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Comment by canada_dry | Hacker News Reader

Adding to this confusing debacle is how HDDs are now being marketed like running shoes: one type for running, one for squash, one for tennis, one for basketball...

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There are only maybe... 3 or 4 elements of HDD designs worth talking about.

* CMR/PMR drives -- Classic hard drive you've been buying for the last 30 years. Not much to say about it.

* SMR Drives -- "Shingled" has very poor write performance, but adds 20% more capacity at lower costs. For "write once, read many" workloads, like archival or backups. This is because "Shingled" drives write data on top of the old data, so that your data is physically overlapping at the microscopic level. Any write must read the old data, rewrite the old data, and then finally write the new data you're planning to store.

* 5400 RPM vs 7200 RPM -- Higher RPM drives are obsolete, because SSDs are faster. 7200 RPM is kind of standard, but 5400 exists for those who want less noise, less electricity usage, and are willing to put up with the lower performance.

That's pretty much it. "NAS" drives tend to have anti-vibration sensors and other such things, but those sensors don't really cost much and I don't really see why it deserves its own category.

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There only needs to be maybe, 3 kinds of hard drives made today. 7200 PMR drives, 5400 PMR drives, and finally 5400 SMR drives.

I don't know if there's any truth to this, but I've read NAS drives also declare themselves "bad" much more readily through SMART, so you replace them sooner.

The point is that the fault recovery is delegated up for handling at the array controller level, rather than being processed by the drive.

NAS drives will more readily return errors for individual IO commands. That's not the same thing as the drive saying it has failed generally, though primitive RAID controllers often treat any IO failure as a whole-drive failure.

It should just be a setting for whether a drive does 0 retries, 1 retry, 5 seconds of retrying, or longer. Not entirely different models.

It probably is a setting. And manufacturers simply make us pay $20-50 more for flipping a bit. I'd be stunned if they have entirely different firmware versions for different products.

I just wanted to add that SMR drives' performance is a bit more nuanced than you described. In most SMR drives, there is a PMR portion allocated as a cache. I've read the cache is usually in the range of 20-50G, depending on the size of the drive, but I'd love to know how accurate that is. So write performance is generally pretty good until you saturate that cache. Then, as you said, it becomes pretty atrocious.

> For "write once, read many" workloads, like archival or backups.

My backup drive gets written to nightly, but reads are very rare (only in case of data loss).

The key aspect of WORM here is that the data you write is not modified in-place or overwritten in part; it stays put until deleted. That's true of most backup strategies, whether or not they generate a lot of read traffic to compute incremental backups. The "read many" part of WORM is usually never about how many actual reads are performed, but about the capability to handle them better than multiple writes.

You are right but the guy you replied highlight a strong point, we want to write once very quickly just to archive something and chance of reading those is low. But we would need to write it. (He did it every night. I just archive it and recycle the storage in my NAS, but not as frequent as every night.). If I want archive reading in the very long term, should use tape or as you say WORM. But not for hard disk usage.

Low cost write quickly. That is the 5400 coming in but not this.

But the key is to tell us so we can choose. Not as scandal like to get such knowledge and still hiding.

"NAS" drives are typically set to not have a timeout mismatch for linux raid.

See here:

https://raid.wiki.kernel.org/index.php/Timeout_Mismatch

How do "surveillance" drives fit into that? They're supposed to be special for being reliable with a lot of writing, several orders or magnitude more the drive's capacity. Apparently normal hard drives aren't reliable enough for recording CCTV footage.

All hard drives have effectively infinite rewrite capability, unlike SSDs. I know I've seen surveillance drives before (WD Purple), but I never figured out what made them special compared to others.

Looking at the WD Purple drive marketing material... they describe "support for 64 cameras". Which means the CCTV community is willing to pay for write bandwidth. It'd probably be a standard 7200 PMR drive, but with assurances and maybe some tests on the write performance.

I've heard WD allude to their surveillance drives having some kind of firmware optimizations to handle multiple streams of writes with good QoS. It seems plausible they they would have a bit of a different write caching strategy based on minimizing seeks and trusting that the data coming into the drive is interleaved from multiple sequential write streams even though at any given moment the command queue will look like a bunch of random writes.

supposedly, part of the difference between the NAS drives and desktop drives (back when they first started segmenting consumer desktop and consumer NAS drives) is that the NAS drives have a much higher inactivity timeout before spinning down. It's supposed to help with longevity since spinning up/down wears out the drive faster and with performance coming back from an idle. I have no idea how much that actually matters or whether that's still true anymore.

10 000 RPM SAS drives absolutely still exists and are relevant. I got my last batch 2 months ago or so. Those SAS drives have nice capacity (2,4TB) and you don't need to worry about DWPD.

What kind of workload do you have where there are enough writes that a 1+ DWPD SSD isn't viable but the performance of a 10k RPM hard drive is adequate?

It would have to be a highly sequential workload for hard drive performance to not be a problem, but that means it's also a workload that's well-suited for QLC SSDs that are cheaper per GB up front and draw less than half the power and are faster in every way. If your worries about write endurance are at all realistic, you must be averaging upwards of 50MB/s of writes over the entire lifespan of the drive, meaning your hard drives are spending at least 20% of their service life actively writing data.

IIRC, PMR(perpendicular magnetic recording) technology is also used on SMR drives, so It's not correct term to differentiate SMR. And the technology have been used around 15 years.

CMR(conventional magnetic recording) is retronym to differentiate SMR.

Can you confidently confirm or cite a source that 7200 SMR doesn't exist? It doesn't sound like I can avoid buying from a sleazy company, but I at least want to avoid this SMR nightmare for a new 8-bay NAS build I was planning right up until all this broke.

I just have to know: which HDD for tennis?

2.5" PATA, more economical and less accidental head trauma.

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