wad at laptop.org
Mon Mar 14 20:29:19 EDT 2011
On Mar 14, 2011, at 3:18 PM, Arnd Bergmann wrote:
> On Monday 14 March 2011 19:50:27 John Watlington wrote:
>> Cards that are in the state you describe are most likely dead due to
>> running out of spare blocks. There is nothing that can be done to
>> rehabilitate them, even using the manufacturer's secret code.
>> In a disturbing trend, most of the cards I've returned for failure analysis
>> in the past year have been worn out (and not just trashed meta-data
>> due to a firmware error).
> Part of the explanation for this could be the fact that erase block
> sizes have rapidly increased. AFAIK, the original XO builtin flash
> had 128KB erase blocks, which is also a common size for 1GB SD and
> CF cards.
> Cards made in 2010 or later typically have erase blocks of 2 MB, and
> combine two of them into an allocation unit of 4 MB. This means that
> in the worst case (random access over the whole medium), the write
> amplification has increased by a factor of 32.
> Another effect is that the page size has increased by a factor of 8,
> from 2 or 4 KB to 16 or 32 KB. Writing data that as smaller than
> a page is more likely to get you into the worst case mentioned
> above. This is part of why FAT32 with 32 KB clusters still works
> reasonably well, but ext3 with 4 KB blocks has regressed so much.
The explanation is simple: manufacturers moved to two-bit/cell (MLC) NAND Flash
over a year ago, and six months ago moved to three-bit/cell (TLC) NAND Flash.
Reliability went down, then went through the floor (I cannot recommend TLC for
anything but write-once devices). You might have noticed this as an increase in
the size of the erase block, as it doubled or more with the change.
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