As for typical dd and emmc, keep in mind that the typical block size is much larger than filesystem blocksize. The physical block, when erased, can only be written to once, before it must be erased again.

What does this mean for badblocks? If a read test finds 1k bad, and you avoid using that sector, but write to anywhere else inside same physical block, it will put that physical block aside, and copy contents to a new one, along with the requested modification. The bad sector is now invisible, but will reappear elsewhere once that same physical block comes up in rotation again.


What's the size of the physical block? That's vendor proprietary and secret information. On the order of 512k to 16M though.

As for edt2 vs ext3, in my experience, ext2 is slower, which suggests it's triggering many read-modify-write cycles, causing excessive write amplification.