Re: cross-platform backup tool Same files from different source dir causes spurious diff files

[Robert Nichols]

On 2/7/22 7:23 PM, Leland Best wrote:
> Hi Cliff,
>
> On Mon, 2022-02-07 at 11:45 -0800, Mr. Clif wrote:
> > Hey Eric,
> >
> > any ideas on this? How do these diff files normally work?
> [...]
>
> I'm not an 'rdiff-backup' developer or anything so all you experts out there
> correct me if I'm wrong but ...
>
> IIRC 'rdiff-backup' keeps inode info as part of the metadata for each file.
> When you mount a filesystem Linux assigns "fake" inode numbers to avoid
> collisions between filesystems on different devices/partitions/etc..  So if you
> change the mount point, every file could potentially get a new inode number and,
> consequently, have changed metadata.  That results in 'rdiff-backup' creating a
> '*.diff*' file for every source file.

Device and inode metadata is kept only for files with multiple hard links. 
That's
to keep track of which links reference the same file. That information is not
needed for files with just a single hard link, and unless something has changed
in the latest release that metadata is not kept. You can look in the
mirror_metadata file (it's compressed ASCII) and see what fields are present
for each file.

> In addition, since 'rdiff-backup' now thinks the files may have changed it
> spends a lot of time checking if anything other than metadata has changed which
> _might_ account for the apparently low throughput.

That would definitely be true, and the presence of all those "zero diff" files
show that it is, in fact, happening.

--
Bob Nichols     "NOSPAM" is really part of my email address.
                Do NOT delete it.