[CentOS] evaluating backup systems: rsync

Fri Jan 11 22:24:04 UTC 2013
Les Mikesell <lesmikesell at gmail.com>

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.

> Google it a bit - it really is hard to wrap your head around.
> A symlink is like a reference "this file is found over there", where a
> hard link is like "I point to that location, the same way the inode that
> was created when the file was points to the location.
> I know that's not right. As I said, try googling.

A symlink references another file name, which is found in another
directory entry so it is not the same concept at all.  Hardlinks can
only exist within the same filesystem - symlinks can reference other
mount points even  to places that don't exist, and operations on it
and the file it references aren't atomic.

   Les Mikesell
     lesmikesell at gmail.com