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Re: SHRED for EXT3?



Mike McCarty wrote:
Robert Nichols wrote:

Mike McCarty wrote:

    One issue is that if I overwrite the file, the location
which contains the original data is *not* the place the overwrite
is done to.



The 'shred' program overwrites the file in a manner that does not reallocate the space. You can also do that with 'dd' if you use


[snip]

Hmm. The man page for shred contradicts what you say.

QUOTE MODE ON

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.)

QUOTE MODE OFF

The context of this discussion is ext3.

Mike

It's easy enough to test. Create a file on an ext3 file system. Unmount the file system, run 'debugfs' and use its "stat" command to see the list of block numbers for that file. Mount the file system again and run 'shred' to overwrite the file without deleting it. Unmount the file system and use 'debugfs' again to examine the list of block numbers. Observe that they are the same. Then repeat the experiment but use 'dd' with the "conv=notrunc" option instead of 'shred'.

--
Bob Nichols         Yes, "NOSPAM" is really part of my email address.


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