How to analyse Time Machine back-ups
GrandPerspective can be used to analyse back-ups made by Time Machine. This can help you to optimise your Time Machine back-up scheme, for example, by identifying files and folders that should not be backed up.
First of all, it is important to understand how Time Machine back-ups are stored. Time Machine makes extensive use of hard links. Each back-up appears to store a copy of the entire drive that is backed up. Yet the amount of disk space needed for a back-up is limited the files that have changed since the preceding back-up. All files and folders that have not changed, are represented by hard links to these items in the preceding back-up.
Below are some ways in which you can use GrandPerspective to analyse your Time Machine back-up. They are simply meant to give you an idea of what you can do. You are of course free to experiment with other approaches.
Scanning all back-ups
Execute Scan folder... on the folder "Backup Volume/Backups.backupdb/My Computer". This may take a while. When scanning is complete, a view is shown showing all back-ups. In the Display panel, select coloring by "Top folder". All files belonging to the same back-up now have the same color.
You will probably see that one back-up is much larger than the rest. This is the back-up folder that was scanned first (it is not neccessarily the earliest back-up). All files in this folder had not yet been encountered during scanning, and have therefore been included. When scanning the other back-up folders, most hard-linked files will have been encountered already in the first folder that was scanned, and so are now exluded.
The folder that was scanned first is actually the least interesting. It simply shows the contents of your hard-drive at the time of the scan (minus the files that you have chosen to exclude from your Time Machine back-ups). So let's exclude it from the view. Start by editing the view's mask. You need to create a new rule. Name the rule "Main back-up", apply it to "files and folders", and let it include the Path test, matching paths that start with "/Volumes/Backup Volume/Backups.backupdb/My Computer/2008-08-08-888888". The parts in italics will be different in your case. You can get the path string you need from the Focus panel. First in the view select any file from the largest back-up folder, go one step "Down", copy the path from the "Folder in view" text box, and go "Up" again in the view.
After having created the "Main back-up" rule, set the mask as follows: "Do not show: Main back-up". You should see that the main back-up is now masked. If this is not the case, you did not specify the mask correctly. Edit it again until it works.
When the mask works correctly, you should filter the view. Use the Filter... command. It should by default be configured the same as the mask, so simply click "OK". A new view window is created, only showing the other back-ups. This new view gives a much better idea of the size required by each incremental back-up. Navigate and explore this view to get a better idea of what is backed up. If you spot files that should not be backed up, update your Time Machine preferences.
Check which files have been backed up during the latest back-up
Initiate a Filtered Scan.... As the folder to scan, pick "Backup Volume/Backups.backupdb/My Computer/Latest". Apply the following filter: "Do not show: hard-linked items". The result is a view showing the files that are new in the latest back-up. The view excludes all files that appeared in the earlier back-ups.
Note: You can apply the same filter to other back-up folders. However, you should then interpret the data in the view differently. In this case, the files that are shown do not appear in earlier back-ups, nor in later back-ups.
. J'espère que cela vous intéressera, il explique assez bien comment cela fonctionne et comment visualiser les dernières sauvegardes.
