How to generate a large file allocating space

Matija Nalis mnalis-ml at
Sun Oct 31 18:46:09 UTC 2010

On Sun, Oct 31, 2010 at 04:34:44PM +0100, Alex Bligh wrote:
> That's /almost/ perfect:
>   9  247808 15069184           14336 unwritten,eof
> testfile: 2 extents found
> I think all I need do is clear the unwritten flag in each of the extents.
> Else I think if I read the file using ext4 later (i.e. after I've
> written directly to the sectors concerned) it will appear to be
> empty. 

Yes, it would appear empty. It is due to security concers others mentioned

> Any idea how I do that?

Sorry, I don't. debugfs(8) only appears to have read-only support for
reading extents, and not for (re-)writing them, so I guess you'll have to
find some function in libext2fs if it exists (or write your own if it
doesn't) to use on unmounted fs.

Opinions above are GNU-copylefted.

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