Timemachine backups to a linux server

If your organization uses Apple laptops, you want to support a way to provide backups. Timemachine is the obvious choice. But what if you do not want to run Mac OS X, but only Linux. Then you need to have an AFP service. This is provided by the "netatalk" package. This is similar to "samba". Online you can find enough tutorials on how to set it up. And since a few years it supports timemachine out of the box.

But is has a few thing you need fix, before it can be used to backup a larger number of clients. They are caused by the fact that everything related to the timemachine backup is setup automatically and for this case the defaults are wrong. Timemachine uses "sparse bundles", a sort of file container, resembling something like zip files. The first problem is that the sparse bundle that is create by is setup to fill the entire partition. This means that you can only have one backup per partition. This is not what you want if you have a 4TB disc in your server, you want to give a few 100 GB to each backup. This can be solved by using LVM. Each backup gets its own volume and by configuring netatalk you can give a different share to each individual user.

The second problem is caused by having the Apple sparse bundle on a linux filesystem. By default, the sparse bundle saves the data in files of 8MB. If you are using the default settings of ext4, you over 40,000 files in one directory and that generates strange error messages. There is a command to change settings of the sparse bundle, but unfortunately that is only available on the mac. So you have to change the "band size" of the sparse bundle. These are the commands to use:

cd /Volumes/TimeCapsule
hdiutil convert hostname.sparsebundle -format UDSB -tgtimagekey sparse-band-size=2097152 -o new_hostname.sparsbundle
mv hostname.sparsebundle hostname.sparsebundle.old
mv new_hostname.sparsebundle hostname.sparsebundle
cp hostname.sparsebundle.old/com.apple.TimeMachine.MachineID* hostname.sparsebundle/

To be precise, before you can change the settings, you need a finished backup. You can do this by excluding almost all directories from the backup. This saves time when you convert the sparsebundle. If the first backup is finished, you mount the AFS share on the mac. Then you open a terminal window and go to the location where the share is mounted, in the "/Volumes" directory. With the "hdiutil" command you change the sparse bundle. For some reason, the size must be set in 512K blocks. If this is finished, you rename the old one in a temporary name. Later you can just delete it. But make sure it does not have the ".sparsebundle" extension. Then you rename the newly created bundle in the original filename. The last step is to copy some configuration files, relating to the timemachine backup, from the old bundle to the new one.

Also make sure you disable the timemachine service when you convert the sparse bundle. When the conversion is finished, you can enable timemachine again and run another backup. You can check the timestamps to see if it is now using the new sparse bundle. If this has succeeded, you can remove the excludes and run a full backup.