M.2 Clarification Requested

[WIEZZY]

Hey, maybe you guys can help with my understanding:

According to the below site, M.2 SSDs should be able to run at speeds of 20 Gbits/sec (when in an "M keyed" configuration), which is about equal to 2.5 GB/s, or 2560 MB/s.

http://rog.asus.com/313352014/labels/guides/buying-an-m-2-ssd-how-to-tell-which-is-which/

However, I have not found any that come anywhere close to even half that:

http://www.newegg.com/Product/ProductList.aspx?Submit=Property&Subcategory=636&N=100008120%20600038492%20600038491%20600136716%20600414703%20600158884%20600476404%20600038499%20600038493%20600038497%20600038502%20600082508%20600488413&IsNodeId=1&IsPowerSearch=1

What am I not understanding? All the M.2 devices on Newegg are about the same speed or slower than my SATA III SSD now. What is the point of the M.2 system if not to be faster?

Thanks!

 

[weilin]

That is correct, M.2 when configured with 4x PCIe 2.0 lanes can support up to 20GT/s (with a 8/10 encoding that's ~2GB/s). That is what the physical medium is capable of supporting. That says nothing about what the device will support. BTW, boards with M.2 that support 4x PCIe are still rare right now; most boards only support 2x PCIe.

Currently, most M.2 devices connect via SATA III (6Gb/s) with a few supporting PCIe (Samsung XP941, Plextor M6e). Most drives can't saturate 2x PCIe 2.0, so why have 4x PCIe 2.0? This gives the interface room to grow in the future.

The fastest drives now are the PCIe devices. As an example, the Plextor is rated for 770MB/625MB read/write which is faster than what SATA III supports (600MB/s). So Plextor opted to use the 2x PCIe standard which gives it the bandwidth it needs. Going to 4x is not necessary because the device can't saturate 2x and going wider would have just been more expensive with no benefit.

The other example is the Samsung XP941. which in its fastest configuration supports 1.2GB/1.0GB read/write. Since 1.2GB is greater than that of 2x PCIe 2.0 (1GB/s), Samsung XP941 implements a 4x PCIe 2.0 interface. This drive will work in a 2x PCIe 2.0 M.2 slot, it would just be limited to only "1GB/s".

Note: 1x PCIe 2.0 M.2 (500MB/s) is less than SATA III (600MB/s), thus you probably won't find a 1x PCIe 2.0 M.2 connector/device.

Edit: I did a bit of research and found the Samsung SM951 which is supposedly only available to OEMS that will support 4x PCIe 3.0 (giving it theoretically ~4GB/s bandwidth). To my understanding, no board supports PCIe 3.0 M.2 yet. If used in today's boards, it would just fall back to 4x (or 2x) PCIe 2.0 which would limit it to 2GB/s (or 1GB/s).

That's not really a problem because the device is supposedly "only" capable of 1.6GB/1GB read/write. Its transfer speed is still under what 4x PCIe 2.0 can do (2GB/s). However, if you only have a 2x PCIe 2.0 M.2 slot, you would bottleneck the drive.

So why ship that drive with 4x PCIe 3.0 then? Well, Samsung is playing the long term game for maximum compatibility. I suspect the next generation chipset will enable PCIe 3.0 M.2 slots. And once again, the common config will probably be 2x PCIe 3.0 M.2 slots. The 2x PCIe 3.0 M.2 slot will support ~2GB/s which is enough to satisfy the drive (PCIe allows support for fewer lanes). Today, the 4x PCIe 2.0 is also enough to satisfy the drive (PCIe 3.0 is backwards compatible with PCIe 2.0). Having 4x PCIe 3.0 support is just a bonus. It doesn't actually contribute anything. Creating a 2x PCIe 3.0 + 4x PCIe 2.0 controller would have been more expensive than just having the 4x PCIe 3.0 controller.

Does that mean you must go out and buy a board supporting 4x PCIe 2.0 M.2 slot now? Well, that's up to you. Do keep in mind though, this drive is not going to be cheap (I expect ~$2 per GB). when the final drive is 400 dollars for a 256GB drive are you actually going to get it?

 

[WIEZZY]

So basically I need to stick with what I have and play the waiting game. Got it, thanks for the explanation.

 

[weilin]

There are a few M.2 SSDs that are faster than your current drive (XP941 and M6e), you probably won't notice the difference in performance to make it worth the premium. The form factor probably doesn't matter to you unless you have a really really small case...

The switch from a mechanical HD to any SSD is typically a huge jump in performance. From that point on, most people won't notice the difference between a mainstream SSD and a top of the line SSD...

 

[WIEZZY]

Ya, that's why I usually just get the cheapest SSD I can find from a reputable company. The only way it would be worth it is if I got the full 20 Gbit/sec speeds, which it looks like isn't happening.