Re: Wear levelling on micro-sd cards

[conover@panix.com (John Conover)]

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, conover@panix.com, http://www.johncon.com/