On Mon, 2006-11-06 at 14:54 -0800, Mark Schoonover wrote:
> >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.
>
> This is true if you're doing it with only filesystem tools, but this system
> is using rsync. What's happening is the cp -al occurs first making hardlinks
> that point to an hourly directory into the current directory, then rsync is
> run to update current. Because rsync will create a new temp file when any
> file changes, the original is deleted with it's data 'pushed' to any
> hardlinks pointing at the original file. Rsync then renames the temp file
> the original file name that has changed, therefore assuring that any
> hardlinks will always have the previous copy of any changed files. With
> rsync running in --delete mode, any files from the source server that get
> deleted, will get deleted out of current in the backup server, causing this
> cascade of hardlinks to get updated with the deleted files data. That's how
> this system can create incremental backups of only changed data, but with
> hardlinks, it looks like full backups are made each and every time. Really
> saves disk space, that's for sure!
>
> Hope this clears things up...
Backuppc is even more extreme in the space savings. It first compresses
the files, then detects duplicates using an efficient hashing scheme
and links all duplicates to one pooled copy whether they came from
the same source or not. It includes a custom rsync on the server
side that understands the compressed storage format but works with stock
versions on the remote side so you don't need any special client
software. And it has a nice web interface for browsing the backup
archive and doing restores.
--
Les Mikesell
lesmikesell@xxxxxxxxx
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