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

[Vadim Kouzmine]

On Fri, 2006-02-10 at 15:53 +0100, Wiebe Cazemier wrote:
> 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 did the same and mtime has changed for me:

dd if=/dev/urandom of=a count=1
dd if=/dev/urandom of=b count=1

stat a b
  File: `a'
  Size: 512             Blocks: 8          IO Block: 131072 regular file
Device: 806h/2054d      Inode: 2094563     Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1001/kouzminv)   Gid: (  440/  vadiki)
Access: 2006-02-10 11:49:56.000000000 -0500
Modify: 2006-02-10 11:49:56.000000000 -0500
Change: 2006-02-10 11:49:56.000000000 -0500
  File: `b'
  Size: 512             Blocks: 8          IO Block: 131072 regular file
Device: 806h/2054d      Inode: 2094711     Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1001/kouzminv)   Gid: (  440/  vadiki)
Access: 2006-02-10 11:50:01.000000000 -0500
Modify: 2006-02-10 11:50:01.000000000 -0500
Change: 2006-02-10 11:50:01.000000000 -0500

cp a b

stat a b
  File: `a'
  Size: 512             Blocks: 8          IO Block: 131072 regular file
Device: 806h/2054d      Inode: 2094563     Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1001/kouzminv)   Gid: (  440/  vadiki)
Access: 2006-02-10 11:51:20.000000000 -0500
Modify: 2006-02-10 11:49:56.000000000 -0500
Change: 2006-02-10 11:49:56.000000000 -0500
  File: `b'
  Size: 512             Blocks: 8          IO Block: 131072 regular file
Device: 806h/2054d      Inode: 2094711     Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1001/kouzminv)   Gid: (  440/  vadiki)
Access: 2006-02-10 11:50:01.000000000 -0500
Modify: 2006-02-10 11:51:20.000000000 -0500
Change: 2006-02-10 11:51:20.000000000 -0500

Notice *** Modify: 2006-02-10 11:51:20.000000000 -0500 ***

I think you miss SECONDS in your lh listing. Looks like "b" was created
15:47 and then you copied a -> b at 15:47.

Vadim

> 
> 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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> Wiki URL: http://rdiff-backup.solutionsfirst.com.au/index.php/RdiffBackupWiki
-- 
Vadim Kouzmine <address@hidden>