[GTALUG] Compressing an image of a microSD card

[Lennart Sorensen]

On Thu, May 14, 2020 at 02:10:26PM -0400, D. Hugh Redelmeier via talk wrote:
> 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.)

According to wikipedia the ATA standard allows choices:

There are different types of TRIM defined by SATA Words 69 and 169
returned from an ATA IDENTIFY DEVICE command:
1) Non-deterministic TRIM: Each read command to the Logical block address
   (LBA) after a TRIM may return different data.
2) Deterministic TRIM (DRAT): All read commands to the LBA after a TRIM
   shall return the same data, or become determinate.
3) Deterministic Read Zero after TRIM (RZAT): All read commands to the
   LBA after a TRIM shall return zero.

Apparently 1 is pretty much only seen on budget garbage drives
2 is most common on consumer SSDs
3 is most common on enterprise SSDs and required by many NAS systems if
you want to run RAID on the drives.

-- 
Len Sorensen