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I've got a collection of HS21s (type 8853) that need a memory upgrade. Crucial's recommended memory kit is

Searching around a bit I've found some "DDR2-667 PC2 5300P ECC Registered" memory for a considerable amount less. Seemingly the only difference is that it's 'registered' vs 'fully buffered'. Is this going to be compatible, and if so, what am I compromising on?



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Registered = Buffered

It's just different electrical terminology. Be sure they're both ECC (though it's very rare to find Registered RAM that isn't also ECC).

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According to the Crucial Memory Expert, there is a difference, and an HS21 requires fully buffered because it "allows for more memory to be installed in a system and to run at greater speeds than ECC Registered memory thus providing better performance while maintaining data integrity and reliability". This is the alternative I've found:… Looks exactly the same, except the price! – Matt Bennett Jul 14 '10 at 13:38
Registers are a type of buffer. A buffer electrically separates two electrical systems, so the electricity used by the ram isn't drained from the memory bus (thus allowing more memory sticks to be used. There is no speed difference, though buffers introduce lag). If registers are used it is fully buffered. Now there are other types of buffers, but none of them are used in memory because they're much slower (or much more expensive). The Crucial Memory Expert likely knows very little about electronics and is just reading marketing hype off his computer screen. – Chris S Jul 14 '10 at 14:01
Mmm, wikipedia has an article on 'FB-DIMM's, which do seem to be a different (and poorly named) technology: – Matt Bennett Jul 15 '10 at 15:22
Correct, FB-DIMMs are quite the different beast from Registered ECC RAM. FB-DIMMs do not have ECC, and each DIMM connects individually to the RAM Controller. The makes it sound like it's very similar to traditional DIMM RAM, which it is not. There's only a very few systems that use it (notably Xeon 5xxx processors), and it was essentially DOA. – Chris S Jul 15 '10 at 15:35
Very confusing! Thanks for the clarifications. – Matt Bennett Jul 15 '10 at 15:54

I'd stick with whatever the current memory is. Don't mix and match.

ECC registered means it is buffered. I think you might find the Crucial memory is the same spec.

Of course, depending on what the servers are actually doing, you might not want to compromise on the quality of the components, which might turn out to be the cost differential here, rather than features.

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