[sf-lug] NVMe, M.2 form factor & subtypes, SATA vs. PCIe, SLC vs. TLC NAND

[Rick Moen]

Quoting Ken Shaffer (kenshaffer80 at gmail.com):

> One other detail to investigate: Is your M2 PCI a 3 or a 4?  The SSD
> devices may come in both varieties, so if you actually have a PCIe4
> slot, spending a little more for a PCIe4 SSD doubles your speed. 

Just to clearify, this is a bandwidth spec _separate_ from how many PCIe
"lanes" the socket gives the device access to.  So, e.g., when I said
that the (desirable) M-keyed M.2 slot/socket on any new motherboard
means it'll support "PCIe x4 devices", that means giving access to four
PCIe lanes (what "x4" signifies) -- the data width of its clock cycle.

Your reference to "PCIe3" and "PCIe4" means the 3.0 vs. 4.0 PCIe
protocol revisions, each generation (as you suggest) doubling the prior
one's per-lane bandwidth:

Revision    Bandwidth    Gigatransfer    Frequency    Encoding[1]
PCIe 1.0    8 GB/s       2.5 GT/s        2.5 GHz      8b/10b
PCIe 2.0    16 GB/s      5 GT/s          5.0 GHz      8b/10b
PCIe 3.0    32 GB/s      8 GT/s          8.0 GHz      128b/130b
PCIe 4.0    64 GB/s      16 GT/s         16.0 GHz     128b/130b
PCIe 5.0    128 GB/s     32 GT/s         32.0 GHz     128b/130b

The PCIe 4.0 standard supports a 16 GT/s bit rate, (roughly 2GB/s per
single lane, or 64GB/s in total) as opposed to 8GT/s bit rate for PCIe
3.0 (1GB/s).

Don't go too crazy about theoretical bandwidth, though, as it has a
history of disappointing unwary buyers too focussed on bandwidth
ceilings as if they had anything to do with reality:  I'd have to do the
math to see if the higher specs even matter for conventional mass
storage, given the bottleneck potential of physical drive hardware.

[1] https://en.wikipedia.org/wiki/64b/66b_encoding