Multiple-pass overwrite of EXT3 file on a journalled fs

[Andreas Dilger]

On Oct 08, 2004  15:18 -0700, sp0ck1701 wrote:
> I am curious if anyone knows why utilities such as
> 'GNU shred' (part of coreutils) and 'wipe' say they
> are not effective on journalled file systems-
> especially EXT3.
> 
> Below is an excerpt from the shred man page which
> specifically mentions EXT3:
> 
> ----
> CAUTION: Note that shred relies on a very important
> assumption: that the filesystem overwrites data in
> place. This is the traditional way to do things, but
> many modern filesystem designs do not satisfy this
> assumption. The following are examples of filesystems
> on which shred is not effective:
> 
> * log-structured or journaled filesystems, such as
> those supplied with
> 
>     AIX and Solaris (and JFS, ReiserFS, XFS, Ext3,
> etc.) 
> ----

I don't know why they would say this.  With very early versions of
ext3 (or filesystems mounted with data=journal) all data writes would
also go to the journal, so you would get two copies of your data on
disk.  The "shred" overwrite would in theory only erase the copy in
the filesystem.  It is also possible to get a copy of the last partial
page of a truncated file being put into the journal.

Of course, this neglects the fact that the journal is continually being
overwritten so the only time there might be data in the journal (in the
non-default data=journal mode) is after it was just written to disk.  In
normal filesystem usage this would be overwritten hundreds of times,
and the user-space overwriting tool would itself cause the journal to be
overwritten in this case (taking relative sizes of files and journal
into account).

I know reiser4 on the other hand is a log-structured filesystem, so
overwriting a file has no guarantee that even the data blocks will be
overwritten.

Cheers, Andreas
--
Andreas Dilger
http://sourceforge.net/projects/ext2resize/
http://members.shaw.ca/adilger/             http://members.shaw.ca/golinux/

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