Memory for server

We have a Dell server (R410) running vSphere ESXi that has the following memory installed:
8GB 1Rx4 PC3L - 12800R (total 48GB)

We purchased 4 more DIMMs, but when they arrived, it says: 8GB 2Rx4 PC3L - 12800R.

I installed the additional memory and everything seems to be working OK. Can you explain the difference between 1Rx4 and 2Rx4, and if the two can be installed on the same server?

Also, is there a performance degradation?
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pzozulkaAsked:
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Seth SimmonsSr. Systems AdministratorCommented:
the 1x and 2x refer to single and dual rank, respectively
this should answer your questions

Singe Rank Memory vs. Dual Rank Memory
http://www.oempcworld.com/support/SinglevsDualRAM.html

as far as performance goes, the speed is probably reduced to 1066MHz though you probably won't notice much of any difference.  it's not a dell thing, it's how the intel chipset was designed.

some additional reference material

PowerEdge R410 Technical Guide
http://www.dell.com/downloads/global/products/pedge/en/server-poweredge-r410-technical-guide-book.pdf
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pzozulkaAuthor Commented:
Why would you say it's reduced to 1066MHz?
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Seth SimmonsSr. Systems AdministratorCommented:
it's the design of the chipset
if there were only a couple modules that were 1600 then it would run at 1600
but when you start populating more slots, the speed is reduced
if it was fully populated it would slow to 800

i had a number of R710's with all 18 modules populated with 8gb 1333 modules (total 144gb) but the speed was reduced to 800

the chipset used in the 12g servers (R420, R720, etc.) don't have that limitation
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pzozulkaAuthor Commented:
Oh I see, so the speed limitation has nothing to do with the memory mismatch -- that was my main concern.

Also, seems like in the article you posted, it's not a bad thing to have dual rank mixed with single rank, and in fact recommended when you have a lot of memory installed. We have 80GB now after the latest upgrade.
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