20 Mar
2005
20 Mar
'05
12:08 p.m.
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