PCIe 4.0 to PCIe 3.0

[rcm024]

TL;DR Is it possible to get 2x PCIe 3.0 out of 1x PCIe 4.0?

 

So with PCIe 4.0, the bandwidth available on the PCIe lanes has doubled even with lane count on the AM4 boards remaining esentially the same (yes I know that the X570 chipset has up to 16 additional lanes but they're only connected to cpu over a PCIe 4.0 x4 connection). I plan on doing a build in the near future (I'm still running my 3770K so I'm not desperate) and would like to use the AM4 platform if possible for my use case. I plan on running multiple high performance VMs simultaneously each with their own graphics card, NVMe drive, USB controller, etc... meaning that I need a lot of addressable PCIe lanes. So my question (as I'm not too knowledgeable on the actual workings of PCIe) is, can a PCIe 4.0 x8 interface be split out on an add in card to 4 PCIe 3.0 x4 interfaces (without dropping the add in card to 3.0 obviously)? My idea, is that I have 4 PCIe 3.0 NVMe drives which is equivalent to a PCIe 3.0 x16 capacity which is equivalent to a PCIe 4.0 x8 capacity. Is something like that possible? I haven't been able to find anything on the market that does this so it makes me think it's not possible, but perhaps someone who's more knowledgeable in PCIe can shed some light on the topic for me. I woould love to go with AM4, (though I'll definitely be waiting for the 16 core part that we all know exists) but if this is impossible, I'll just wait for Threadripper 3000 I guess.

 

[Jurrunio]

I dont think such adapter card exists. Changing PCIe version is free of charge, that and increasing lane count to make use of the max bandwidth is expensive as you will need an extra chip to handle a signal conversion.

 

But speed isn't really what I'm concerned about, a boot drive dont need high transfer speed. Low latency is all that matters, in that case PCIe lanes and alterations dont really matter, neither do SATA or NVMe.

[Firewrath9]

You'd need a PCIe switch, which costs money.

[Electronics Wizardy]

You can use a plx bridge for this, but it won't be cheap.

 

Id just get a singel fster ssd, and if you want a faster drive for vms look at optane.

 

What are these vms doing, 4x nvme drives are probably overkill esp if you doing passthrough of gpus as it seems.

[rcm024]

19 minutes ago, Electronics Wizardy said:

You can use a plx bridge for this, but it won't be cheap.

 

Id just get a singel fster ssd, and if you want a faster drive for vms look at optane.

 

What are these vms doing, 4x nvme drives are probably overkill esp if you doing passthrough of gpus as it seems.

Multi user (my roommate and I + potentially a guest) gaming rig + dedicated server(s) for games and other services + maybe a router vm (this will probably end up on a different box)

 

The SSDs are plenty fast, but I want to be able to potentially hit them all simultaneously, hence the question. Does a plx bridge allow you to do individual device passthrough? i.e each vm gets its own nvme drive even though they are all on the same physical card?

[Electronics Wizardy]

1 minute ago, rcm024 said:

Multi user (my roommate and I + potentially a guest) gaming rig + dedicated server(s) for games and other services + maybe a router vm (this will probably end up on a different box)

 

The SSDs are plenty fast, but I want to be able to potentially hit them all simultaneously, hence the question. Does a plx bridge allow you to do individual device passthrough? i.e each vm gets its own nvme drive even though they are all on the same physical card?

Id stay away from using one pc for multiuser like this, It often about the same price to build 2 pcs and is much easier to manage and work with.

 

Nothing you listed needs much io speed, Id just run a single or a few sata ssds.

 

Why do you need disk passthrough? Id just make a virtual drive and use that for the vms, makes things like snapshots and backups much easier.

[rcm024]

22 minutes ago, Electronics Wizardy said:

Id stay away from using one pc for multiuser like this, It often about the same price to build 2 pcs and is much easier to manage and work with.

I'm sure it is, but that's not what I want to do.

26 minutes ago, Electronics Wizardy said:

Nothing you listed needs much io speed, Id just run a single or a few sata ssds.

Sure, it doesn't "need" it, but i WANT it. Also, I already own the NVMe drives (got them on sale real cheap) so I'm not going sata.

26 minutes ago, Electronics Wizardy said:

Why do you need disk passthrough? Id just make a virtual drive and use that for the vms, makes things like snapshots and backups much easier.

Virtual drive is a possibility, but it still doesn't negate the need for full bandwidth, especially if I decide to stripe the array. Also, I'm not worried too much about snapshots and backups. I have on and offsite backup in place for important files and if something catastrophic happened to the machine, I'd just start from scratch anyway.