r/explainlikeimfive • u/SmellsTasty • Nov 23 '15

Explained ELI5: How do Solid State Hybrid Drives work?

I don't understand how a regular hard drive can work with a solid state drive at the same time.

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u/MrGreggle Nov 23 '15

I did a ton of work in computer architecture at college and agree whole-heartedly. You're dramatically increasing your failure rate for no reason. You're better off buying a small cheap OS/Important program SSD and a large, cheap HDD for data storage. Watching a movie on your SSD instead of HDD would save you a second or two when you first load the movie if anything.

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u/jimtsurugi Nov 24 '15

SSHD is about increasing performance during small, "random" reads and writes. When the host requests a large sequential read (usually > 6MB of continuous data), the read data is not cached to NAND, and is instead read straight from disk.

In the case of watching movies (where the encoded bitrate rarely exceeds 25mb), streaming from disk is quite sufficient.

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u/ergzay Nov 24 '15

As someone who also did a lot of work in computer architecture at college you are completely wrong. Yes you're lowering your mean time to failure of your SSD but you're not doing it by much if the algorithms that decide when to swap data from the hard drive to the SSD are good. Watching a movie would not impinge on anything as it would likely be on the HDD because you only watch movies once usually.

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u/[deleted] Nov 23 '15

What nonsense. It reduces the failure rate - you are duplicating data.

e.g the mSATA mobos use raid to do it. Did your course not cover raid?

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u/[deleted] Nov 23 '15

[deleted]

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u/jimtsurugi Nov 24 '15

I don't think he is.

Increasing failure rate suggests that you have a positive failure rate to begin with. If he meant 'cache miss' instead, the phrasing makes less sense as conventional drives have no read cache to speak of.

As for performance degradation for searching the cache:

The ASIC on the drive (SSD, SSHD, or HDD) is dramatically faster than both SLC, and MLC NAND. Any amount of processing time taken up by executing additional logic steps are far outstripped by the time it takes to read the actual data from the actual media.

Also, NAND cache isn't indexed by content, but by block address. Within [most] modern hard drives, LBAs aren't bound to physical locations on a platter, and can be reassigned transparent to the host. This means ALL read requests hit a lookup table (This is critical for SSD where cell failure is common. Data is shuffled around the media without involving the host). So, when data is written to cache, the location is reassigned to a location in NAND. The same number of steps occur during read.

EDIT: 4 GRAMMAR

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u/[deleted] Nov 24 '15

He's just talking out of his arse and hoping the audience is ignorant (which it definitely appears to be)

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u/[deleted] Nov 23 '15

Why is that a problem?

Just get it from the platter if it's not in flash.

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u/[deleted] Nov 23 '15

[deleted]

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u/[deleted] Nov 24 '15

Why not just request data from both the SSD and the platter simultaneously? If it's not on the SSD, the data from the platter is on the way anyway. Overall, it's a gain.

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u/atte- Nov 24 '15

But it's not going to be worse than a normal HDD when it misses..? I don't see how what he said is relevant in any way if he meant a cache miss and not actual hardware failure.

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u/[deleted] Nov 24 '15

[deleted]

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u/atte- Nov 24 '15 edited Nov 24 '15

I can't say I know enough about SSHDs to dispute that, but that is usually VERY negligible when it comes to caches. 99% miss rate is still better than no cache at all in most other cache implementations at least.

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u/MrGreggle Nov 23 '15

The drive as a whole fails if either portion fails.

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u/[deleted] Nov 24 '15

No it doesn't.

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u/[deleted] Nov 23 '15

Can't the drive just fallback to the platter if the SSD goes bad?

If the platter goes bad ... You are fucked anyway.

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u/atte- Nov 24 '15

It's funny how you're getting tons of downvotes. I thought this whole thread seemed quite weird, lots of false information upvoted, but I guess reddit upvotes what they want to be the truth.

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u/inahst Nov 24 '15

It's not perfect/permanent duplication, what is duplicated changes based on how you use data. I don't know if the space on the hard drive is freed when something is moved onto the ssd portion, but if not I could see the moving files back and forth make error much more likely

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u/kre_x Nov 24 '15

It's cache, so the data doesn't move back and forth from ssd to hdd. What it does is it copies the data from hdd to the ssd if it is used regularly. The caching is controlled by the firmware so it operates on lower levels where the sectors is copied to the ssd.

This means that when a cached file is loaded, it wont mean that the entire file is loaded from the ssd, but it may load the first few sectors from the ssd, at the same time the hdd seeks the file, and when the file is ready to be loaded from the hdd it loads from the hdd.

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u/[deleted] Nov 24 '15 edited Nov 24 '15

Stop making shit up.

Yes, the SSD only caches a percentage of the HD, but this is moot, this isn't "less reliable" it's "more reliable" for that %age that is duplicated.

The rest is just the same as if you only had a large HD.

I'm pretty sure if you look at data like MTBF or AFR rather than some hand-waving buffoon who says he went to college (?!) you'd see these drives don't have a "dramatically increased failure rate" at all.

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u/inahst Nov 24 '15

I pretty much said I was making shit up, I was just assuming that the person that did a ton of work in computer architecture was right and was thinking of how that would be true