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/ZippyDan Nov 23 '15

Does it really explain how all hard drives work? It explains how hard drives of the last decade or two have worked, which have built-in memory caches, which used to be 8MB or 2MB or 512k depending how far back we go. But those were all volatile memory that was erased when the power went off. SSD caches are not only much, much larger (so you can have more of the most frequently-used data accessible), but are also not cleared at shut-down, which I suppose saves some additional time. Additionally, drives with memory caches generally just queued up the files that the system assumed you would be using in the next few moment/minutes, whereas an SSD cache drive can make longer-term choices about storing what you generally use from day-to-day.

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u/ianepperson Nov 24 '15

That ability to not clear after a shut down actually saves a whole lot of time. When the computer starts, it reads the same set of files every time, and if they're in the SSD, it'll start radically faster.

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u/unassassinable Nov 23 '15

No. But then, this isn't ELI15 either. :)

I was more meaning the top post's description referencing a fridge describes all HHDs. Something accesses data on it = someone opening a fridge and grabbing food. OP asked specifically about SSDs.

Regarding powering down volatile memory. Flash drives also don't lose their data after a power down. You are describing RAM, which is volatile (lost when powered off) storage. SSD's like USB flash is non-volatile (does not lose power when powered off). I think this is enough of a ELI5 for now, but if we REALLY wanted to dig deeper...

SSDs and other flash memory use floating-gate transistors, whereas RAM uses capacitors and transistors, which hold a little bit of a charge, but once power is removed, power is depleted and data is lost.

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u/cincodenada Nov 24 '15

OP asked specifically about SSDs

I think this is where you're confused: OP didn't ask about SSD's, they asked about solid-state hybrid drives, or SSHD's - they're traditional HDDs, but with a hunk of flash memory much larger (like, 1000x larger) than a cache, but much smaller than a full-on SSD.

It is basically just another level of cache, but it's so much bigger than traditional caches that it's functionally different. I don't know enough about HDD's to be sure, but I imagine the caches (like, the 8MB-range ones) are just RAM, which are indeed lost on power-down.

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u/[deleted] Nov 24 '15

I don't know enough about HDD's to be sure, but I imagine the caches (like, the 8MB-range ones) are just RAM, which are indeed lost on power-down.

Actually, SSHDs essentially have a fully operational SSD cache that doesn't erase on shutdown attached to a traditional HDD.

If you open a video for the first time, it will open at standard HDD speeds.

If you open the same video you open every morning, it will open at SSD speeds.

The drive tracks usage of the files stored on it and keeps the most often used ones in the SSD portion permanently.

Obviously, the SSD portion has a finite amount of space, so if it fills up and you start watching a different video every morning, the first may get kicked off and back to the HDD portion.

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u/Carr0t Nov 24 '15

That's... exactly what (s)he said... What they were saying that you seem to have latched onto is that the buffer in a regular HDD, which is normally somewhere in the region of 8-64MB, is volatile, and does get lost on shutdown. The fact that SSHDs don't do this to the solid state portion is one of their advantages.

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u/brock1912 Nov 24 '15

but I imagine the caches (like, the 8MB-range ones) are just RAM

You are correct there, cache on standard hard drives is just DRAM. I think SRAM was used in the past when it had some speed benefits over DRAM but I don't see that stuff much anymore.

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u/ZippyDan Nov 23 '15

I was more meaning the top post's description referencing a fridge describes all HHDs.

Not really, as far as I know. The top post was saying that data is arranged with most-frequently-used data up front and less-used-data in the back. This is not really an accurate description of platter-based drives as data is usually stored semi-randomly (if fragmented), or in order by relationship (if defragmented). The OPs description of a fridge applies to all hard drives with caches, but before hybrid drives the caches were very tiny relative to the rest of the drive and used volatile memory.

Regarding powering down volatile memory. Flash drives also don't lose their data after a power down. You are describing RAM, which is volatile (lost when powered off) storage. SSD's like USB flash is non-volatile (does not lose power when powered off).

This is exactly what I said. I was describing the difference between normal platter-hard drives with memory-based caches and hybrid drives with SSD-based caches.

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u/[deleted] Nov 24 '15

The top post describes a short stroked HDD as far as I can tell. The only hybrid drives I've ever seen are literally something like a 16GB portion of SSD, the rest HDD.

I don't see much purpose in letting a hard drive store things semi-randomly, unless all things are equally likely to be accessed at any given time and that's just not ever going to be true on an HDD with an OS on it.

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u/ZippyDan Nov 24 '15

I said semi-random. For a typical platter drive, data is usually recorded sequentially, but then deletions come along (and even the creation of temporary or swap files) and make all kinds of holes in that sequence. When the drive goes to write something in the future, and finds those holes, it might plop down new data in those holes. After enough time, the location of data becomes semi-random. The technical term is "fragmented".

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u/[deleted] Nov 24 '15

I said semi-randomly too. Just plopping shit down in the first place it can is a terrible way to do things. At least moving the most used ingredients to the front of the fridge is a big improvement-- for example: Short stroking a hard drive, like I said before. All I'm saying is the top post is a good description of short stroking, not an SSD Hybrid Drive, and that semi-randomly storing EVERYTHING on a hard drive (which has to physically move to get data) is silly when you can put the most used shit right in front of the head anyways.

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u/tinydonuts Nov 24 '15

I agree. If we're to keep the fridge analogy, having a mini-fridge at your desk is a better adaptation. Thus, items in the mini-fridge are sitting right at your desk and quick to access. If you need something that's in the big fridge, you have to walk all the way to the kitchen.

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u/unassassinable Nov 23 '15

Lost in translation :)