Wandering Thoughts archives


About DRAM-less SSDs and whether that matters to us

Over on the Fediverse, I grumbled about trying to find SATA SSDs for server OS drives:

Trends I do not like: apparently approximately everyone is making their non-Enterprise ($$$) SATA SSDs be kind of terrible these days, while everyone's eyes are on NVMe. We still use plenty of SATA SSDs in our servers and we don't want to get stuck with terrible slow 'DRAM-less' (QLC) designs. But even reputable manufacturers are nerfing their SATA SSDs into these monsters.

(By the '(QLC)' bit I meant SATA SSDs that were both DRAM-less and used QLC flash, which is generally not as good as other flash cell technology but is apparently cheaper. The two don't have to go together, but if you're trying to make a cheap design you might as well go all the way.)

In a reply to that post, @cesarb noted that the SSD DRAM is most important for caching internal metadata, and shared links to Sabrent's "DRAM & HMB" and Phison's "NAND Flash 101: Host Memory Buffer", both of which cover this issue from the perspective of NVMe SSDs.

All SSDs need to use (and maintain) metadata that tracks things like where logical blocks are in the physical flash, what parts of physical flash can be written to right now, and how many writes each chunk of flash has had for wear leveling (since flash can only be written to so many times). The master version of this information must be maintained in flash or other durable storage, but an old fashioned conventional SSD with DRAM had some amount of DRAM that was used in large part to cache this information for fast access and perhaps fast bulk updating before it was flushed to flash. A DRAMless SSD still needs to access and use this metadata, but it can only hold a small amount of it in the controller's internal memory, which means it must spend more time reading and re-reading bits of metadata from flash and may not have as comprehensive a view of things like wear leveling or the best ready to write flash space.

Because they're PCIe devices, DRAMless NVMe SSDs can borrow some amount of host RAM from the host (your computer), much like some or perhaps all integrated graphics 'cards' (which are also nominally PCIe devices) borrow host RAM to use for GPU purposes (the NVMe "Host Memory Buffer (HMB)" of the links). This option isn't available to SATA (or SAS) SSDs, which are entirely on their own. The operating system generally caches data read from disk and will often buffer data written before sending it to the disk in bulk, but it can't help with the SSD's internal metadata.

(DRAMless NVMe drives with a HMB aren't out of the woods, since I believe the HMB size is typically much smaller than the amount of DRAM that would be on a good NVMe drive. There's an interesting looking academic article from 2020, HMB in DRAM-less NVMe SSDs: Their usage and effects on performance (also).)

How much the limited amount of metadata affects the drive's performance depends on what you're doing, based on both anecdotes and Sabrent's and Phison's articles. It seems that the more internal metadata whatever you're doing needs, the worse off you are. The easily visible case is widely distributed random reads, where a DRAMless controller will apparently spend a visible amount of time pulling metadata off the flash in order to find where those random logical blocks are (enough so that it clearly affects SATA SSD latency, per the Sabrent article). Anecdotally, some DRAMless SATA SSDs can experience terrible write performance under the right (or wrong) circumstances and actually wind up performing worse than HDDs.

Our typical server doesn't need much disk space for its system disk (well, the mirrored pair that we almost always use); even a generous Ubuntu install barely reaches 30 GBytes. With automatic weekly TRIMs of all unused space (cf), the SSDs will hopefully easily be able to find free space during writes and not feel too much metadata pressure then, and random reads will hopefully mostly be handled by Linux's in RAM disk cache. So I'm willing to believe that a competently implemented DRAMless SATA SSD could perform reasonably for us. One of the problems with this theory is finding such a 'competently implemented' SATA SSD, since the reason that SSD vendors are going DRAMless on SATA SSDs (and even NVMe drives) is to cut costs and corners. A competent, well performing implementation is a cost too.

PS: I suspect there's no theoretical obstacle to a U.2 form factor NVMe drive being DRAMless and using a Host Memory Buffer over its PCIe connection. In practice U.2 drives are explicitly supposed to be hot-swappable and I wouldn't really want to do that with a HMB, so I suspect DRAM-less NVMe drives with HMB are all M.2 in practice.

(I also have worries about how well the HMB is protected from stray host writes to that RAM, and how much the NVMe disk is just trusting that it hasn't gotten corrupted. Corrupting internal flash metadata through OS faults or other problems seems like a great way to have a very bad day.)

tech/SSDsUnderstandingDramless written at 23:15:41; Add Comment

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