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I'm browsing for a few low-end servers for a startup and I'm a bit confused about the different memory types. The advantage of ECC is clear - single-bit error correction. When it comes to registered memory it seems more vague, especially in systems that support both registered and unbuffered memory.

A Google search mostly finds copies of the Wikipedia article, which states that registered memory chips "...place less electrical load on the memory controller and allow single systems to remain stable with more memory modules than they would have otherwise". However I can't find any quantification of this.

What I'm wondering about is:

  • Is registered memory an improvement over unbuffered when it comes to soft error rate, or is it purely about the maximum number of modules supported?
  • If yes, at what point (amount of modules or GB of memory) do these improvements start to become noticeable?

For a specific example, the HP ProLiant DL 120 G6 server manual states that maximum supported memory configuration is 16 GB unbuffered (4x4GB) or 12 GB registered (6x2GB). In this case I'd rather have the extra 4GB of memory if the reliability difference is negligible.

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Two reasons leap to mind but both are due to the same technology.

Basically Registered memory can wait for writes to 'fill' the available buffer, it's a bit like a synchronous link in that respect. This means that all of the reads or write collecting at the start or end of a edge signal - this makes it faster overall than unbuffered.

Also because the clock management is more accurate/adhered-to-better more modules per channel/bus can be used as the signals don't get out of sync or degrade, meaning more memory per channel/bus. That's why very often a server can take say 18 x Registered modules but only say 6 unbuffered ones. Of course unbuffered is cheaper so if you want a low-memory server it may be just fine but if you think you're likely to go over the server's 'unbuffered-limit' then you should go straight for Registered on day-one.

It's got little to do with reliability by the way - just speed and expansion.

Hope this helps.

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Thank you, this confirms what I was starting to suspect - that for a low-end server where max memory is usually ~16 GB, one might as well go for the cheaper unbuffered. Especially in the HP case mentioned where the registered memory limit is actually lower than the unbuffered :-) –  odd parity Feb 22 '11 at 21:02

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