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I was just reading a question on backups with flash drives, but I was not quite sure why SSDs would be better than flash drives for backups.
To me, solid-state drives look like larger versions of flash drives. But surely there must be something that makes the former much more reliable than the latter.
What's the difference between an SSD and just a regular flash drive? For example, do they both use NAND?
oldmud0
Posted 2015-05-24T23:41:00.450
Reputation: 3 858
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Both Flash and SSD are based on NAND-based flash memory, which retains data without power, and so can be labelled as Flash memory.
Technologically, the main differences between the two are in :
NAND technologies
NAND technologies diverge on two points : Speed and price.
On the one hand one finds MLC (Multi-level cell) which is a memory element capable of storing more than a single bit of information. Most MLC NAND flash memory has four possible states per cell (or even more with TLC), so it can store several bits of information per cell. This reduces the number of required transistors, so reducing size and manufacturing costs, while also reducing speed and increasing the possibility of errors.
On the other hand one finds SLC (single-level cell), where each cell can exist in one of two states, storing one bit of information per cell. This increases the access speed, while also increasing manufacturing costs and electricity usage.
An MLC cell is typically rated at 10,000 erase/write cycles, while an SLC cell might last 10 times that before failing.
Because of these differences, MLC is typically used in slower and cheaper media, accessed typically via USB. A good SSD will use SLC and be costlier, but faster, have a longer life-time and be typically accessed via SATA 2 or 3.
Memory controller
A USB mass storage controller has only a small micro-controller with a small amount of on-chip ROM and RAM.
An SSD controller is much more complicated. The controller is an embedded processor that executes firmware-level code and is one of the most important factors of SSD performance. Some of the functions performed by the controller include:
In a hybrid SSD, the controller will also manage a small classical hard disk.
Connector
A flash stick normally uses a standard-A USB plug that provides the physical interface to the host computer. These can now go up to USB-3 speeds for the more costly models, or USB-2 for the common ones.
SSD technology uses electronic interfaces compatible with traditional block input/output (I/O) internal hard disk drives. Additionally, new I/O interfaces, like SATA Express, have been designed to address specific requirements of the SSD technology. Most SSD cards are typically much faster than classical hard drives.
Summary
A Flash stick typically has less memory capacity, is slower, cheaper and is also less dependable than an SSD.
There are of course always devices that bridge these differences by using compensating technologies.
References :
harrymc
Posted 2015-05-24T23:41:00.450
Reputation: 306 093
+1 But not all, but most, flash and SSD storage are NAND based – Keltari – 2015-06-28T19:23:21.883
@Keltari Can you give me an example of an actual SSD product that is not using NAND? I am curious, because I have never seen one. What kind of capacity would a NOR based SSD have? I mean I know that NOR based flash memory exists. It's been invented by Intel in 1988. But currently, all SSD drives I know of are using NAND flash, which was invented by Toshiba in 1989. We know that NAND based SSD drives are leading the way, in terms of low cost and high capacity. I don't think NOR based SSD will catch on anytime soon. Thus my question, to have a look at a finished NOR based SSD product. – Samir – 2015-06-28T19:58:22.540
3Errata: Intel was first to introduce a NOR based flash chip in 1988, and Toshiba followed in 1989 with its NAND based chip. It was invented earlier than that though, by Dr. Fujio Masuoka and his team at Toshiba, and first presented in 1984. – Samir – 2015-06-28T20:15:01.643
Very nice answer! Took the highest-repped person in SU to write it, too ;) – oldmud0 – 2015-06-29T01:37:43.303
According to the wikipedia article for CompactFlash, they were originally NOR flash, but have since switched to NAND. – Brian Minton – 2015-11-23T14:50:09.350
The summary itself was worth a +1 for me! – josh – 2016-08-01T15:36:29.100
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pyrocrasty
Posted 2015-05-24T23:41:00.450
Reputation: 1 332
1DRAM is certainly faster than NAND, but is volatile even when powered and hence needs refreshing. – sawdust – 2015-05-25T05:06:10.023
@sawdust: Yes. I can't tell if you're just elaborating or if you disagree with something I said. (I only ask because someone downvoted this answer.) – pyrocrasty – 2015-05-25T14:37:05.553
Both. I downvoted your answer because DRAM is not used in SSDs for non-volatile storage as you wrote. – sawdust – 2015-05-25T18:28:54.883
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I didn't claim they were used for nonvolatile storage. I said they were used in some more expensive SSDs, which is true. See here under 'ultrafast rackmount SSDs': "In today's market the fastest rackmount SSDs on the SAN are always RAM SSDs.". Another article here.
– pyrocrasty – 2015-05-25T19:03:42.760SSD in general mean any drive with no moving parts so the RAM is indeed SSD , but you can't compare RAM to flash drive so he definitely mean the non-volatile SSD and so your answer is wrong and i think you are confused between these two SSDs – Robert – 2016-03-27T18:41:30.450
@sawdust i don't think they will ever try to do that with DRAM , if they want to make super fast non-volatile from volatile one ( and give it continues power ) , then it must be the SRAM not the DRAM , it's need less power and a lot faster – Robert – 2016-03-27T18:47:30.717
@robert: The question asked whether both SSD and flash drives use NAND. I just answered it. I don't see any reason to assume the OP meant to specifically restrict the question to non-volatile SSDs. Even if they did, I can't see how my being "confused about these two SSDs" would follow. – pyrocrasty – 2016-03-27T19:05:13.527
SSD refer to two different things , first is the old and general term solid state drive which is any storage that doesn't have any moving parts but that's are not used any more , the second is the non-volatile SSD storage which is the question about – Robert – 2016-03-27T20:18:46.477
@robert: No... it doesn't. The term is used most often to refer to non-volatile SSD drives, but that doesn't mean there's some separate restrictive meaning in play. – pyrocrasty – 2016-03-27T20:37:01.247
that's what i meant , so DRAM has nothing to do with SSD – Robert – 2016-03-28T00:10:50.503
@pyrocrasty -- I had forgotten about a board in the 3.5" form-factor that used DRAM modules as a RAM-based disk. FWIW I've used (i.e. written drivers for) battery-backed SRAM as non-volatile storage devices. If you edit your answer and elaborate/clarify on what you mean by "DRAM", I'll remove my (hasty) downvote. As it stands, the answer could be misunderstood, e.g. as if SRAM and DRAM drives are available in 2.5" form factor just like NAND flash (and platter) drives. – sawdust – 2016-03-28T02:38:16.010
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There are a few articles out there about the differences between SSDs and Flash drives.
https://danielmiessler.com/blog/the-difference-between-ssd-and-flash-hard-drives/ http://www.mynetworks.me/2010/12/20/ssd-solid-state-drive-vs-flash-drive-usb-drive/
Alex Atkinson
Posted 2015-05-24T23:41:00.450
Reputation: 2 845
1The points don't sound logically correct, though. For example, #1 is not a difference since hard drives use platters whereas SSDs use Flash memory, more of a digression to the difference between SSDs and HDDs. – oldmud0 – 2015-05-25T13:48:37.940
However, the second article is very useful. – oldmud0 – 2015-05-25T13:50:22.323
"...compare Flash to SSD ... latter is a type of the former". That's the wrong way around. You mean "SSD to Flash". – pyrocrasty – 2015-05-25T14:30:18.483
@pyrocrasty No, not really. At its core, SSD is a form of flash based storage device. Without the key component, there would not be any SSD to talk about. Just like with the Li-ion, which is a form of battery. So no, it's "latter is a type of the former". In other words, SSD is to flash based memory what Li-ion batteries are to... well, batteries. Don't confuse the generic term "flash drive" (as in USB flash drive) with "Flash" (flash memory). – Samir – 2015-06-28T20:22:35.937
@sammyg: Flash is not the only type of solid state memory. At the very least, memory must be non-volatile to be called flash memory. However, volatile solid-state memory (RAM) also exists (and is sometimes used in hard drives, for that matter). Any flash-based drive is an SSD, but not all SSDs are flash-based. – pyrocrasty – 2015-06-29T23:56:51.890
But that is just too broad for a definition of what SSD is. Keep this up and you might as well start calling it "memory" or break it down to "atoms". I was referring to the current use of the term SSD. So... magnetic tape is non-volatile... does that make it an SSD? Lack of moving parts (solid) and ability to retain data when powered off is not what makes it an SSD. Please... show me an SSD product in any category (consumet, enterprise, SATA, SAS, PCI-E, ...) that is not using Flash memory? – Samir – 2015-06-30T00:52:01.070
@sammyg: Whether it's volatile or not has nothing to do with being solid-state. Here are a couple of articles on DRAM-based storage.
– pyrocrasty – 2015-07-02T08:10:56.530@sammyg (cont): Here is an article on a specific line of DRAM disks (ZeusRAM). I think the ZeusRAM disks would be classed as "hybrid" SSDs: they use DDR memory for day-to-day operation but also have Flash-based backup memory built in - and enough power stored in capacitors to dump the disk contents to Flash in case of power loss.
– pyrocrasty – 2015-07-02T08:11:19.470@pyrocrasty Oh you got me! ;-) I don't know what I was thinking. Erase the "PCI-E" part from my question, and the "not using Flash" part. The articles you linked to are very interesting, and those are some interesting products. But those are RAM disks! At least in my mind, an SSD must use some type of conventional HDD interface like SATA or SAS, and use non-volatile, Flash based chips. I have hanged up entirely on the term "SSD" or "solid state disk" here, not so much on the solidity (solid nature) of it. – Samir – 2015-07-02T22:45:31.383
@pyrocrasty I believe this is a marketing term introduced only recently to describe the new kind of "HDD" so that regular people would sort of "get it" (understand what it is). If you go back in history I don't think you will encounter this term anywhere. Yes, if you are reading the news articles and ads, you will see DRAM based products referred to as "SSD". Even some books and articles on the subject will refer to primary storage devices from 1970s and 80s as "SSD". But this term is used in retrospect. This is not what the people of the day would have described it as. – Samir – 2015-07-02T22:47:19.987
@pyrocrasty So I am questioning the technical term for this type of device, not the solid nature of it. I believe the correct term for them was "RAM disk" back in the 1990s at least. But they were volatile, and they were too expensive and impractical for everyday use in the consumer market. They were specialty products for high-end servers. But in retrospect, I would consider them as solid state disks, of course. It's just that it's not what we called them. – Samir – 2015-07-02T22:50:54.007
@pyrocrasty My guess is that it's with Flash based RAM disks that plug into PCI-E slots that the term "solid state disk" was coined, which eventually took the form factor of a 3.5 inch and 2.5 inch HDD kind of shiny metallic box, which is now seeing a continued drop in prices and increase in adaptation rate among consumers and businesses alike and are slowly taking over from HDD disks and becoming mainstream tech. But of course, DRAM based "solid state disks" (which I call RAM disks) are still superior to SSD (Flash based) in terms of performance. – Samir – 2015-07-02T22:58:08.060
Battery backed ram disks have been around a while, and still count as non-volatile, and solid-state. For instance, http://www.thessdreview.com/our-reviews/allone-cloud-disk-drive-101-ramdisk-review-500k-iops-ddr3-storage/ is a review of a 32GB one. As @pyrocrasty stated, it's solid state because of no moving parts. And yes, I think they do write the RAM contents to some flash-based media in the case of a power failure.
– Brian Minton – 2015-11-23T14:49:47.730
3Not a definitive answer: speed and wear-levelling technology. On the latter, flash-drive cells have a limited number of times they can be written to; so SSDs are designed with heavier write-loads than are typical flash drives. They achieve this by having redundant writable cells and algorithms to rotate between these cells. Having said that, there's no technical reason flash-drives can't have these. – Otheus – 2015-05-24T23:58:40.780
1And I would like to know (in addition to question) whether there is a difference between NAND flash storage in Android phones and the SSD? – Firelord – 2015-05-25T00:37:09.077
Differences are and not limited to the quality of the flash memory cells and the memory controller. Usb flash drives typically use the lowest quality flash memory. – Moab – 2015-05-25T13:56:33.097