How to pick the per­fect SSD


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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.


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 9.5mmthick 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.


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 triple­level cell (TLC) tech­nol­ogy that stores three bits of in­for­ma­tion in ev­ery cell. This is of­ten part­nered with a bu„er of SLC me­mory or DRAM, to give write op­er­a­tions an ex­tra boost.

If you’re buy­ing an SSD for an en­ter­priseg­rade 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.


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 half­ter­abyte 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 so­called “over pro­vi­sion­ing”. 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 over pro­vi­sion­ing.


SSDs don’t need de­frag­ment­ing. In­deed, disk op­ti­mi­sa­tion tools can shorten the life of a solid­state 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 on­board con­troller hard­ware keeps track of the state of the drive, and will warn you when its demise is im­mi­nent so you can copy o„ 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 o„er 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 built­in 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 built­in BitLocker en­cryp­tion sys­tem: as long as the SSD sup­ports Mi­crosoft’s eDrive stan­dard, BitLocker will use its built­in 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 e„ect 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.

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