archiving data, was RE: Media Longitevity/Care

[Eric Smith]

Ethan wrote:
> Flash isn't a perfect replacement for rotating magnetic media for any
> OS that has a swap partition... heavy use will begin to deplete
> regions of the Flash media.  Before anyone says that it will take "X
> million years..."

Not millions of years, but more than a few.

> I want to comment that I've seen it happen myself in
> heavy use situations... specifically, I rigged up an old Olympus
> camera as a webcam for my "EarthDial".  I used this camera because it
> was the end of summer at the South Pole and there wasn't time to get
> get something more suitable before our 8 month winter started.  The
> camera was controllable via RS-232 serial line and gphoto.  The one
> weakness was its 8MB SmartMedia storage.  I took several thousand
> pictures, one every 5 minutes for several weeks.  As the weeks wore
> on, the capacity of the card diminished by about 5%.

That's because "SmartMedia" is a horrible misnomer.  It really should
be called "DumbMedia", because there's no intelligence in the card
at all.  So it does no wear levelling.

CF and SD cards have a built-in intelligent controller that performs
wear levelling automatically.  Some (or all?) cards also have automatic
replacement of bad blocks with spares.  They will last much longer than
SmartMedia in an environment that rewrites certain blocks frequently,
such as swap space.  They'll still eventually were out, of course, but
it will take quite a while.

The exact details of the wear-levelling and bad block replacement
are unspecified by the CF and SD standards.  Similarly to the SCSI
and ATA standards, CF and SD just present a logical array of good
blocks, with the geometry details hidden.  Different manufacturers
may have different algorithms.  Sandisk has a white paper describing
theirs in general terms.

If I were buying a card for a high-wear application, I'd want to buy a
well-known reputable brand such as Sandisk or Lexar, rather than the
cheapest off-brand card at Frys.

Eric