Would a Transcend USB Flash Module of 2GB work?

[Jeremy Chadwick]

On Wed, Jun 25, 2008 at 12:56:28PM +0200, Alwin Roosen wrote:
> I want to install FreeBSD onto an USB Flash Module from Transcend. The
> links below are the product page and datasheet in PDF. It says it should
> be compatible with OS that supports USB standard.
> Can anyone confirm that this would actually work?
> http://www.transcend.nl/Products/ModDetail.asp?LangNo=0&ModNo=122

This looks like nothing more than a USB flash drive (sometimes called
USB pen drive), except instead of a standard Type-A USB connector, it
uses a 10-pin connector like what's on PC motherboards.  (I also like
how their connector isn't even keyed, which means you could indeed
plug the adapter in backwards; all present-day internal USB cables
are keyed, so why aren't those?)

The PDF contains absolutely no information about what type of flash is
used, how flash I/O is performed, or if any form of wear levelling is
implemented.  There's also no MTBF, which is disappointing, although it
does list erase cycle counts (100K).  The closest thing to MTBF there is
"Data Retention", which says "10 years".  There's too much ambiguity
there for me to believe it.

> I am also wondering if it is a good approach to use USB dongles, instead
> of real Solid State devices on ATA.
> ...
> My hardware supplier recommends the USB dongles, because they
> are cheaper and easier to install (just plug it in an 10-pin USB port on
> the motherboard).

SSD is different than the product you list above.  The most important
aspect of present-day (past 2-3 years) SSD is how wear levelling is
implemented.  It's very well done on present-day SSD devices, while on
USB storage devices like what you list, I strongly doubt it's done
similarly.

I would recommend you read the following documents:

http://www.storagesearch.com/ssdmyths-endurance.html
http://en.wikipedia.org/wiki/Wear_levelling

With regards to the Wikipedia article, now you see why knowing what sort
of wear levelling mechanism and "logic" is used in those USB devices.  I
would easily trust present-day SSD over a USB stick (which is really
what those things are), period.

Then there's the performance aspect.  Those USB devices are going to top
out at 33MByte/sec, and I'm willing to bet they don't even achieve that.
Take a look at pages 6-8 of the following SSD review (compared against
present-day hard disks):

http://www.tomshardware.com/reviews/ssd-memoright,1926.html

> I see a lot of USB problems in the Mailing list Archives (see link
> below as example). I don't understand all of that, and I know there
> are many USB devices which can cause these issues.

Is there a question in here somewhere?  :-)  I think what you're
indirectly trying to ask is "is USB on FreeBSD stable?"  IMHO, the
answer is a big fat no.  I would not trust the present USB stack with
the same reliability as, say, the SCSI CAM or (I'll regret saying this,
I just know it) ATA subsystem.

What goal are you trying to accomplish?  Do you want to build a
FreeBSD system using standard PC parts which doesn't involve storage
media that has moving parts?

-- 
| Jeremy Chadwick                                jdc at parodius.com |
| Parodius Networking                       http://www.parodius.com/ |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.              PGP: 4BD6C0CB |