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The hidden downside of cheap SSDs no one warns you about
If you've ever seen NVMe SSD reviews online, you've probably noticed that some reviews mention these drives as having DRAM inside. And you've probably wondered—why would an SSD have RAM?
As it turns out, not only is it common, but it can be very important. Here's why.
Why would my SSD have RAM?
It's actually quite an important part of your SSD, if you happen to have it. And it actually serves a slightly different purpose than the RAM in your PC's RAM slots. Perhaps the most critical function of DRAM on an SSD is to store the "logical-to-physical" mapping table, or the translation layer. When your OS saves a file, it assigns it a logical address, but the SSD does not store that file linearly; instead, it scatters the data across various physical NAND flash chips to maximize speed. Without a dedicated DRAM cache, the SSD controller would be forced to search the slower NAND flash memory every single time it needed to locate a file, creating a bottleneck that would significantly increase latency and result in a sluggish user experience. Meanwhile, by keeping said map in that high-speed DRAM, the controller can look up data locations almost instantly.
DRAM in your SSD also serves as a vital buffer for write operations, acting as a sort of staging area for incoming data. Because NAND flash memory has strict rules regarding how data is written and erased, writing small chunks of data directly to the flash is inefficient and destructive. A DRAM cache allows the SSD to hold these small, random writes temporarily and group them into larger, sequential blocks before committing them to the permanent storage. This process, often referred to as write coalescing, drastically reduces "write amplification," a phenomenon where the drive performs more internal writes than what you actually requested. And by minimizing the frequency of these erase-and-write cycles, the DRAM cache protects the NAND cells from unnecessary wear.
Omitting DRAM reduces manufacturing costs (especially so these days), but its presence is the key differentiator that ensures an SSD operates with consistent high speed and endures years of heavy usage without premature failure.
How do DRAM-less SSDs work, then?
While it's a very good thing to have, it's also technically optional. Not that I recommend skipping it, especially if you can get something that actually has DRAM, but there are DRAM-less SSDs in the wild as well. These compensate for the lack of onboard memory primarily through a feature called Host Memory Buffer (HMB). Introduced with the NVMe 1.2 specification, HMB allows the SSD to borrow a tiny slice of your computer's system RAM—usually less than 100MB—to store the critical address mapping table. Instead of relying on a dedicated DRAM chip on the drive itself, the controller reaches out over the PCIe bus to access this "borrowed" map in your system memory.
This adds a slight amount of latency compared to onboard DRAM (which might or might not result in a performance penalty, depending on your setup), but it's significantly faster than reading the map from the SSD's own slow NAND flash, preventing the drive from stalling during complex file operations.
As far as writing to the drive goes, these rely on SLC caching for this. Modern SSDs typically use TLC or QLC flash, which stores multiple bits per cell and is relatively slow to write to. To compensate, the controller programs a portion of this storage to operate as Single-Level Cell (SLC) flash, which only stores one bit per cell but is much faster. Incoming data is written rapidly to this "pseudo-SLC" zone first, acting as a high-speed buffer. And during idle moments, the drive quietly moves this data to the permanent, slower storage areas.
If this cache fills up during a massive file transfer, the drive's speed will drop dramatically to the native speed of the slow flash (after all, the drive is making some serious gymnastics to make up for not having DRAM), but for most daily tasks, this combination of HMB and SLC caching might effectively mask some of the hardware's limitations.
Should I get a drive with DRAM?
It really comes down to what you're going to use the drive for, and your specific budget. The gap between DRAM-equipped and DRAM-less SSDs has narrowed in recent times, but that's almost certainly not lasting with the global RAM shortage about to have an effect on pretty much all of our electronics.
You should get a drive with DRAM if you're going to use it as your main drive. After all, your OS performs thousands of tiny background operations constantly. DRAM handles this small-file traffic much more smoothly, making the computer feel snappier. And because the DRAM cache absorbs many small writes, it reduces wear on the flash memory, generally allowing the drive to last longer.
If you just need a place to dump photos, movies, or documents, though, you won't notice the difference. Sequential read speeds (opening a large video file) are often just as fast on DRAM-less drives. Likewise, if you're reading this article at a time when DRAM-equipped SSDs have already started going up in price, then you don't have much of a choice other than going DRAM-less. These are some pretty wild times we're living in.
