Mix and match RAM for server?

EnderW

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I have a mixture of DDR4 ECC UDIMMs - 2x Kingston 16GB, 1x Hynix 8GB, 1x Hynix 32GB. I think some are different speeds as well.

I have tested the single sticks and the matched pair independently in my Asrock C246 motherboard with Passmark’s MemTest86 and all passed a full round as well as all 4 sticks at once.

I know all RAM will run at the lowest speed, but is there any other reason not to use all 4 modules simultaneously? The planned use for this system is a TrueNAS file server.
 
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If there were anything else I'd say YOLO and send it but for a TrueNAS machine I'd really try to stick to not only matched sticks but motherboard vendor qualified sticks. ZFS is very memory dependent (as I'm guessing you know since you are here.) It will probably work at some janky half bandwidth speed or something but this is your data....

The 2x 16GB Kingston sticks are probably more than enough.
 
In my experience server ram is actually a little more forgiving in the sense you don’t have stuff that’s way off from the JDEC spec and as long as the voltage and type matches up I haven’t had any issues.

Someone at STH recommended 16+16+32 and said I would get dual channel. I’m still trying to wrap my mind around how that works but I’ll probably try it.

I also feel pretty good about stability for any config that’s passed the full gamut of MemTest86 testing.
 
I don't get the dual channel comment. DIMM slots are hardwired to controller memory channels; a stick itself has nothing to say about what channel it connects to, other than what slot it's in. If you plug two DIMMs into two slots each connected to a different channel, you are necessarily running dual channel. Perhaps they were thinking of channel or rank interleave; mixed memory might disable some kinds of access interleave, depending on exactly what processor you're dealing with.
 
Watch that you still get dual channel memory.

From my experience server boards are quite forgiving when stuffing in random selections of RAM sticks.
 
With 32GB next to 8GB you would only have dual channel for the first 16GB of data while the remaining 24GB will be accessed at single channel speeds.
That is if the MB supports running dual channel with different sized sticks. If not then it will just be single channel all the way.

With 16, 16, 32GB you would have dual channel for the first 32GB and single channel for the remaining 32GB.
As for if the data only lands on the single channel kit when the dual channel kit is full or if it just randomly goes wherever I have no idea.
 
With 32GB next to 8GB you would only have dual channel for the first 16GB of data while the remaining 24GB will be accessed at single channel speeds.
That is if the MB supports running dual channel with different sized sticks. If not then it will just be single channel all the way.

With 16, 16, 32GB you would have dual channel for the first 32GB and single channel for the remaining 32GB.
As for if the data only lands on the single channel kit when the dual channel kit is full or if it just randomly goes wherever I have no idea.
As I noted above, channels are hardwired to DIMM slots. There's no such thing as a "dual channel kit" per se. You can have a kit of two sticks, and if you put one stick in a channel A slot and one in a channel B slot, you are running dual channel.

If you have 8 GB in a slot that's wired to channel A, and 32 GB in a channel B slot, it's correct that 16 GB total will be potentially accessible with channel interleave. (Whether the memory controller actually does interleaving in this situation is up to the memory controller, and I don't know what current desktop processors do.)

I'm not exactly sure what was meant by "16, 16, 32 GB", but if say one 16 GB stick is in a channel A slot, and the other two sticks are in channel B slots, then 32 GB total is potentially channel interleaved and the remaining 32GB is not.

I guess what I'm trying to make clear is that there's a difference between running "dual channel" and running with interleaved access to both channels. The channel that a stick is connected to is dictated by what slot it's in. Whether the memory controller can interleave access, i.e. conduct data transfers on both channels simultaneously, is going to be up to the controller; when the channels see the same number, sized, and rated sticks, channel interleave should always be possible; when not, it's going to depend on the controller, and I don't know what current controllers do.
 
I installed as follows: A1 16GB, A2 16GB, B2 32GB.

BIOS and CPU-Z are reporting dual channel mode and a qiuck AIDA64 bench mark is showing read/write speeds comprable to other DDR4-2400MHz dual channel results.
 
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