CrashPlan and Time Machine complement each other very well and can be used on the same computer, side-by-side, backing up your live data. But you don't want to try to integrate the two. Read on for the nitty-gritty technical details.
The short answer is that backing up Time Machine data with CrashPlan does not work very well. There is no advantage to sending your Time Machine data offsite if you're already using CrashPlan to back up remotely.
The long, technical answer that follows describes results when we tested backing up Time Machine data with CrashPlan. This chart compares the Time Machine backup archive size before and after backing up to CrashPlan.
|Time Machine Size||Time Machine Size in CrashPlan|
|Start||53 GB after initial backup complete||--|
|End (7 days later)||63 GB||303.5 GB|
Because it's not practical to make a full copy of the file system every hour, Time Machine works by creating hard links to directories and data that have not changed since the previous backup.
A file system contains both content files (e.g. documents, photos, MP3s) and files that store index information about those content files (e.g. size, permissions, type). The files that store index information are called directories (or folders). The link between the content file and the entry for the file in the directory can either be “soft” or “hard.”
A hard link points directly to the address on the disk where the file is actually stored. A soft link is really just a file that contains the path of the target file. A Windows “shortcut” is an example of a soft link.
A soft link has the advantage of being able to reference a file on another device or file system, whereas a hard link can only refer to files on the same file system. A soft link is independent of the target: if a soft link points to a file that is later deleted, the soft link continues to exist.
A target file can have multiple hard links and soft links that point to it. As long as at least one hard link still exists, the file continues to exist. Once the last hard link is deleted, the file is deleted. A soft link that refers to the deleted file no longer works.
CrashPlan follows hard links. In other words, if a file has 10 hard links, the file count increases by 10 and the file selection size increase ten fold. When CrashPlan follows those 10 hard links, the file count is multiplied by 10 and the file size of every file in the directory also is multiplied by 10. That's a lot for CrashPlan to scan! After running Time Machine for just one week, CrashPlan said the Time Machine volume had 7,984,818 files (includes both files and hard links) and had grown to 303.5 GB! The Mac OS measured the Time Machine volume at 63 GB (hard links excluded).
What matters is not the discrepancy between Time Machine and the OS; rather in following those hard links, CrashPlan is scanning an abnormally large number of files rather frequently. Remember, there were almost 8 million files after only a week of using Time Machine. Depending on your CrashPlan settings, this scanning could take a very long time or seriously bog down your system.
As this test illustrates, Time Machine and CrashPlan can work very well side-by-side. Just don't try to integrate the two.
Time Machine was designed for local backup only, while CrashPlan was designed for local and remote backup. If you want remote backup and just one system, you'll probably be happiest using CrashPlan both onsite and offsite. If you prefer to use two systems, use CrashPlan for remote backup of your files only (not Time Machine data) and use Time Machine (or Time Machine and CrashPlan) for local backup.
To ensure that Time Machine data isn't included in your CrashPlan backups, review your backup selection to confirm that the box next to your Time Machine destination is deselected. It should look like this:
If your entire Time Machine is excluded from the backup, then you don't have to worry about any of this data being backed up by CrashPlan. However, if you choose not to exclude the entire drive, then at a minimum you should confirm that the Backup.backupsdb folder for Time Machine is excluded from your CrashPlan backup. The location of this file can vary:
When you specify a volume as a Time Machine destination, it erases the volume first. So in order for CrashPlan/Time Machine to work on the same volume, set up Time Machine first and then point CrashPlan to the volume as a backup destination.
Jonathan DePrizio provides a useful diagram of hard links vs. soft links in his blog posting on the topic.
For general information about setting up Time Machine to perform multiple backups on a single computer, Christopher Breen's article provides a quick overview. Additionally, Cory Bohon provides a step-by-step tutorial on configuring Time Machine for multiple backups.
Finally, if you're interested in learning more even more, James Pond wrote a great article explaining how hard links work with Time Machine. Pond's article also includes information about locating your sparse bundle files. For further information on sparse bundle files, see Steve Sebban's blog post on the topic.