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Comment by jeffbee | Hacker News Reader

SSD erase blocks vary, could be anywhere from 64KiB to several MiB. Their logical blocks (visible to the host operating system) are usually either 512 or 4096 bytes.

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NAND flash erase blocks passed 1MB a long time ago. For mainstream TLC NAND, 16 to 24 MB is currently typical, and QLC NAND has gone as high as 48 and 96 MB. NAND page sizes are usually 16kB, but often with support for faster partial page programming for the sake of 4kB operations. Logical block sizes of 512 bytes are purely a fiction for the sake of compatibility and SSDs don't actually track allocations at that granularity. They do track things at 4kB granularity even though that's not quite large enough to be a good fit for today's flash.

Where does one find information on erase granularity, and, like, doing the optimizations offered by NVMe ZNS on a "normal" SSD? I assume if you throw a sufficiently uncomplicated ZNS-like workload at them, they don't try to ruin your efforts?

I guess arguably this comes from the lack of availability of ZNS drives, especially ones that aren't outcompeted by a Samsung 980 series drive on workloads that don't exceed the 980's warranty's 600 drive writes over 5 years. And depending on what you're doing, you may even use such drives for the IO-heavy load and retire them after 90% of their TBW to something like fileserver duty.

For recent drives, if you're lucky the vendor has implemented the Preferred Write Alignment, Preferred Write Granularity, and corresponding Deallocate alignment and granularity reports, added in NVMe 1.4 (2019). Those values may not exactly correspond to the underlying NAND page and erase block sizes, but can probably be taken at face value for their intended purpose if the drive actually reports values larger than 4kB. Otherwise, finding that kind of low-level NAND specifications often comes down to trawling through TechInsights teardowns and conference proceedings (especially ISSCC, though what gets presented doesn't always match what goes into mass production).

A zoned-like workload that is append-only within large chunks and uses a Deallocate (Trim) or Write Zeros command to clear chunks should perform well even on standard SSDs. I don't think the exact zone size has much impact, as long as it's comfortably above the erase block size.

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