How NetBackup determines when UNIX files are due for backup
Incremental backups on UNIX clients consider all files and directories to determine if a backup is due based on a reference date. (That is, back up all the files that have changed since date_x).
The following types of time are associated with UNIX files and directories:
When NetBackup reads the data for a file that is included in a backup, it does not affect the file modification time. It does affect the access time of the file. For this reason, NetBackup saves the atime and mtime of the file before it reads the file. Then NetBackup resets the atime and mtime with the utime(2) system call. NetBackup does not cause problems for storage migration products or the administrator scripts that use file access times (atime) as criteria for their operations. While this benefit is obvious, a side effect is that it does update the ctime of the file.
Note:
Customers can configure NetBackup so that it does not reset the access time of the file after it reads a file. Customers can choose to have NetBackup use the ctime and the mtime of the file to determine what files to include in an incremental backup. Normally, these two options are used together, but there may be some sites that want to use one without the other. By default, NetBackup uses only the mtime of the file to determine what files and directories to back up.
When a file is moved from one location to another, the ctime of the file changes, but the mtime remains unchanged. If NetBackup uses only the mtime to determine the files that are due during an incremental backup, it does not detect these moved files. For sites where using the mtime might create a problem, use the ctime to determine files due to be included in an incremental backup. The ctime is used if the bp.conf file contains the USE_CTIME_FOR_INCREMENTALS and DO_NOT_RESET_FILE_ACCESS_TIME entries.
See USE_CTIME_FOR_INCREMENTALS option for NetBackup clients.
See DO_NOT_RESET_FILE_ACCESS_TIME option for NetBackup clients.
When a directory is moved from one location to another, the ctime of the directory changes, but the mtime remains unchanged. Neither the mtime nor the ctime are changed for the files or directories within the moved directory. No reliable method using file timestamps can determine that files within a moved directory need to be included in an incremental backup.
In either case, these moved files and directories are included in subsequent full backups.