[colug-432] SSD vs SD

[Matt Simmons]

Sorry, I missed "external USB SSD" at first. I don't believe that
external USB drives will be able to take advantage of TRIM, since TRIM
is a SATA/SAS command, and I don't think (?) that gets passed to USB
storage. If someone knows better one way or another, I'd love to
learn.

--Matt

On Sun, Mar 17, 2013 at 9:24 AM, Matt Simmons
<standalone.sysadmin at gmail.com> wrote:
> There are a lot.
>
> First, lets talk about what they have in common.
>
> Both use flash memory.
>
> OK, now lets talk about the differences.
>
> A MicroD card (or a USB thumb drive) have either a single flash chip
> on them, or a very small number of flash chips, probably not more than
> two. This is for a few reasons. 1) cost. More flash chips are
> obviously more expensive. 2) Controller complexity. Both the size and
> the cost need to be kept to a minimum. This means that the controllers
> talking to these chips are kept as simple as possible. More chips
> means more complexity and more physical size in many cases. So smaller
> is better all the way around.
>
> An SSD, however, doesn't have size as much of a physical constraint
> (at least comparatively). Also, cost becomes less important than other
> factors like performance, capacity, longevity, and so on. What you end
> up with is a device in the disk drive form factor that is actually
> very powerful.
>
> An SSD controller is very fast - the processors are up to 800mhz (and
> probably above in some models by now), and in the case of enterprise
> SSDs, have multiple cores.
>
> They also use an array of flash chips, several per bus, with multiple
> busses. Each individual flash chip isn't actually that fast. What
> makes an SSD incredibly fast is that the controller stripes reads and
> writes across its busses, and across all of the chips in its busses, a
> little bit like RAID-0, effectively. By spreading the reads and writes
> across a large number of chips, you can increase performance. This is
> why you can compare datasheets of the same line of drives, and often
> you'll find that the larger capacity SSDs are rated faster than the
> lower capacity models. They have more physical chips to spread writes
> across.
>
> Also, many of the advanced features available over the SATA/SAS
> protocols aren't available to USB/MicroSD interfaces. Things like
> TRIM, which allows the drive to know which blocks aren't in use. It's
> a common mistake to think that TRIM actually frees up blocks in SSDS -
> what TRIM actually does is marks blocks as unused so that Garbage
> Collection (GC) can come through later (at a presumably not-busy time)
> and clean up the blocks marked as deleted. USB/MicroSD cards don't do
> GC either.
>
> MicroSD and USB devices also don't have any kind of battery backing.
> If you were to pull out a card without unmounting it, the writes have
> the potential to be lost. If you were writing a file, it's maybe not a
> big deal, depending on what filesystem you were writing (because the
> filesystem might be able to clean up after itself if it's journaled)
> but if you're writing filesystem metadata? Hard to tell. SSDs
> (especially enterprise SSDs) have either batteries (or more likely
> supercapacitors)  that can power the flash chips for long enough to
> guarantee writes. This way if the power goes out to the server or
> array the drive is in, the writes are guaranteed.
>
> Another big factor is data retention.The underlying way that flash
> memory writes is essentially to put an electrical charge into a small
> deposit of doped silicon. Over time (months typically), if left
> unrefreshed, that electrical charge will dissipate and the controller
> won't be able to determine whether the bit is a one or a zero. For
> this reason, SSD controllers periodically go through the old data and
> "scrub" it, refreshing the electrical charge. This is really hard to
> do if the flash chip is on a USB keychain in your backpack.
>
> As far as your question about write lifetime, the write lifetime is
> related to the number of times a particular "cell" of data is written
> to. That might sound obvious, but it implies a lot of things. First,
> how many bits are stored in each cell? You may have heard of SLC and
> MLC - single layer cells and multi-layer cells. These refer to how
> many "bits" each individual doped cell hold (the same small area of
> doped silicon in the previous paragraph). SLC has one, MLC has two.
> Some new flash chips are TLC - three layer cells (and those aren't
> even just used for cheap things like USB sticks - My Samsung 840 uses
> TLC (the 840 Pro uses MLC)). The big differences are in terms of wear
> leveling.
>
> Wear leveling is (as you probably know), spreading writes across
> several areas, so that one set of bits doesn't get wore down and
> become unable to be read. This kind of physical damage happens over
> thousands and thousands of writes, but sometimes computers need to
> write a lot. So to prevent this, the write are spread over as many
> chips as possible (remember the performance gains up above?). And the
> USB / MicroSD cards only have one or two chips...so there's not much
> wear leveling possible. Fortunately, you're not actually writing all
> that much to one of these devices. Probably, anyway. I suppose if you
> ran a VM from a USB stick, it would write enough to eventually cause
> problems. You're much more likely to break the stick physically than
> to run into a flash write limit. At least, that's been my experience.
>
> There are some other differences, too, but this is getting really
> long, and I've covered the major stuff.  To sum it up, there are very
> significant differences, and the SSD isn't just an expensive group of
> thumb drives taped together :-)
>
> If you have any further questions, I'd be happy to explain anything
> else I know.
>
> --Matt
>
>
>
> On Sun, Mar 17, 2013 at 8:27 AM, Scott Merrill <skippy at skippy.net> wrote:
>> Aside from form factor, what differences exist between an external SSD
>> drive and a MicroSD card?
>>
>> USB 3 for an SSD drive may introduce speed differences.
>> SSD drives likely draw more power.
>>
>> Is there a meaningful difference in the expected life of each medium?
>> Is there a meaningful difference in the wear leveling technology
>> applied to each?
>>
>> What other differences may exist?
>>
>> Thanks!
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>
>
>
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