Comment by scottlamb | Hacker News Reader

Comment by scottlamb | Hacker News Reader

Are these host-managed SMR HDDs available for individual purchase, or are they enterprise-only? Last time I looked it seemed like the latter. I'd be curious to know if anyone has found a way to purchase them individually, and if they're significantly cheaper or higher-capacity than the closest equivalent PMR drive.

I've played with device-managed SMR drives and decided I hate them. Too much I/O happening behind my back, so a low workload they can sustain before hitting a horrible performance cliff. Also, in my very limited sample, poor reliability even if you stay within the rated workload. (2/2 drives that were in continuous use started accumulating bad sectors within a couple years.)

In theory, host-managed SMR drives with the right software might have much less write multiplication. The NVR software I'm working on more or less uses them as a big ring buffer (or several big ring buffers, one per stream) which seems ideal. But I suspect getting host-managed SMR to work optimally isn't worth the effort unless you have datacenters full of drives.

It would be enough if there was a way to disable device managed mode. Then the drive would still work on legacy machines, but if the host supports it, it could perform better.

> It would be enough if there was a way to disable device managed mode.

They exists, and they are called host-aware SMR drives. Sadly, device-managed drives exists because drive manufacturers want to both achieve cost-cutting and increased profits (and was relatively unknown until the whole WD Red (and companions) DMSMR brouhaha last year forced every hard disk manufacturer to state if the drive is DMSMR or not).

Do you have an example model available for purchase by individuals?

Sorry for this late reply but as I knew it it is exclusively an enterprise product (I know, it should be available also to prosumers).

Host-managed SMR is indeed not available in consumer drives yet FAFAIK. They're all drive-managed SMR, which is the worst of both worlds.

I wonder if a single bad bit could permanently destroy a whole ~16MiB sector? So SMR drives would need significantly more reserves to compensate for that.

Shingled drives can still be read sector-by-sector. The shingling just changes how the data is written; it doesn't change how the data is read.

So no, a single bad bit would only affect a single sector. And in fact, a single bad bit wouldn't do any harm, as all drives use error correcting codes that can handle a lot more than a single bit going bad. You'd actually lose a whole sector at time (512 bytes or 4096 bytes, depending on the drive).

The difference between a regular and a SMR drive is that the SMR drives have a narrower read head compared to the write head.

This means that when writing data it overwrites neighboring tracks, and hence writing is partially overlapped like shingles.

Apart from that they're more or less the same as normal drives as far as I know, firmware aside. So a single bad bit shouldn't affect more than in a non-SMR drive.

Am I mistaken? If so I would appreciate a correction.

I based my post primarily on what I recalled from this talk[1] by a HGST engineer.

[1]: https://youtu.be/a2lnMxMUxyc?t=556

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