U.2, SAS, SSD, NVMe .... I'm completely confused.

Hello, thanks for your question. Your question isn't quite troubleshooting related so I will mostly copy and paste some information I searched online for your understanding.

U.2  SAS  SSD
U.2= a form factor https://dell.to/46LHG4F
SAS= interface
SSD= storage 
 U.2 has a size of a 2.5 inch disk.

https://dell.to/3Ajzmgh
There’s gen 1 to gen 4.
Main difference: speed.
you can join older device to newer port and it will work. [for example gen 1 device to gen 4 slot]
Newer gen device will work, just not as fast in older pcie slot [gen 4 card in gen 1 slot]
The speed difference between pcie 3 and 4 is usually minimal. But there’s also lanes that are more important.
Pcie 3.0 with 16 lanes are better than pcie 4.0 with 8 lanes.
But pcie 2.0 with 16 lanes will usually not be better than pcie 4.0 with 8.
It is often written as pcie 4.0/3.0/2.0/1.0 16/8/4/2x with first is the pcie gen and second are the lanes.

https://dell.to/46LHGBH
The difference between PCIe x1, x4, x8, x16 and x32
The short answer is:
‘PCIe x1’ connections have one data lane
‘PCIe x4’ connections have four data lanes
‘PCIe x8’ connections have eight data lanes
‘PCIe x16’ connections have sixteen data lanes
‘PCIe x32’ connections have thirty-two data lanes (currently, these are VERY rare)
The more data lanes in a connection, the more bandwidth between the card and the host.  However, there is usually a cost increase incurred with higher lane counts.

PCIe is an updated version of the PCI protocol.  Similar to PCI/PCI-X interfaces, PCIe was developed for peripheral component interconnection.  PCIe differs from PCI/PCI-X in several ways, but this blog won’t cover most of those differences.  However, one key difference will allow us to better understand the differences between the variations of the PCIe protocol (x1, x4, x8, x16 and x32).  That key difference is ‘parallel’ versus ‘serial’ data transmission.
In PCI and PCI-X architecture, all of the cards share parallel data lines to and from the host.  Differences between card-speeds and slot-types regularly result in throttled data speeds.
In the PCIe architecture, each card has it’s own dedicated serial data connections (lanes) to the host.  This allows each card connection to achieve a bandwidth independent of other cards that may be active in the system.  The number of lanes are indicated by the suffix of the PCIe protocol (×1, ×4, ×8, ×16, ×32).  Each lane is capable of speeds from 250-1969 MB/s, depending on the version of the PCIe protocol (v1.x, v2.x, v3.0, v4.0).
PCIe cards can always operate in PCIe slots with the same or more lanes than the card.  For example, an x8 card can operate in a slot with x8, x16, or x32 lanes.  Similarly, an x1 card can operate in any PCIe slot.
Sometimes, a PCIe slot operates with fewer data lanes than the mechanical slot-type indicates.  For example, sometimes a motherboard manufacturer will use an x16 mechanical slot even though the data connection is only x8.  In these cases, the higher-bandwidth card still may work (for example, an x16 card in an x16 slot with only x8 connection), if the card doesn’t require that extra bandwidth.  The following image shows the PCI connections of the motherboard used in DuroPC’s IMB540-4.  Notice ‘SLOT 1/3’ are PCIe x16 slots but only Slot 3 has an x8 connection. 

Hope this helps.

Respectfully,

Hi,

I'm thankful for your efforts to write a reply, but it's not answering to anyone of my question: those are general info but a asked specific questions about specific drives and specific interfaces/protocols.

1. is Ultrastar DC SS2200 SAS SSD a U.2 drive? if yes, why it's labeled SAS SSD? and then how to connect it to SAS controller if it's a U.2?

2. As I found when googling: U.2 is for NVMe. But if I take the same example, the sepcs sheets indicates clearly that is not supporting NVMe!

3. MVMe should be faster than a SAS SSD. Why a Dell P4610 speed is 4Gb/s and a no SSD SAS is at 12Gb/s

4.  For PCIe, You said that more lanes  = more speed. But why if I'm using a x16 adapter, I'm not getting any performance gain compared to x4 adapter? (https://www.dell.com/community/en/conversations/precision-fixed-workstations/u2-ssd-on-pcie-30-x16/66a7d3f2c5a33c14814542fd?commentId=66a7dd47c5a33c14814544ec).

Many thanks.

Hi, 

As I uınderstood Ultrastar DC SS2200 is a SAS SSD and not a U.2 NVMe drive. 4GB/s (Gigabytes per second) = 32Gb/s (Gigabits per second) https://dell.to/4dkKoR5 speed you mentioned, 4GB/s for the Dell P4610, is in gigabytes per second (GB/s), not gigabits per second (Gb/s). This is a common point of confusion. U2 can be used for both SAS and NVMe drives but your specific drive is SAS-only. Interface = how it plugs in(U.2, SAS, PCIe), Protocol = how it talks. (SAS, NVMe, SATA)

@DELL-Erman O​ 

Hi Erman O,

many Thanks cause I start to figure out how things is done due to your reply.

Just to be sure, the Ultrastar SS2200 should be an SAS SSD with U.2 interface? cause I counted 28pin (in SAS interface, there is a "blank" in the middle. SS2200 has a 8 pin in its middle). If it is a U.2 (no NVMe), it is possible to plug it in a a standard SAS controller (in a T430 or in a T7910 for example)?

For speed, It's clear that my confusion come from the Byte vs bit.

Last thing, If I allow more lanes for a U.2 by using a U.2-PCIe adapter x16, can this allow highest speed? If no, can I have a quick explanation? and in your opinion, why there is such x16 adapters.

Many many thanks.

Thanks a lot.

10/10

Finally I get anwers I need.

I'm really thankful for your help Emran O.

Have a nice day.