[CentOS] evaluating backup systems: rsync

[m.roth at 5-cent.us]

Les Mikesell wrote:
> On Fri, Jan 11, 2013 at 4:05 PM,  <m.roth at 5-cent.us> wrote:
>>>
>>> Mark, maybe you could explain what a "parm" is and how using hard links
>>> saves space.
>>
>> A hard link isn't easy: it's an inode that is referenced by more than
>> one other inode. In effect, it's a pointer, rather than a reference, so
that
>> it really, in effect, acts like the real file, and is almost
>> undistinguishable from one. You don't actually delete the real file
>> until all hard links pointing to it are gone.
>
> Close... A directory entry has a pointer to an inode and the inode has
> the information about the file attributes and location.   A directory
> entry is a 'link' with a name.   There can be 0 or more links to and
> inode and a link count is maintained atomically in the inode as links
> are added or removed.  A 'hard link' is the scenario when two or more
> directory entries (names) point to the same inode.  The file data is
> not removed and the space freed until the inode link count is zero
> _and_ there are no open file handles that reference it.
<snip>
Thanks, Les.

At any rate, the point is that the hard links point to *exactly* the same
file on the disk, so it *looks* as though they take up equal space, but in
reality, there's only one copy.

So, if you're copying a directory to a timestamped named directory, and
all the files except one or two being backed up haven't changed, then
there's entries in both backup directories, but there's really only one
physical copy, that of the one or two that are changed.

     mark