[rdiff-backup-users] File change detection using hashes

[Wiebe Cazemier]

Ben,

A while back I suggested a feature to include hash-checks to determine
if files have changed or not, instead of the mtimes+size combo. I trust
you remember. I've been away from the list a while, and I see there has
been a poll. I didn't see this feature in the poll, but I'd like to
emphasise that it is quite important. I've just discovered another
situation in which mtime+size is not reliable. I'll explain:

if you have two files of equal size, but different contents, and copy
one over the other, the mtime of the target file is preserved. Now,
because the size and mtime haven't changed, rdiff-backup doesn't see it
as a change and doesn't back it up. This if course, would be very
undesirable.

Here is an example of what I mean:

================================
# lh
total 8.0K
-rw-------  1 [owner] users 16 Feb 10 15:46 a
-rw-------  1 [owner] users 16 Feb 10 15:47 b

# cat a;cat b
this is a file.
this is b file.

# cp a b

# lh
total 8.0K
-rw-------  1 [owner] users 16 Feb 10 15:46 a
-rw-------  1 [owner] users 16 Feb 10 15:47 b

# cat a;cat b
this is a file.
this is a file.
================================

I really think this feature should be part of the next stable release,
because now I can't fully trust my backup to be accurate.

On a sidenote, the wiki at
http://rdiff-backup.solutionsfirst.com.au/?SuggestedFeatures doesnt seem
to work, the articles are not accessible.

Regards,

Wiebe Cazemier

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