Tim Woodall writes:
>
> Do these cards have wear levelling? Have I just got unlucky that it's
> the start of the card that is unwriteable and so I cannot continue on
> the 12GB of space that has never been part of a partition?
>
Almost all SD cards from the major manufacturers in the last 5 years
use wear leveling.
Each "bit" can be re-written about 6K times before failures start.
So, the more unused SD space is better, since wear leveling writes to
a "bit" that has been written to fewer times.
To test, say with a 16 GB SD, fill the SD to all except the last 1 KB,
and with a looping script, write 1KB of 1's to the remainder of the
SD, erase the "bits," then 1KB of 0's, erase the "bits", and so on;
the SD card will fail within hours to a few days, (with luck-note that
MTBF is mean time between failures, meaning that by MTBF, half will
have failed, half still running; its a stochastic/probability issue;
it does NOT mean that all are expected to last at least 6K writes.)
Doing the same test without filling to the last 1 KB, and the SD card
will last a very long time, (about 16 million total writes.)
Obviously, a VM thrash is to be avoided on SD cards that are running
near capacity.
On the other hand, using an SD card for archival storage, (or backup,)
in a write once, and store STP to protect the plastic out gassing
failures, the failure rate is determined by quantum mechanics, and is
quite long, (usually quoted as in excess of a century, to 3 sigma
total recovery of data.)
SSDs have related issues, too.
John
--
John Conover, cono...@panix.com, http://www.johncon.com/
