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[Bug-tar] Trying to figure out why unchanged file gets backed up in leve


From: Bret Mckee
Subject: [Bug-tar] Trying to figure out why unchanged file gets backed up in level 1 backup
Date: Wed, 1 Sep 2004 16:46:35 -0600

Greetings:

I'm having a problem with incremental backups.  I'm using amanda to
manage the backups, but this looks like a  gnutar issue to me.

I did the first backup, and of course it did a level 0. I then did a 
second backup and it did a level 1 backup.  Because I have a large
/home disk, I back up all the users directories separately.

My home directory (/home/mckee) is about 5GB, and the level 0 seemed
large enough to have gotten to it all.  The level 1 backup was run
almost immediately (as part of testing the new install), and virtually
nothing changed. I was really surprised to see that the level 1 was
about 500Mb or 10% of the level 0 (I had expected it to be much
smaller). 

First, a bit of version information:
$ uname -a # I'm running RH Enterprise ES
Linux hostname 2.4.21-15.ELsmp #1 SMP Thu Apr 22 00:18:24 EDT 2004
i686 i686 i386 GNU/Linux

$ tar --version # And because it might matter:
tar (GNU tar) 1.13.25

amanda, unpacked from:
-rw-rw-r--    1 mckee    mckee     1383528 Jun 22 06:50
amanda-2.4.4p3.tar.gz

I used amrestore | tar -tv to get a list of the files in the level 0
and level 1 archives, and discovered that several things that almost
certainly had not changed were backed up. I then went and read all
about gtar's --listed-incremental mode and *think* I understand how it
is supposed to work (famous last words :-), and I still can't explain
why these files were backed up.

Picked as an example, one file that didn't change but that was backed
up was:

/home/mckee/proj/proj-2.3/client/pubring.gpg

which was backed up relative to /home/mckee as:
./proj/proj-2.3/client/pubring.gpg

Trying to figure out why it got backed up, I looked in the gnutar-list
files for the path to both files:

hostname_home_mckee_0:26641 31851568 ./proj
hostname_home_mckee_1:26641 31851568 ./proj

hostname_home_mckee_0:26641 4637093 ./proj/proj-2.3
hostname_home_mckee_1:26641 4637093 ./proj/proj-2.3

hostname_home_mckee_0:26641 37028138 ./proj/proj-2.3/client
hostname_home_mckee_1:26641 37028138 ./proj/proj-2.3/client

Note that device/inode didn't change for any of the directories in the
path (which would have triggered tar to back up the files)

Here are the entries for the tar -tv output, and again the dates/sizes
didn't change:
mckee.list.0:-rw------- root/root           1692 2004-03-23 10:04:03
./proj/proj-2.3/client/pubring.gpg
mckee.list.1:-rw------- root/root          1692 2004-03-23 10:04:03
./proj/proj-2.3/client/pubring.gpg

I'm looking to understand this behavior, both because it is a waste of
tape 
(even virtual) to backup unchanged bits and because I can't help
wonder 
if some files are not being backed up at all (i.e. if it doesn't
correctly 
decide what to back up, it could easily be missing files too).

If anyone can explain this behavior, or if you need additional
information to try and explain it, please let me know.

Many thanks in advance,

Bret






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