Duplicity backs up unchanged files daily (lp:#839048)

[Original report](https://bugs.launchpad.net/bugs/839048) created by **Chris Stankaitis (cstankaitis)**
```
We are backing up a netapp volume which is mounted over NFS to our backup
server.  duplicity seems to be backing up files which are unchanged in the
daily incremental.

backup line:

duplicity --encrypt-key "7049C7BA" --archive-dir "/var/duplicity"
--name="bt" --tempdir "/var/tmp" --exclude "/mnt/bt/.snapshot" -v9 --full-
if-older-than 1M /mnt/bt file:///backup/vol/bt >> /tmp/backup-output-bt.txt

example file:

# ll static/ad_assets/var-2.8.0/cache/.nfs000000000019bcc50000372e
-rw--w--w-. 1 nobody nobody 220 May 20  2010
static/ad_assets/var-2.8.0/cache/.nfs000000000019bcc50000372e

[root@backup1]# stat
static/ad_assets/var-2.8.0/cache/.nfs000000000019bcc50000372e
  File: `static/ad_assets/var-2.8.0/cache/.nfs000000000019bcc50000372e'
  Size: 220             Blocks: 8          IO Block: 65536  regular file
Device: 1dh/29d Inode: 2780177     Links: 1
Access: (0622/-rw--w--w-)  Uid: (   99/  nobody)   Gid: (   99/  nobody)
Access: 2011-08-30 01:43:35.497401000 -0400
Modify: 2010-05-20 14:50:30.000000000 -0400
Change: 2011-05-25 23:25:12.938675000 -0400

As I understand it that file should be in my full backup, and then not
touched again.  There are a lot of these old un-touched files which appear
to be getting backed up every day wasting a lot of space on our backup
server.

As you can see duplicity things the file was modified as of last nights
backup

# cat backup-output-bt.txt.1 | grep .nfs000000000019bcc50000372e
Selecting
/mnt/bt/static/ad_assets/var-2.8.0/cache/.nfs000000000019bcc50000372e
Comparing ('static', 'ad_assets', 'var-2.8.0', 'cache',
'.nfs000000000019bcc50000372e') and ('static', 'ad_assets', 'var-2.8.0',
'cache', '.nfs000000000019bcc50000372e')
Getting delta of (('static', 'ad_assets', 'var-2.8.0', 'cache',
'.nfs000000000019bcc50000372e')
/mnt/bt/static/ad_assets/var-2.8.0/cache/.nfs000000000019bcc50000372e reg)
and (('static', 'ad_assets', 'var-2.8.0', 'cache',
'.nfs000000000019bcc50000372e') reg)
M static/ad_assets/var-2.8.0/cache/.nfs000000000019bcc50000372e
Selecting /mnt/bt/static/ad_assets-
backup-2010-10-19/var-2.8.0/cache/.nfs000000000019bcc50000372e
Comparing ('static', 'ad_assets-backup-2010-10-19', 'var-2.8.0', 'cache',
'.nfs000000000019bcc50000372e') and ('static', 'ad_assets-
backup-2010-10-19', 'var-2.8.0', 'cache', '.nfs000000000019bcc50000372e')
Getting delta of (('static', 'ad_assets-backup-2010-10-19', 'var-2.8.0',
'cache', '.nfs000000000019bcc50000372e') /mnt/bt/static/ad_assets-
backup-2010-10-19/var-2.8.0/cache .nfs000000000019bcc50000372e reg) and
(('static', 'ad_assets-backup-2010-10-19', 'var-2.8.0', 'cache',
'.nfs000000000019bcc50000372e') reg)
M static/ad_assets-
backup-2010-10-19/var-2.8.0/cache/.nfs000000000019bcc50000372e

# ll backup-output-bt.txt.1
-rw-rw----. 1 root root 14163064063 Aug 31 21:57 backup-output-bt.txt.1


Duplicity version: 6.11
Python version 2.6.5
OS Distro and version Redhat Enterprise Linux 6
Type of target filesystem: Linux
```

Story points: 20.0