[colug-432] SSD vs SD

[Rob Stampfli]

On Sun, Mar 17, 2013 at 12:21:58PM -0400, Rick Hornsby wrote:
> Hmm.  Interesting.  So would a logical conclusion to this be that archival
> storage on an SSD is not recommended?  As in, for example, don't stick an
> ssd into an external storage bay, throw it in the closet and expect the
> data to be readable in a year?  Also, don't stick an ssd into a system that
> you only use (and then power down) a couple of times/month (or less
> frequently)?  Basically - the SSD needs power.
...
> Several folks that I've talked to have said that they install their OS and
> programs onto their SSD and put everything else onto spinny storage (what
> is the right term to reference a spinning disk to differentiate it from an
> SSD?).

Another consideration is that solid state devices fail in different ways
than spinning disks.  With disks, if you can read it, the data is likely
good.  With NAND devices, it's not always clear.

I have a Sheevaplug that I have dedicated to being a media server.  It
has a large spinning disk attached (for holding the "media"), but the OS
is located on an SDcard.  The system has always run flawlessly, but after
some period of time, I started seeing lots of strange file system
corruption problems on the root FS when I'd take it down for some reason
and then fsck it as a precaution.  At first I thought it was an OS problem,
but eventually I laid out several backups side-by-side on my main box and
compared notes.  What I found was that additional new bit errors were
cropping up in files with each backup, but the card wasn't reporting any
errors.  It is unlikely that a small number of such errors will affect a
file system in a way that is catastrophic.  Most data is in little used
areas such as /var/cache/apt.  The system kept running with these errors,
but occasionally they would affect inodes and directories and of course
that would send fsck into a tizzy.  But, the bottom line is that I really
had to throw away my backups and build a good system again, since the
backups could no longer be trusted.

So, a word to the wise:  Be careful with NAND devices, or at least be aware
of their idiosyncrasies.

Rob