[CentOS] Best backup software for linux

Les Mikesell lesmikesell at gmail.com
Mon Nov 6 19:18:32 UTC 2006

On Mon, 2006-11-06 at 18:42 +0000, Peter Crighton wrote:

> You wrote "Hardlinks are key to this backup strategy. Using cp -al
> creates hardlinks to files, and this simple command is what does all
> the heavy lifting for daily and weekly backups. Wikipedia has a very
> good explanation on how hardlinks work. In a nutshell, when there's a
> hardlink pointing to a file from the hourly directory, to a file in
> the current directory, and that current file gets deleted, all the
> links that point to that now deleted current file gets the file data
> 'pushed' back towards all the links. I'll have to think how to explain
> this better."
> Do you mean that the hourly files are written when created, the
> hardlink for the daily doesn't actually copy the file (simply makes a
> link), but if the file is set to be deleted from it's location
> (because it's gone from the server) then it is actually moved so that
> it still exists in the daily backup  but is removed from the hourly?
> --

Think of all directory entries as links.  The real entries that
map disk space to files are inodes and links are names pointing
to the inodes.  There can be any number - including 0 - of links
to an inode.  The space is not released for re-use until the
link count goes to 0 and no process has the file open.  So hardlinks
are just multiple names pointing to the same data, and the data
doesn't go away until the last name is removed.  Note that this
only works as a backup if the original filename is removed.  If it
is overwritten or truncated instead, all links now point to the
changed version.

  Les Mikesell
   lesmikesell at gmail.com

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