Re: PATCH - ext2fs privacy (i.e. secure deletion) patch

[Valdis . Kletnieks]

On Wed, 04 Feb 2004 00:33:37 GMT, the grugq said:

> All that said, the user's content is something that the user could be 
> considered responsible for erasing themselves. The meta-data is the part 
> of the file which they dont' have access to, so having privacy 

Actually, I have encountered file systems where two successive
write() calls from userspace to the same offset in the file wouldn't
end up in the same physical location on the disk (AIX's JFS with compression).
It would LZ compress each 4K block, and then find a contiguous set of
512-byte sectors to write it.  So one write might compress down to 6 sectors
and be written in one place, the next time it doesn't compress as well
and takes 7 - so it ends up elsewhere because the previous hole was
exactly 6 - and if you try to zero it by writing a block of zeros, that
would compress down to 1 sector and fail to overwrite the others...

If we ever do a filesystem with compression, we'll have the same issue.

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