So, did we agree that an SSD does NOT need to be fragmented, and that the OP's questions have been put to rest? Or, is this still a black hole that cannot be escaped from? It's very interesting, but meaningless. Since none of us know what is going on within the controller, speculation of this kind will remain just that: speculation.
Neither, because of that last sentence. SSDs can surely use plenty of tricks to get sequential access a little faster, but fragmentation, with SSDs, and probably future HDDs, for that matter, is a 3-layered problem:
1. FS fragmentation, which needs to modification of the FS to handle well (basically, defrag while writing new contents of files being overly-fragmented, or otherwise prevent it from getting too bad w/ some files).
2. Mapping structure complexity. Different SSDs do it different ways, and there's no one true method of arranging them. Sorted trees can often be compacted to be fast, but may get larger/deeper, and thus slower, when many small writes occur, as one hypothetical I can think of. Random-access type B-trees would have the same kinds issues, in that the device could very well slow down while being used, if the trees are always kept balanced. But, smaller structures would mean more of the mapping data being inside the controller, rather than out in DRAM. If that could be done, that itself might be a net win, compared to having to wait on DRAM, since even at a only several hundred MHz, SRAM accesses and ALU cycles are generally cheap compared to DRAM addressing. There's not a perfect solution here, and different methods will influence (but not truly
decide) the drive's random access performance.
As an aside, I wouldn't be surprised if some drives start using the pseudo-SLC modes available as a means of not using DRAM, and simply go from SRAM straight to pseudo-SLC space, as a means to save cost and power (or, using that to be able to use
less DRAM, and power it off when idle, without sacrificing as much performance as doing that with plain MLC or TLC would).
3. NAND v. LBA mapping (NAND fragmentation). Repeated accesses to different blocks and/or dies on the same flash channel are going to be slower than accessing another. But, simple RAID-like, or effectively random, channel mapping to LBAs is a must for performance with lager accesses or sparse accesses. Ideally, you will get one access to one channel, than the next to another, so they can be overlapping in the SSD itself. A deterministic pseudo-random layout, FI, could prevent the problem Lorne talks about, at the cost of lower plain sequential performance.