I am planing to upgrade my memory from 2GB to at least 16GB for running 1 or 2 VMs.  I keep reading that with unbuffered ecc memory you should not populate more than 2 memory slots.  I have a Asrock C226 WS Motherboard which does not support buffered or registered ecc memory and only supports 8GB per slot for a total of 32GB.  I originally bought 2GB of RAM because I was only planing to use this a just a file server with no plugins but with the up coming v6 I want to run some VMs.  One of the VMs will be a TV tuner server haven't decided if it will be windows or not and don't know if I would run a second VM yet.

 

How much ecc unbuffered memory should I get?  should I get only 16GB and only use 2 slots or should I use all the slots and have 32GB?  What would the downside of using all 4 slots?

I think garycase had a nice video linked somewhere that explains the electrical background that represents the problem you name.

In simple words, as I understood it, driving that many signal lines in an un-registered setup can present an electrical problem

that will/can result in signal degration and thus resulting in driving the modules improperly.

 

AFAIK most manufacturers (should) know  about this.

A typical countermeasure IMHO is that RAM speed will be reduced if more modules are present.

 

..what does the board manual say?

You should actually ask the support for a tested memory module in a 4-modules config.

IMHO it should be fine then.

Re-using your existing modules in a config that is not tested by the manufacturer is something I'd rather not do and just go for 2x8GB then.

Currently there are no listed 8G ECC DIMMs. You are limited to 16GB ECC memory and that is using all four DIMM slots.

 

http://www.asrockrack.com/general/productdetail.asp?Model=C226%20WS#Memory%20QVL

 

A few thoughts ...

 

Currently there are no listed 8G ECC DIMMs. You are limited to 16GB ECC memory and that is using all four DIMM slots.

 

Not true.  This just means AsRock hasn't tested any 8GB ECC modules and added them to their qualified memory list.    It's VERY likely (nearly certain) that any 8GB module from a quality vendor (Crucial, Kingston, Corsair, etc.) will work just fine.    I'd definitely buy 8GB modules -- personally I'd just buy 2 (for the reasons I'll note below) ... but 4 should work fine.

 

 

How much ecc unbuffered memory should I get?  should I get only 16GB and only use 2 slots or should I use all the slots and have 32GB?  What would the downside of using all 4 slots?

 

I'd only use 2, as you'll have a more reliable memory subsystem if you only install 2 modules.  Two double-sided ECC modules are presenting 36 "loads" to the memory control and data buses -- which degrades the signal quite a bit.  Installing 4 modules bumps that to 72 "loads" ... which has a much more substantial impact on the signaling waveform.    You can see this pictorially in Corsair's excellent tutorial in Item #10 here:  http://www.xlrq.com/stacks/corsair/153707/index.html

 

HOWEVER ... since you're using ECC modules, it IS true that any single-bit errors will be automatically corrected, so the risk of installing 4 modules is somewhat lower than it would be without ECC.  But the things that are often needed to run 4 modules reliably will still be required ... i.e. the BIOS may restrict the speed of the modules; may add a cycle or 2 to the latency settings; etc.

 

I'd install 2 8GB modules and just run with 16GB unless you find that you actually NEED more (i.e. you're running out of memory, or decide you want to run additional VM's and require more memory for them).

 

It's a shame that desktop motherboards don't support buffered modules -- with these there's effectively no limit on how much memory you can install, as there's only ONE load per module      But you need a higher-end server board to get support for these ... not really worthwhile for most UnRAID setups.

 

The listed motherboard is a server motherboard from ASRock, still no support for ECC FBDIMM.

 

When seeking support for problems you could end up following the motherboard manufacturer provided guidance. It is a conservative approach, but also meets your needs.

 

Typically, I would agree that memory from a quality vendor should work. Kingston specifically faced problems with 8GB UDIMMs and Haswell.

http://forums.freenas.org/index.php?threads/fyi-kingston-no-longer-listing-8gb-modules-for-most-supermicro-x10-motherboards.18773/

 

Clearly you need unbuffered RAM ... that's what this discussion has been all about.  [Not sure why the comment r.e. FBDIMM support]

 

On the last two systems I've built using SuperMicro Haswell boards, I've used these memory modules with no problem:  http://www.newegg.com/Product/Product.aspx?Item=N82E16820139979

 

I'd certainly think they'd work fine on the AsRock board ... but there's certainly no guarantee -- there ARE occasional compatibility issues with certains board and some RAM modules, so anytime you use memory not on a QVL it's possible you'll find some issue (for that matter, I've seen the same thing with modules that WERE on a QVL).