XO-1.5 users: need your SD card data

Daniel Drake dsd at laptop.org
Tue Apr 19 10:42:08 EDT 2011

On 18 April 2011 23:46, James Cameron <quozl at laptop.org> wrote:
> No, ext* does not require unused filesystem blocks to be zeroed out or
> to read back 0s if read.

Ok, you convinced me :)

What I was getting confused with is the requirement for inode tables
to be zeroed out. (this is probably what takes most of the time during
mke2fs). I might be wrong on that count as well though - its just
something I picked up in a discussion which I could have
misunderstood. Probably easy to test: create a file of all 0xff bytes,
run mke2fs on it, count the number of 0x00 bytes.

So if we were simply to assume that zeroed data is unused, we might
end up with problems where cards that erase to all-ones wouldn't have
the required all-zeroed inode tables. But I don't think that's what
you're suggesting.

It seems like it might be something worth investigating further. I
guess the possible complications are converting the free block numbers
on the partitions into offsets into the disk itself, and then into
zblock indexes. The questionable suitability for zerofree for ext4 is
a small concern as well - we do use file extents.


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