[GTALUG] Compressing an image of a microSD card

[D. Hugh Redelmeier]

| From: Lennart Sorensen via talk <talk at gtalug.org>

| Certainly something like clonezilla knows how to deal with filesystems
| and partitions and ignore the parts of the filesystem that are not in use
| (even if not zeroed).

If you use fstrim on a filesystem, what happens when a dd accesses an
unallocated block?  Is that well-defined?  An I/O error?  Are all
unallocated blocks identical?  Is this up to the manufacturer or some
standard?

(I'm too lazy or too busy to experiment.)