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Author Topic: what are the merits and limitations of the different types of flash memory?  (Read 3861 times)
Target
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« on: July 19, 2011, 12:46:30 AM »

Like most everyone these days I have a handful of USB drives that I use for all sorts of things, however I just bought a netbook to do a bit of stuff on and I was wondering why I couldn't use an SD card instead of my usual key.

I assume the internals are pretty much the same as USB keys, but the SD cards form factor is a bit more user friendly (it goes in the slot, out of harms way)

What I don't know is if there are any limitations on using these things like a 'normal' drive.  It doesn't seem logical to me that there would be, but then what do I know

I've had a hunt around and I don't seem to be able to find anything that discusses the relative merits of the different types of removable flash memory, though of course having said that, the first thing you guys will do is google the answer straight away  mad

Anyone able to shed some light on the subject (don't limit yourselves just to SD cards and/or USB keys either)
 
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« Reply #1 on: July 19, 2011, 01:50:56 AM »

Down to the metal it's all flash memory inside, just the electrical interface and usually some controller to interface between the chips and the interface that makes the difference. Depending on memory and controller speed the storage speed will be different.
It's just that some form-factors are offered cheaper than the others, what makes them more popular amongst consumers, as well as the physical shape, a micro-SD card is rather annoying to take with you as it's so fragile and small, so a more robust USB stick with a keyring attached just feels better/safer. Except when it's put into a computer(-like) device. A Micro-SD card is much more convenient in a mobile phone then a USB stick...
And then there's the case of connectors: a USB plug is designed to be connected/disconnected quite often, while a micro-SD card wasn't designed for that. And where I wrote micro-SD you could replace with any of the other card-like memory devices, though the bigger sizes often have a bit more robust connectors.
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« Reply #2 on: July 19, 2011, 03:04:23 AM »

I've got a Kingston MicroSD USB reader that's about as small as I imagine they can make it while still being large enough to actually handle.  Similar to:

  - http://www.kingston.com/u...sh/usb_microsd_reader.asp

It's on my key ring and has a 4GB MicroSD card in it so it can be used as an always handy USB memory 'stick'.  I also try to buy MicroSD cards for my cameras since they'll slide into a full size SD Card adapter for use in the camera, and the reader on my key ring is always available as a photo reader.

I imagine one day I'll upgrade the 4GB media with 16GB, but by the time I feel the need there will probably be some newfangled format that has made SD obsolete and I'll have to fork over my $20 for 512GB in the new format instead. Actually, I probably won't need anything since there will be free, unlimited storage in the cloud with unimaginable transfer rates.  Why, I won't even need a keyring anymore, since the RFID chip in my nasal cavity will let me into (and let me start) my car, as well as let me in my house.

Now, where did I leave my nasal cavity???
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Ath
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« Reply #3 on: July 19, 2011, 04:35:09 AM »

Why, I won't even need a keyring anymore, since the RFID chip in my nasal cavity will let me into (and let me start) my car, as well as let me in my house.

That would give 'putting your nose in' a whole different meaning Grin
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cranioscopical
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« Reply #4 on: July 19, 2011, 08:58:48 AM »

the RFID chip in my nasal cavity will let me into (and let me start) my car

That's nothing to sneeze at!

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« Reply #5 on: July 19, 2011, 10:06:53 AM »

A severe nosebleed, or chronic sinusitis could endanger the chip.
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« Reply #6 on: July 19, 2011, 06:17:30 PM »

I have a micro SD card usb adapter as well, but i like the idea of using an SD card cos it goes straight in the slot (no adaptor required) and nothings exposed, and if needs be I can take it out and put it in the laptop at work too.  And if I need to access it at home I can use a card reader

Just seemed a neater solution as USB keys could be vulnerable in certain situations, but you never seem to see them 'promoted' this way - it's always this is a camera card, this is a phone card, etc
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« Reply #7 on: July 19, 2011, 06:45:08 PM »

Just seemed a neater solution as USB keys could be vulnerable in certain situations, but you never seem to see them 'promoted' this way - it's always this is a camera card, this is a phone card, etc

This is to make sure you buy two of them...

smiley
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« Reply #8 on: August 21, 2011, 10:05:05 PM »

I was doing a bit of 'netshopping' for an SD card reader in PCMCIA form to slide into my laptop and receive installation folders and free up some hard drive space.  I have an SSD in this old Pentium3 laptop I'm typing on because it's in a very vulnerable spot and can't be relocated.  SSD's will (hopefully) take much more bangs and bumps before losing data.

I went through the excercise of modifying a slipstream XP installation setup for a USB stick a few years ago, and the results were disappointing to say the least.  The speed was HORRIBLE!  If I'm going to have the same problem with an SDHC, I'll have to scuttle the whole plan.

I was looking around for write speed info on some of those SDs, and haven't found any so far.  I'm worried when I do find some detailed speed specs they're going to be in a language I don't understand.

I don't use USBs or flash for keys, but I'd love to hear in a general sense anyone's impressions.

I've seen so many SD adapters that look to be used for OS installations, for instance plugging into IDE cables, but people claim to use USB XPs, and I'm not going down that road until USB 3.0 is every where, at the least.

A quick check online just now found sata models available as well, although in much smaller numbers than the IDE styles.
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« Reply #9 on: August 22, 2011, 02:26:09 AM »

SD Cards in general are quite slow. The most commonly found cards are Class 4, which means a write speed of 4MB/s - compare that to a decent USB flash drive which is around 10 - 20 MB/s. Note that there exist Class 10 and 16 cards as well, but naturally they're much more expensive. More importantly, the card reader should be able to support those speeds as well. An even more important fact is that the *read* speed is often only just higher than the write speed, which is upto 9MB/s for a Class 6 card - which is incredibly slower than your run-off-the-mill USB flash drives. (Note that sequential read speed is around 20MB/s but you won't be seeing those speeds unless you're doing long sequential transfers, such as installing Win7 or reading other large files like movies, ISOs etc. XP install will be slow because it reads a large number of small files, which takes ages). Random write speeds are even worse - usually in order of KB/s.

Also, there are issues booting from SD cards on a lot of computers. My Dell netbook for example doesn't even see the card when it's inserted in the internal reader, however, it's able to boot off it just fine when a USB based external reader is used.

Finally, it's usually cheaper to just buy a USB stick than go for an expensive yet comparatively slower SD card. Besides, it's really easy to loose those tiny microSD cards! Imagine loosing 16gigs of stuff because it fell out of your pocket and you never even noticed it...


1. http://en.wikipedia.org/w...igital#Speed_Class_Rating
2. http://chdk.wikia.com/wiki/Benchmarks
3. http://www.ocworkbench.co...0-16GB-SDHC-Review/p2.htm
« Last Edit: August 22, 2011, 02:17:39 PM by [deXter] » Logged

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« Reply #10 on: August 22, 2011, 12:37:22 PM »

Thank you [deXter]!  Those Class ratings I noticed a couple of days ago in the ads I was looking at on SD cards were in the spot I was hoping to see read/write speeds.  It's good to learn they directly correspond to MB/s.  I guess the SD Association will keep that correspondence in the future as well.  Maybe the SD card will catch up to useable speeds for PCs, but until then I'm steering clear.

I appreciate the information and links.
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« Reply #11 on: August 22, 2011, 03:02:55 PM »

No wonder that random-writes are slow, especially considering Windows defaults to not enabling write cache for these kind of removable devices... but interesting that 4k random reads are so slow!
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- carpe noctem
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« Reply #12 on: August 22, 2011, 09:38:14 PM »

No wonder that random-writes are slow,...
Writes are slow in flash memory for important reasons. For today's flash, you can only write in the same physical memory location 15,000 times before that location will fail (because of a silicon metal state change). (That number was 10,000 times about 6 years ago.) So to prevent writing in the same location all the time, there's memory management (MM) firmware to map the writes evenly across the entire physical address space. So you need to factor the execution time of that MM firmware into the write speed.

This is why solid state drives are purchased primarily for read applications (e.g. a web server) and not write applications (e.g. a database server). The solid state drive will really speed up any read-application heavy work.
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« Reply #13 on: September 01, 2011, 12:02:39 PM »

No wonder that random-writes are slow,...
Writes are slow in flash memory for important reasons. For today's flash, you can only write in the same physical memory location 15,000 times before that location will fail (because of a silicon metal state change). (That number was 10,000 times about 6 years ago.)
I wonder if anybody have reliable numbers for the erase cycles - I've seen a lot of different figures mentioned. And with the move to smaller production scales, those numbers tend to go down and not up!

So to prevent writing in the same location all the time, there's memory management (MM) firmware to map the writes evenly across the entire physical address space. So you need to factor the execution time of that MM firmware into the write speed.
I do wonder if the usb pendrive style flash devices do any of this remapping? At least in my mind, there's a big difference between those and solid-state drives with a SATA interface... even if they both use MLC modules smiley

quote author=superticker link=topic=27369.msg259223#msg259223 date=1314067094]This is why solid state drives are purchased primarily for read applications (e.g. a web server) and not write applications (e.g. a database server). The solid state drive will really speed up any read-application heavy work.[/quote]I can assure you that today's solid-state drives are purchased for write-intensive applications as well, and they really do shine there compared to magnetic storage drives too smiley - the management firmware doesn't "just" do remapping to reduce wear & tear, they also stripe the data across flash channels to achieve higher speed.

But there's that detail with pendrives vs. sata devices again.
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- carpe noctem
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« Reply #14 on: September 06, 2011, 11:44:04 PM »

This is why solid state drives are purchased primarily for read applications (e.g. a web server) and not write applications (e.g. a database server). The solid state drive will really speed up any read-application heavy work.
I can assure you that today's solid-state drives are purchased for write-intensive applications as well, and they really do shine there compared to magnetic storage drives too smiley - the management firmware doesn't "just" do remapping to reduce wear & tear, they also stripe the data across flash channels to achieve higher speed.

But there's that detail with pendrives vs. sata devices again.
It might be the SATA drives use DRAM (with battery backup) rather than flash; otherwise, you would have a really high failure rate in write intensive applications. I do know the SATA solid-state drives fail much more than their mechanical counter parts, and they fail without warning. That seems odd to me for a flash failure, but it would make sense in a DRAM design if the battery backup suddenly failed.

Come to think about it, it makes more sense to use DRAM over flash in a SATA drive design just because flash write speeds are so slow and their write times can be somewhat non-deterministic because of the MM firmware execution involved.
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« Reply #15 on: September 07, 2011, 07:52:58 AM »

I do know the SATA solid-state drives fail much more than their mechanical counter parts, and they fail without warning. That seems odd to me for a flash failure, but it would make sense in a DRAM design if the battery backup suddenly failed.
It's definitely not the flash cells that are worn out, those spurious drive deaths happen much too fast for that. For the OCZ drives, it seems to be the firmware that goes into a panic-state after some power-cycle. The Intel bug seems to be along the same lines. But it's really discouraging that these things happen, as solid state drives were supposed to fail gracefully. Oh, btw, only the "enterprisey" SSDs have battery backup.

FWIW, I doubt they "fail much more than their mechanical counterparts" though, the shop I bought my Vertex2 from told me they had around 1% fewer RMAs on SSDs than mechanical disks.

Come to think about it, it makes more sense to use DRAM over flash in a SATA drive design just because flash write speeds are so slow and their write times can be somewhat non-deterministic because of the MM firmware execution involved.
If flash memory really was that slow, a ram buffer (which all the drives do have) would only allow for high burst speeds - but the decent drives allow for high sustained speeds. My X25-E does something like 150MB/s sustained.
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