Question What's the point of multiple M.2 slots on a MoBo if they eat into PCIe lanes?

[Mick-e]

I wasn't sure if this was a storage question of motherboard question.
I'm looking at building a new CAD rig, and I'm driving myself nuts with configuration options.
I'm set on an X870E board, and yes I know they are almost pointless compared to X670E's but I'm trying to future proof the rig as much as possible as I can usually milk 6 years out of a machine, and maybe the newer chipset gets me a little further down the line.
This amazing document has all of the (most) AMD boards compared
https://docs.google.com/spreadsheet...9O-y_6hv8sPs/edit?gid=755628141#gid=755628141
I've talked myself out of running a RAID1 since the only reason I want to do that is to backup my system drive (because I've had SSDs fail) but I don't need RAID to do that I just need to make a disk image back up, and I found software to schedule that.
From what I can tell with all of the X870, and X670 chipsets as soon as you add a second M.2 SSD you start to eat into PCIe lanes and different boards at different rates.
Or at least that is true for the 5.0x4 mode.
So in this example

ROG STRIX X870E-E GAMING WIFI | Gaming motherboards｜ROG - Republic of Gamers｜ROG Global

The ROG Strix X870E-E Gaming WiFi elevates builds with 18 power stages, DDR5 slots and extensive PCIe 5.0 connectivity. It features five M.2 SSD slots with heatsinks, USB4® support, WiFi 7, AI Overclo

rog.asus.com

Where this board has 3 5.0x4 M.2 slots.
What is the point of having those slots if you lose half of the PCIe lanes?
Again using this board as a reference it looks like slots M.2 slots 4 and 5 are 4.0x4 and don't share the PCIe lanes .
If I were to use the linked ROG then the best scenario is to put a primary OS drive in M.2 5.0x4 Slot 1, a disk image drive in M.2 4.0x4 in Slot 4, and a storage drive in M.2 4.0x4 Slot 5.

What I'm trying to do is have a machine that is as fast as possible for the CAD software to work with OS and files stored. And hopefully have this be good enough for a few years to come.

Am I over thinking this?

 

[USAFRet]

Your "disk image" drive really does not need to be on a fast M.2 port.

Done correctly, this is all automated, in the wee hours and in the background, when you're not using the system.
My Images happen overnight, to spinning drive in my NAS, across the network.

 

[JohnBonhamsGhost]

Where this board has 3 5.0x4 M.2 slots.
What is the point of having those slots if you lose half of the PCIe lanes?

using this board,
if you're only interested in the fastest drives available then you likely don't care about having all PCIe slots populated with all lanes available.

but,
for gaming you'd see no difference vs SATA 6Gb/s and those can be had much cheaper.
so why use up the PCIe lanes with another M.2 drive?

 

[CountMike]

What's the point of number of PCIe lanes if they are not used?
in case of having to choose between PCIe and M.2 slots,more the merrier so you have a choice which is better than not having it. in case some have to be shared, so be it again you have a choice.

 

[rgd1101]

yup. instead of custom motherboard for each individual need

 

[Eximo]

Back when HEDT was more affordable, that would be the choice of the person who wanted lots of lanes.

AMD has a decent stopgap with the E class boards that have multiple chipsets. You can hook up lots of stuff, you just don't have the bandwidth to use it all at once.

These days an 8x PCIe 4.0 connection is plenty good for your average GPU, leaving room for 1 or 2 additional PCIe M.2 drives if needed.

 

[Viking2121]

Well I mean its options, I got 3 on my x570, if I use all 3, the vary bottom slot it disabled, but if I needed the lower x16 length lane down there, I just wont use the lower m.2 slot. I'm fine with that compromise as I bought the board knowing that and what I was going to use it for.

You'd have to go with a more exotic kind of build to see less compromises, but going that route can get expensive.

 

[Mick-e]

I muddied this question up about my personal build but the root question is still what's the point of having multiple M.2 slots if adding additional M.2 drives slows them down?
It seem counter productive.
Different boards behave in different ways so it's not easy to generalize.

My specific use is for CAD software that is CPU intensive, and depending on what it's doing GPU intensive.
I will only use 1 PCIe card for the GPU.
I'll play some games but that is not the primary reason and I'm not looking for every single FPS I can get.
So it seems I should have 1 M.2 5.0x4 for C drive, 1 M.2 4.0x4 for storage drive, and 1 Sata HD for disk image.

 

[rgd1101]

ROG STRIX X870E-E GAMING WIFI | Gaming motherboards｜ROG - Republic of Gamers｜ROG Global

The ROG Strix X870E-E Gaming WiFi elevates builds with 18 power stages, DDR5 slots and extensive PCIe 5.0 connectivity. It features five M.2 SSD slots with heatsinks, USB4® support, WiFi 7, AI Overclo

rog.asus.com

** M.2_2 & M.2_3 slots share bandwidth with PCIEX16(G5). When M.2_2 & M.2_3 are occupied with SSD devices, PCIEX16(G5) will run x8 only.
it only slow down the gpu

 

[JohnBonhamsGhost]

what's the point of having multiple M.2 slots if adding additional M.2 drives slows them down?

filling all M.2 slots doesn't usually slow down other M.2 slots.

just possible that installed PCIe cards will have their available lanes reduced.
only an issue if you have cards installed that may benefit from the full/max available PCIe bandwidth. and if you have that card installed in a slot that may reduced.