Tuesday, October 19, 2010

rsnapshot - it's for pulling, obviously!

It's been a few months since I learned about rsnapshot, a neat little utility that uses hard linking to create multiple point-in-time snapshots of your data. It combines the neat features of rsync into a package that does more than just mirror data.

If you've ever had clients who suffer from "data decay", you'd know that just having the most recent version of a file can be useless. An Excel spreadsheet could have been corrupted weeks ago, and people will continue to write changes to it--despite warning messages--until it really fails hard, which is always too late.

Conceptually I was having a hard time understanding the topology of rsnapshot server and the clients it's supposed to back up. Are you supposed to have rsnapshot on every client machine, connecting to the server using only rsync? Actually, no.

Quite foolishly I was modifying the snapshot_root directive in /etc/rsnapshot.conf to point to a network location (CIFS share), even though the comments clearly state that it's supposed to be a local root. I guess this should have been a no-brainer, but in my skimming of the documentation it wasn't clear why I couldn't set the snapshot_root to be a network location!

Only when I tried to use rsnapshot in conjunction with a TeraStation Live did I learn the truth! Behold, rsnapshot sits on the server, manages it's own backup root, and communicates with clients using only rsync, or rsync over SSH. This makes sense because the rsnapshot server must scan its local repositories for changes, and it does folder rotation. For example:


Say you set retention to 5 days. This means that the next daily backup will cause a rearrangement. daily.2/ becomes daily.1/ and daily.1/ becomes daily.0/, and daily.0/ disappears. On a local filesystem, these reflect simple inode reference changes for moves and renames, but on a remote FS, all bets are off.

On a related note. I'll have a quick guide on how to get root access to a TeraStation LIVE, and how to install rsnapshot.

A second post will go over how to initiate a VSS snapshot of a Windows drive over SSH in preparation for a pull backup from a Windows client to a Linux server.

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