diary at Telent Netowrks

Backing up a workgroup server with rsnapshot#

Thu, 10 Feb 2011 21:52:00 +0000

Lately I have been changing the way that $WORK backs up their office server. It should be pretty simple, right? Removeable USB drive they can take home at night, rsnapshot, win.

Almost. The problem is user-proofing.

The disk should not be mounted when not in use

Really, I don't want end-users unplugging a mounted drive. So automounting on drive plugging is a non-starter.

The disk has to be mounted before doing the backup

You'd think you could use cmd_preexec in rsnapshot.conf for this. No. If cmd_preexec fails (e.g. because the disk is not plugged in), rsnapshot prints a warning (which nobody will read) and then goes on and does the backup anyway.

There is a neat option no_create_root which you'd think was designed for this: "rsnapshot will not automatically create the snapshotroot directory. This is particularly useful if you are backing up to removable media, such as a FireWire or USB drive". However, it does this test before running @cmdpreexec@, which is entirely the wrong order of events for our purposes

The disk must be idle before being unplugged

Well, there's not really any physical way to stop people unplugging it during a backup, but we could at least give ourselves a sporting chance by telling them when the backup's happening.

There are several parts to this: first we spend an inordinate amount of time fighting with zenity to make it do something vaguely sane before giving up and creating a workaround that lets it do what it wants instead. The result looks a lot like rsync-wait.sh in which, if your external HDD is not called Fred, feel free to amend the text messages. If you're one of the vanishingly small number of people who has to use GNOME but isn't on a Linux kernel or for some other reason don't have inotify, you'll have to replace inotifywait with e.g sleep 10

We want this to be run for every user, because they all have physical access to the server and any of them might be charged with taking the disk home. So we need a way of launching this script on login and stopping it on logout - surprisingly the latter is harder. If you write an X client program in a real language it will probably eventually notice (or just die with SIGPIPE) when its X server socket closes, but a shell script doesn't necessarily have a persistent X connection open - so if we start the script from Xsession.d or similar and if you log out and in again the process won't die, and you end up with two copies running at once. Double bubble trouble.

To save you the afternoon that this took me to figure out, the simplest answer is "start it from xterm". So, create a .desktop file and drop it in /etc/xdg/autostart directory

dan@carnaby:~$ cat /etc/xdg/autostart/rsync-wait.desktop 
[Desktop Entry]
Name=Backup Disk notifier
Comment=System tray icon for notifying rsnapshot running
Exec=/usr/bin/xterm -iconic -e /usr/local/bin/rsync-wait.sh

So far, this is working. I think.