Will z390 (or 370) chipset support a PCIE-3.0 x8 SAS controller/HBA?

[defcon]

I'm building my first Unraid system in a 24 drive 4U chassis (6G backplane with 6 sff-8087 ports so 4 drive lanes each) and have settled on intel 9th generation CPU (prob a 6-core i5) as the sweet spot for me for cost & performance. Goal is storage, plex transcoding and dockers. Maybe a future VM for fun (but not for gaming). I'd like to keep it as solid state as possible when idle (to save power) so am intending to pool two M.2 drives (x4 each) for dockers and array's cache pool (might be overkill). I have an HP220 SAS HBA (x8) and an HP PCIE SAS expansion card that just uses PCIe for power from what I understand (so x0). I'll only use CPU's integrated GPU.  So I'm looking at 370 an 390 mobos and going for "z" version figuring I want that third physical PCIE x16 slot just in case I want it later.

QUESTION:  When I look at intel's spec sheet for z390 chipset it says 

Expansion options >

   PCI Express Configurations is x1, x2, x4 [NO x8!!]

I/O Specifications >

   Supported Processor PCI Express Port Configurations is 1x16 or 2x8 or 1x8+2x4 [HAS x8!!]

 

What's going on?  Will a z390 board like the MSI MAG Z390 Tomahawk support my x8 SAS HBA card or not (it doesn't say it supports x8)? I don't want my whole drive array to run at half speed. Is this the difference between PCIe slots supported by CPU vs the chipset?  Is z370 better for this (harder to find dual M.2s and I don't need Wifi). Should I be looking at enterprise mobos like c246 (says its for Xeon CPUs but still has same 1151 socket)? I'm def more familiar with the consumer CPUs and want to limit power usage as much as possible.  While I'm add it should I just use SATA SSDs instead of M.2s for my cache? Thanks.!

Edited February 13, 2022 by defcon
typo

[Hoopster]

19 minutes ago, defcon said:

Will a z390 board like the MSI MAG Z390 Tomahawk support my x8 SAS HBA card or not (it doesn't say it supports x8)?

The board you linked supports:

• 3 x PCIe 3.0 x16 slots (support x16/x4/x1 modes)
• 2 x PCIe 3.0 x1 slots
• 1 x M.2 slot (Key E) for an Intel® CNVi wireless module only

Most x8 HBAs can go in an x16 physical slot.  Many boards do not have x8 physical slots and have some combination of x16, x4 and x1.

 

Depending on what you plan to attach to the HBA, x4 may be enough anyway.  HDDs rarely need more than 200 MB/s.  x4 in PCIe 3.0 provides 4 GB/s bandwidth (really closer to 3 GB/s by the time you factor in overhead.)  Let's just say 3.2 GB/s to make the math easy.  If the HBA supports 8 drives, that is 400 MB/s per HDD even with x4.  You don't want to connect SSDs to HBAs because most (with a few exceptions) don't support TRIM properly.  If you were connecting 8 SDDs to the HBA, they would likely be bandwidth constrained, but you don't want to do that.

 

I have my x8 HBA in an x16 slot on my MB.  My HBA is only PCI 2.0 (half the bandwidth of PCIe 3.0) but even with 8 drives attached, it would provide enough bandwidth as they only get up to 200 MB/s in rare cases.

[UhClem]

You do NOT want a Mobo with the PCIe config like that MSI you linked. It has all 16 of the CPU's PCIe lanes going to ONE of its x16 slots. (The other two x16-wide slots, one of them x4 lanes & the other x1 lane, are connected to the chipset.)

Nothing wrong with the Z390 chipset itself, but look for a mobo that allocates the CPU's 16 lanes among two slots, either as x16/x0 (when that 2nd slot is empty) OR as x8/x8 (when both are occupied).

 

Even better, but I've never seen it!!, would be three CPU-connected slots, configured as: x16/x0/x0 or x8/x8/x0 OR x8/x4/x4.

 

 

[defcon]

7 hours ago, UhClem said:

Nothing wrong with the Z390 chipset itself, but look for a mobo that allocates the CPU's 16 lanes among two slots, either as x16/x0 (when that 2nd slot is empty) OR as x8/x8 (when both are occupied).

Thanks for the comments. I'd still like to have the x8 capable slot. So is the 390 chipset incapable of doing what I want, or do I just need to find another 390 (or 370) motherboard that is setup to do x8?

[JorgeB]

2 minutes ago, defcon said:

or do I just need to find another 390 (or 370) motherboard that is setup to do x8?

If you want a desktop board look for a board with SLI support, those will have two slots that can be configured as x16/x0 or x8/x8, with server boards from for example Supermicro they usually always come with two slots configured for x8/x8, and usually one of them can be bifurcated to x4/x4.

 

 

[Hoopster]

3 hours ago, JorgeB said:

with server boards from for example Supermicro they usually always come with two slots configured for x8/x8, and usually one of them can be bifurcated to x4/x4.

@defcon  I was mistaken in my earlier post.  My ASRock Rack mATX C246 server MB has one physical x16 slot, one physical x8 slot and one physical x4 slot.  With the x8 HBA in the x8 slot as it is now, the x16 slot automatically switches to x8 electrically.

[Sheikh]

On 2/14/2022 at 1:19 AM, Hoopster said:

The board you linked supports:

• 3 x PCIe 3.0 x16 slots (support x16/x4/x1 modes)
• 2 x PCIe 3.0 x1 slots
• 1 x M.2 slot (Key E) for an Intel® CNVi wireless module only

Most x8 HBAs can go in an x16 physical slot.  Many boards do not have x8 physical slots and have some combination of x16, x4 and x1.

 

Depending on what you plan to attach to the HBA, x4 may be enough anyway.  HDDs rarely need more than 200 MB/s.  x4 in PCIe 3.0 provides 4 GB/s bandwidth (really closer to 3 GB/s by the time you factor in overhead.)  Let's just say 3.2 GB/s to make the math easy.  If the HBA supports 8 drives, that is 400 MB/s per HDD even with x4.  You don't want to connect SSDs to HBAs because most (with a few exceptions) don't support TRIM properly.  If you were connecting 8 SDDs to the HBA, they would likely be bandwidth constrained, but you don't want to do that.

 

I have my x8 HBA in an x16 slot on my MB.  My HBA is only PCI 2.0 (half the bandwidth of PCIe 3.0) but even with 8 drives attached, it would provide enough bandwidth as they only get up to 200 MB/s in rare cases.

To be honest. I totally agreed your opinion. It's quite helpful for me as well as other too. Overall, Z390 is an extremely powerful motherboard.