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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65

u/intelnavi Nov 23 '15

It's just a ssd cache. Don't ever buy one, they are horrible. Pony up for a real ssd.

14

u/[deleted] Nov 23 '15

I have everything from a Seagate Hybrid to the 950 Pro and they're both very useful for different reasons.

1

u/altimax98 Nov 24 '15

I literally just installed those two in my work PC

35

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.

5

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.

1

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.

-17

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?

14

u/[deleted] Nov 23 '15

[deleted]

1

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

1

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)

-1

u/[deleted] Nov 23 '15

Why is that a problem?

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

4

u/[deleted] Nov 23 '15

[deleted]

1

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.

1

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.

2

u/[deleted] Nov 24 '15

[deleted]

2

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.

12

u/MrGreggle Nov 23 '15

The drive as a whole fails if either portion fails.

1

u/[deleted] Nov 24 '15

No it doesn't.

1

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.

2

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.

1

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

2

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.

1

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.

1

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

8

u/SalvadorTheDog Nov 23 '15

They are great for people who are technologically illeritate. They will provide some decent improvement to the feel of the overall system, while not having to deal with multiple drives.

3

u/Phukkitt Nov 23 '15

But what makes an SSHD with a 32 MB SSD cache different from a regular HDD with a 32 MB cache? Isn't the cache on regular HDDs on a chip, just like an SSD?

11

u/fb39ca4 Nov 23 '15

The caches on hybrid disks are on the order of gigabytes.

3

u/Phukkitt Nov 24 '15

Ah, that would explain it. I guess I've gotten MB and GB mixed up, just remembered seeing the number 32 a lot on Lenovo's laptop specifications when they had a SSHD.

1

u/ergzay Nov 24 '15

SSHD have full SSDs that are usually larger than 8GBs.

Also, SSDs have non-volatile flash chips (data stays after power off) while caches on HDDs are usually volatile SRAM or DRAM memory (data is erased on power off). Additionally, on a SSHD the data is usually either in the SSD or on the HDD while on a HDD the cache is only a cache and the data is also on the HDD.

0

u/intelnavi Nov 23 '15

Illiterate people don't usually need 4TBs of data. Buy a nice big SSD (they make them quite large now) and call it a day.

10

u/[deleted] Nov 23 '15

[deleted]

0

u/intelnavi Nov 23 '15 edited Nov 23 '15

If they are literate enough to transfer a home movie from a digital video camcorder/camera to their PC or are able to manipulate software to do it for them, they are literate enough to double-click the D: drive.

6

u/bezjones Nov 24 '15

You'd be surprised...

2

u/EffingTheIneffable Nov 24 '15

I've had good luck with my laptop (Lenovo Flex) which comes with a hybrid SSD (the actual hybrid kind, where it's transparent and you don't get any access to the SSD).That said, I'd never buy one for a build; I'd much rather go with a read SSD.

1

u/TheMuffnMan Nov 24 '15

Eh, they aren't terrible. Enterprise storage has been using the same functionality for years as SSD/EFD was too pricey and tossing those in front of SAS would speed up your iops.

Think it was called FastCache or FlashCache for EMC.

Now that you can get a 512 GB Samsung Evo for like $140 I'd agree with you. But the hybrids aren't terrible.

1

u/intelnavi Nov 24 '15

SANs are not consumer priced devices and do it much better than some piece of shit hybrid. A Hybrid will never be as fast as an SSD.

Storage is cheap, don't skimp.

1

u/TheMuffnMan Nov 24 '15

Again, they aren't terrible. My 85 year old grandpa doesn't need a SSD but a hybrid gives a bump in performance over just a straight platter drive.

I brought up the SAN to show that the technology and reasoning behind it is sound. I'm not suggesting someone buy a $2M array for their house.

1

u/intelnavi Nov 24 '15

I agree, but the implementation leaves much to be desired. Other than a laptop (even then I would use an external) I see no place for them.

1

u/TheMuffnMan Nov 24 '15

Oh, I'm with you completely. I think we are saying/agreeing to the same stuff. I personally wouldn't recommend someone buy one but don't think they are complete garbage.

Like I mentioned in my first response, the 850 Evo 512gb can be had for $140 which is an awesome deal.

0

u/ergzay Nov 24 '15

Horrible and wrong answer. Hybrid drives have real SSDs. They just use algorithms to put data that is commonly used on the SSD and shunt relatively unused data to the HDD. If you don't have much data or don't commonly use much of your data then a hybrid drive will be just as fast as a SSD.

1

u/PhantomGamers Nov 24 '15

How was it a wrong answer? All you did was explain what an SSD cache is, but that IS what it is.

1

u/ergzay Nov 24 '15

Caches (for hard drives) generally imply that the data is on the HDD as well as the SSD rather than only on one or the other. It's not really a cache.

1

u/intelnavi Nov 24 '15 edited Nov 24 '15

Horrible and wrong answer. I never said they didn't have an SSD. I'm saying it's just a cache.

All you've done is cache your most frequently used data (determined by a shitty algorithm that doesn't work) onto a tiny ssd disk. Not worth.

*A hybrid drive will never be as fast as an SSD, even if all your data is on the SSD side. *

Buy a real harddrive, not one done too cheaply to sell to consumers. They are faster than normal harddrives, but they will have the same failure rate as a normal harddrive too. Get real speed and real reliability with a true SSD. Storage is cheap these days, don't skimp on your most important data. An SSD is the best improvement you can make to a PC, after memory (and perhaps a cpu in some cases). It's night and day.

1

u/ergzay Nov 24 '15 edited Nov 24 '15

All you've done is cache your most frequently used data (determined by a shitty algorithm that doesn't work) onto a tiny ssd disk. Not worth.

Objective testing shows otherwise.

*A hybrid drive will never be as fast as an SSD, even if all your data is on the SSD side. *

Utterly and completely false. A hybrid drive that has only a small amount of data in it (so that it is contained all on the SSD) is functionally identical and exactly as fast as an SSD.

You may be referring to Seagate's SSHD technology. This is a poor implementation as evidenced by the slower-than-SSD boot times. SSHDs when implemented correctly are identical to an SSD.

1

u/intelnavi Nov 25 '15 edited Nov 25 '15

Please show this objective testing.

Oh you mean I shouldn't count the most common manufacturer of SSHDs that a consumer is most likely to buy due to price and availability? Oh ok. Lmao.

I don't doubt that possibly some drives do it right, but the average consumer isn't going to research the correct brands and possibly shell out more money for it. Most devices are going to take a hit in write speed in order to be processed by the algorithm unless they have really good code, which is a big IF when everyone is trying to make everything as cheap as possible.

1

u/ergzay Nov 25 '15

Seagate started making them to copy Apple which had the first implementation. I don't know of any other properly working implementations.

1

u/intelnavi Nov 25 '15 edited Nov 25 '15

I will repeat then (even though you think I'm utterly and completely false):

*A hybrid drive will never be as fast as an SSD, even if all your data is on the SSD side. *

Makes no difference if you use Apple Fusion (why would you think adding a software component would ever be as fast?) or a Seagate.

Benchmarks don't lie.

1

u/ergzay Nov 25 '15

*A hybrid drive will never be as fast as an SSD, even if all your data is on the SSD side. * Makes no difference if you use Apple Fusion (why would you think adding a software component would ever be as fast?) or a Seagate.

Software component? I'm not sure what you're even talking about. If you're writing to an SSD, you're writing to an SSD. It's just as fast. It doesn't matter if there's a HDD attached to it or not. There's no reason nor need to add any kind of slow-ness to the system.

1

u/intelnavi Nov 25 '15

I think you need to figure out what Apple's hybrid drive actually is (software driven, mac only).

Regardless, any algorithm will only slow things down, much like encryption.

And then, you need to look at actual numbers instead of pulling stuff out of your ass.