Issue

Incremental backups are the same size or larger than the full backups, even if an incremental backup ran an hour after the full completed.

Issue description

If this behavior is seen, it is likely that the file set for this backup has Preserve Access Time selected, while Only Check Modification Times cleared.

On the Unix-like operating systems, when you do a differential or incremental backup, files will be included for backup if the ctime (change timestamp) or mtime (modification timestamp) is greater than the time of the last backup (so if the file has been modified or changed). Opening the file to read it for backup, updates the file’s atime (access time). The Preserve Access times option tells the agent to reset the atime on the file to the value it was before the file was backed up. Setting the atime is a change to the file, so the ctime gets changed. Because of that change, the next backup finds the file has a newer ctime, and so it gets included in the backup again. The Browse and Restore dialog shows the file’s mtime (which contributes to the confusion).

Steps to resolve

There are two options for making incremental and differential backups work properly for a Linux client.

One is to clear Preserve Access Times. When a file gets backed up, its atime will be set to the time of the backup. With this option off, the atime will not get reset, to the ctime remains unchanged. Some programs do not like having the atime changed on their files, so turning this option off could have mildly unpleasant consequences if such a program is being used.

Second is to select Only Check Modification Times in addition to having Preserve Access Times selected. This will ignore changes to the ctime, and only back up the file if the mtime is newer than the last backup. Changing the properties of a file without modifying its contents will only update the ctime of that file. So with this option, changing the owner, group, or permissions of a file (or other file attributes) will change the ctime, but that change will not be noticed and backed up until the next full backup.

Unless it causes a problem for some software you are using, the first option is recommended.