CCC reported that the destination is full. What can I do to avoid this?

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By default, CCC starts with a SafetyNet pruning limit that will establish 25GB of free space on the destination at the beginning of each backup task. CCC will increase that limit automatically as necessary. If you are not using CCC's default SafetyNet settings and you're running into a "destination is full" error, then you may need to apply a more liberal pruning limit in Advanced Settings. The amount of free space required on your destination depends on the size of files that you normally edit during the day. In general, you should have as much space available at the beginning of the backup task (e.g. right after pruning is complete) as you ordinarily see copied during a backup task. So if CCC ordinarily copies 9GB of data, maybe with a spike to 14GB every once in a while, you should configure your pruning settings to accommodate that maximum value (e.g. leave at least 15GB of free space). Especially if you modify large files on a regular basis, the nominal amount of data copied each time could be fairly high. If you use a Windows virtual container that is 80GB on a daily basis, for example, the nominal amount of data copied during your daily backup task will be at least 80GB, so you will have to accommodate that with your pruning settings.

To change CCC's SafetyNet pruning settings, select your task in CCC's main application window, then do the following:

  1. Click the Advanced Settings button
  2. In the Before Copying Files section, indicate how CCC should prune the SafetyNet folder, e.g. based on free space available on the destination, age of the archives, or size of the archives.
  3. Specify a limit.
  4. If you selected the free space option, consider checking the Auto adjust checkbox so CCC can manage this value for you automatically.
  5. Save the changes to your task.

Why does CCC report that the destination is full when it appears to have enough room for newer files?

To prevent overwriting a good backup file with a corrupted file on the source, CCC uses a special file copying procedure called an atomic copy. If a file has changed since the last backup, it will be copied to the destination using a temporary filename, e.g. .filename.XXXXXX. When CCC has finished copying the file successfully, CCC deletes (or moves to the SafetyNet) the older version on the destination, then renames the updated file to the correct filename.

Because CCC uses this special procedure, the destination volume must have, at minimum, enough free space to accommodate all of the data that will be backed up plus enough room to accommodate a temporary copy of the largest file on the source volume. If you frequently modify very large files, such as movies, disk images, or virtual machine containers, you should designate a backup volume that has considerably more space than is consumed by your source volume to avoid running out of space during a backup task, and you should configure CCC's SafetyNet pruning settings to accommodate a temporary copy of the largest file on the source volume.

An example to illustrate the dilemma

Consider the following scenario:

  • 500GB source volume
  • 500GB destination volume
  • 450GB of data on the source
  • The largest file on the source is 75GB

If the destination is empty, the math is easy — 450GB of data easily fits on a 500GB disk.

Now let's go to a subsequent run of the backup task. Suppose no changes have occurred at all on the source, except to that 75GB file. How shall we proceed to copy that file to the destination? The destination has only 50GB of free space at this point.

Option A: Fast and loose

  • Delete the 75GB file from the destination
  • Copy the newer 75GB file from the source to the destination

Option B: Atomic copy

  • Copy the newer 75GB file from the source to the destination
  • Delete the 75GB file from the destination

Option B is impossible in this scenario. But, Option A is foolish. CCC never uses option A, that's just gambling with your data. This isn't theoretical either, we've heard stories of people losing data in this manner with other "backup" software.

CCC uses the atomic file copying method. Rather than deleting a file that will be replaced, and then copying the replacement file, CCC copies the replacement file to the destination first (using a temporary file name). When the file has been copied successfully, CCC then removes (or archives) the older version of the file, and then renames the temporary file to its correct name. This is particularly important should CCC discover that the source file is unreadable due to a media error. With the "Option A" copying behavior, you'd be left with no good copy of the file on the destination and of course the corrupted copy on the source. The downside to the atomic copying method is that the destination needs to have enough free space to accommodate the old version of the file and the replacement version of the file.

If you find yourself in a similar scenario, you have a couple options:

One last note – CCC's "Run a deletion pass first" troubleshooting option does not contradict the atomic copying procedure, so it is not applicable in this scenario. The deletion pass removes files from the destination that are no longer present on the source, it does not remove files that will be getting updated during the backup.

I have SafetyNet turned off, how could the destination be too full?

If you have disabled CCC's SafetyNet setting, note that deletions occur as the items to be deleted are encountered. CCC traverses the files and folders on your source and destination volumes in alphabetical order, so it is possible that CCC will attempt to write new files to the destination before deleting items that were deleted from the source. If you have made large organizational changes on the source (e.g. renamed or moved folders, deleted and created many items), you may want to try the following steps to proactively clear space on the destination:

  1. If you did not choose the option to delete the SafetyNet folder from the destination when you disabled the SafetyNet option, choose Delete a SafetyNet Folder... from the Utilities menu. Drag the _CCC SafetyNet folder from the Finder onto the Delete a SafetyNet Folder window to remove that folder.
  2. Click the Advanced Settings button.
  3. Uncheck the box next to Protect root-level items on the destination.
  4. Check the box next to Run a deletion pass first in the Troubleshooting Options box.
  5. Save and run the backup task.

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