Re: [Bug-tar] Trying to figure out why unchanged file gets backed up in level 1 backup

[Joerg Schilling]

Sergey Poznyakoff <address@hidden> wrote:

> Bret Mckee <address@hidden> wrote:
>
> > If anyone can explain this behavior, or if you need additional
> > information to try and explain it, please let me know.
>
> What was the exact command line given to tar when making level 0
> and level 1 dumps?

The main problem with GNU tar's incrementals is that GNU tar does not
put enough (meta) information for a single file on the archive.
As a result of this fact, GNU tar is unable to handle renamed files.

While single renamed files may be handled by just adding the new
file to the level 1 dump, this does not work for renamed directories.
If a directory is renamed, GNU tar archives _all_ files that are part
of the directory tree under that directory. this makes GNU tar incrementals
extremely big.


        Is it possible that your size problem is a result of a renamed 
        directory? 


Unfortunately, GNU tar even has bugs with extracting the incrementals and
a renamed directory results in a different tree on the filesystem where the
incrementals are extracted :-( It seems that during the past 12 years, nobody
did run tests for even the simple cases: If you rename a directory and
create a new directory with the old name, the content of the old directory
is not removed. There may be more bugs in GNU tar, I stopped testing after the
first problem occurred.

If you are interested in a portable OS and filesystem independent solution
for incremental backups, it make sense to check star. Star archives all file
metadata and this way is able to use a similar method as ufsdump/ufsrestore
use. Star does not make more assumptions on the filesystem than GNU tar,
only a POSIX compliant filesystem with inode numbers is required.

Star incrementals are of similar size as a ufsdump incremental. Star has been 
verified to handle all simple cases correctly, so the current implementation
in star is already more complete than GNU tar's current implementation.

Please check star-1.5a47 on:

ftp://ftp.berlios.de/pub/star/aplha

Jörg

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