Comment by jorvi
2 days ago
It's not really the SSDs themselves that are incredibly fast (they still are somewhat), it's mostly the RAM cache and clever tricks to make TLC feel like SLC.
Most (cheap) SSDs their performance goes off a cliff once you hit the boundary of these tricks.
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It can be good to know that SSDs are fast until you exhaust the cache by copying gigs of files around.
It doesn’t hurt to be aware of the limitations even if for the common case things are much better.
We're talking about devices capable of >2GB/s throughput, and acquiring footage <.5GB/s. No caching issues, but I'm not buying el cheapo SSDs either. These are all rated and approved by camera makers. It wasn't brought up because it's not an issue. For people that are wondering why camera makers approve or not particular recording media, this is part of the answer. But nobody was asking that particular question and instead the reply tried to rain on my parade.
Not sure what warranted such an aggressive response.
I grew up in the 90s, on 56kb modems and PCs that rumbled and whined when you booted them up. I was at the tail end of using floppies.
I never said I didn't love the speed of SSDs, and when they just started to become mainstream it was an upgrade I did for everyone around me. I made my comment in part because you mentioned dumping 4K into the SSD and/or editing it. It can be a nasty surprise if you're doing something live, and suddenly your throughout plummets, everything starts to stutter and you have no idea why.
It wasn’t. IMHO the best feature and only real reason I hang around HN is exactly because I get to interact with others who know more.
The magic is in knowing how the trick works.
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> once you hit the boundary of these trick
Tell me more. When do I hit the boundary? What is perf before/after said boundary? What are the tricks?
Tell me something actionable. Educate me.
Your tone is quite odd here. I'm having difficulty parsing your intention, but I'm going to assume you're being genuine because why not.
For the RAM cache, you hit the boundaries when you exhaust the RAM cache. It performs faster, but is smaller and once full, data has to be off/loaded at the rate of the slower backing NAND. It might not be RAM, either, sometimes faster SLC NAND is used for the cache.
It's not really possible to describe it much more concretely than that beyond what you've already been told, performance falls off a cliff when that happens. How long "it" takes, what the level of performance is before and after, it all depends on the device.
There are many more tricks that SSD manufacturers use, but caching is the only one I know of related to speed so I'll leave the rest in the capable hands of Google.
Two of the main ones actually aren't really DRAM related but how full the drive is.
For most (all?) SSD drives they need a good 20% of the drive free for garbage collection and wear levelling. Going over this means it can't do this "asynchronously" and instead has to do it as things are written, which really impacts speed.
Then on top of that on cheaper flash like TLC and QLC the drive can go much faster by having free space "pretend" to be SLC and write it in a very "inefficient" size wise but fast method (think a bit like striped RAID0, but instead of data reliability issues you get with that it only works when you have extra space available). Once it hits a certain threshold it can't pretend anymore as it uses too much space to write in an inefficient fast way and has to write it in the proper format.
These things are additive too so on cheaper flash things get very very slow. Learnt this the hard way some years ago when it would barely write out at 50% of HDD speeds.
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Tone's hard to read in text, but I read the tone as "eagerly excited to learn more". I am also interested and appreciate your comment here.
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My tone is a combination of genuine curiosity and moderate annoyance at a dismissive but unhelpful comment.
RootsComment: SSD speed is miraculous! Jorvis: well ackshually is just RAM and tricks that run out Me: your comment provides zero value
I am annoyed by well ackshually comments. I’d love to learn more about SDD performance. How is the ram filled? How bad is perf when you cache miss? What’s worse case perf? What usage patterns are good or bad? So many interesting questions.
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Toms Hardware usually includes a "Sustained Write Performance and Cache Recovery" test
The test measures the write cache speed and the time to the fall to the native NAND write speed. There are usually irregularities in the sustained write speeds as well.
https://www.tomshardware.com/reviews/wd-black-sn850x-ssd-rev...
The other test I've seen is based on writing and using up free space, SSD performance can drop off as the free space fills up and garbage collection efficiency goes down. I think this impacts random writes particularly
In the enterprise space, drives tend to keep more over provisioned NAND free to maintain more consistent performance. Very early on the SSD timeline, it was advisable to only allocate 80% of consumer drives if you were using them outside of desktops and expected the workload to fill them.