbage collection routines enabled by the controller should ensure heavy use doesn't slow the drive with time, while wear levelling helps prevent excessive strain on too few data blocks.

Hubris is common from SSD makers, who insist their drive is perfect and will not need any refinement. In truth, faults and bugs, some critical, are discovered after the drive has been released. To resolve such defects you may be expected to update the controller firmware on the drive; but bewarned that several brands have pledged to support just Microsoft. Of the six SSDs tested here, only two (Crucial and Plextor) can be fixed without using a Windows PC.

Conclusion

With the possible exception of the PNY XLR8 PRO, these are all startlingly fast solid-state drives, with the PNY still ticking the damn-fast box, and worthy of pursuing if its price was a little more competitive, below the £200 mark. The Corsair Neutron GTX is a surprise outsider that really impresses, and while its priced at the higher end at around 79p/gigabyte it can easily recommend itself for great small-file juggling. Laptop weight-wathcers may also note it has the lowest mass of just 47g. Toshiba is only now starting to address the consumer retail market, but its THNSNH drive has great promise with top-tier sequential and good read IOPS, if a little tardy with write IOPS. Samsung's 840 Pro is one of the swiftest solid-state storage devices you'll find, but platform support is poor and there are some unresolved question marks over its compatibility. A longer warranty would make our recommendation of the Crucial M500 stronger, but as it stands this is the new budget-performance champion. It already offers keen pricing and while not the leading speed demon is plenty quick enough for most mortals. Decent firmware upgrade support just adds to its appeal. The same latter consideration applies to Plextor's top drive the M5 Pro. We were impressed with the QA story here, and outstanding IOPS figures. Combined with very competitive pricing for all this performance and assured quality, it deserves our Best Buy as the finest all-round SSD of the six.

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