Wikipedia

Ranking

Sometimes memory modules are designed with two or more independent sets of DRAM chips connected to the same address and data buses; each such set is called a rank. Since all ranks share the same buses, only one rank may be accessed at any given time; it is specified by activating the corresponding rank's chip select (CS) signal. All other ranks are deactivated for the duration of the operation by having their corresponding CS signals deactivated. DIMMs are currently being commonly manufactured with up to four ranks per module. Consumer DIMM vendors have recently begun to distinguish between single and dual ranked DIMMs.

After a memory word is fetched, the memory is typically inaccessible for an extended period of time while the sense amplifiers are charged for access of the next cell. By interleaving the memory (e.g. cells 0, 4, 8, etc. are stored together in one rank), sequential memory accesses can be performed more rapidly because sense amplifiers have 3 cycles of idle time for recharging, between accesses.

DIMMs are often referred to as "single-sided" or "double-sided" to describe whether the DRAM chips are located on one or both sides of the module's printed circuit board (PCB). However, these terms may cause confusion, as the physical layout of the chips does not necessarily relate to how they are logically organized or accessed.

When you buy memory you only see the "published" specifications. Memory manufacturers often change what components are used to build their memory modules. With the first production run of a particular part number a certain set of components kay be used, but in subsequent production runs (of that exact same model number) different components may be used. While these different components will result in the same "published" specifications, there may be most likely will be differences in unpublished specifications. (Note that rank and channels are two completely different things.)

My guess is that you've fallen into this trap - matching published specs but different unpublished specs (ie your new and old memory are not a matched set of four sticks (even though each set of two sticks may be matched)

It is odd though that memory with the same part number would have differing rank configurations.

In the "olden days" rank was identified by the specification "single sided" (single rank) or "dual sided" (dual rank) specification. All the ICs (the big black squares) being on one side (single sided) or split between two sides (dual sided). But modern manufacturing techniques allowed both single and dual sided to be either single rank or dual rank. The "sided" specification is sometimes still used, but what it actually indicated is ambiguous now.

 

It sounds like you are right. The first revision was from Feb 2012 (dual rank) and the latest from Sept 2016 (single rank).

The question is: should I toss the old sticks and buy another two new ones? RAM is cheap, but it sounds like this doesn't matter too much. The wiki seems to indicate single rank should technically be faster, but is it actually?

 

Quote:

Originally Posted by Celeras

It sounds like you are right. The first revision was from Feb 2012 (dual rank) and the latest from Sept 2016 (single rank).

The question is: should I toss the old sticks and buy another two new ones? RAM is cheap, but it sounds like this doesn't matter too much. The wiki seems to indicate single rank should technically be faster, but is it actually?

If you buy another two sticks, hoping that the match the original two, which there is no guarantee of, you may end up worse off than you are now.
"Fool me once, shame on you. Fool me twice, shame on me."

To see if or how much of a performance hit there is you could run a memory intensive benchmark and compare the results with just the original sticks installed to the results with just the new sticks installed. SuperPi would be my choice.

Once you know that you can decide if replacing memory is the way to go. Then the issue whether to buy a 4 x 8 Gig matched set or to try your luck again with another 2 x 8 Gig matched set.

You haven't said what brand/model memory this is. Some memory manufacturers will add a revision number to the part number when they change the components used. Corsair does this and even identifies which ICs were used with which revision . The problem is that the revision number is only seen on the sticker on the sticks, not on the retail packaging, so, if you can't see the sticks themselves before you buy it's still a crap shoot.

Pros and cons:
Keep what you have -
Pro - Free
Con - may be a performance hit

Buy 2 x 8 Gigs
Pro - may match
Con - doubles you cost (perhaps sell the unused 2 x 8 gig sticks)
Con - may not match, heck, may not even be bootable!

Buy 4 x 8 Gigs
Pro - Be assured of a match
Con - triples you cost (again sell the 4 x 8 Gigs you have, but you should sell then a two matched sets not a 4 stick matched set)

Good luck on whatever you decide to do..............