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Old 11th September, 2006, 03:45 AM
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FB-DIMM Memory Question

I recently ordered one of the new Mac Pro computers that Apple put out in the past month or so. The default memory configuration is simply two sticks of 512 MB PC 667 DDR2. While a gig of memory is enough to start out with, I wanted to add at least another gig or two. So I began looking around at prices, only to find that this memory seemed astronomically expensive compared to memory I've shopped in the past.

This memory is different from past memory I've owned in three ways: it's ECC, it's Fully Buffered, and most importantly (I think) they're FB-DIMMs. What is the difference between a normal DIMM stick and an FB-DIMM? For that matter, what are the relative advantages/disadvantages of having Fully Buffered ECC memory over Non-Buffered Non-ECC memory?
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Old 11th September, 2006, 04:26 PM
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FB-Dimms have an extra chip on the ram stick itself which is an additional buffer. This apparently removes a problem of the memory signal degrading (thus performance falls) as more sticks of ram are inserted into the machine.

The advantages

- More robust / error tolerant memory
- Alot more ram in the same system with no loss in speed (some Xeon boards already support 64 Gigs of ram!)
- Because lots of sticks can be used, things like Quad channel ram can be used. Along with Dual FSB

Another advantage is the chip forms part of the memory controller, to upgrade the type of ram a board uses you just update the fbdimm chip (was the plan, to allow ddr2 boards to take ddr3/4 without any further upgrade)

Disadvantages

- The chip requires power, so overall system power is up about 5w per ram stick used.
- This power means heat, fb-dimms run hot
- Costs, the chips are not cheap and fb-dimms are far from mass market at present.

However the current target market is high end workstations and mid-low servers that uses the new intel xeons.
When you're paying £1500 per cpu, an extra £50-100 per on ram is nothing.

The mac pro's are essentially aimed at being a workstation system and have so jumped on the bandwagon. They are very quick machines however!

A pair of dual woodcrest xeons are very fast especially the 3GHz versions. Not cheap tho!
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Old 11th September, 2006, 05:29 PM
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FBDIMM is expensive, as Keith points out, due to the additional processing onboard. The "Buffer" (AMB - Advanced Memory Buffer) sits on a serial memory bus and communicates with parallel memory chips. There are two independant channels, one pushing commands/data up to the RAM, and one pushing responses/data down from the RAM.

As it's specifically designed for a market that wants more than 4 DIMM slots per board, along with the reliability, expect to have to pay for it. It's also designed to support multiple links, making dual channel RAM look rather slow by comparison.
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Old 12th September, 2006, 02:07 AM
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Although I don't necessarily like the price point of this memory, it sounds like they perform at a high standard. I bought this computer because although I wanted to stick with OS X for my main computing platform, I also wanted to take full advantage of Boot Camp and play games on a Windows installation. By all appearances, I should be able to install a fully working copy of XP pro (after slipstreaming SATA drivers into the install disk... there's been a big headache over this) and get some enjoyment out of the latest and greatest games. I'm quite interested in seeing if the X1900XT I configured into it will be worth the extra cash....
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Old 12th September, 2006, 09:43 AM
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Server memory generally is not designed to operate as fast as overclockers memory. Instead, they're designed to be fault tolerent and support error correction so that the servers are as robust as they can be. That usually means that the memory subsystem is slower - certainly the latency with FBDIMMs is higher than that of standard DDR2.
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Old 12th September, 2006, 08:32 PM
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Still having two woodcrest cpu's (4 total cores) its going to be killar quick and it sounds as if they've wired it the same as the server line with dual independant fsb's (1333mhz as well) upto quad channel ddr2 memory etc. It will be quicker than the upcoming quad core kentsfield chips
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Old 13th September, 2006, 03:58 PM
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Some more info on the mac pro and its FB Dimm's http://www.anandtech.com/mac/showdoc.aspx?i=2832
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