Overview of Incremental Filesystem Backups
Background and description of what makes up a filesystem incremental backup.
An incremental backup is one that backs up only those files which have changed since the last backup (which can be either full or incremental itself, this is defined within the job).
For the purpose of the incremental backup a “changed file” is defined as follows:
A file is included in an incremental backup on UNIX, LINUX or MAC when its last modified or last changed time is more recent than the last recorded backup.
These can be viewed by using the command "stat filename" on most flavours of UNIX and LINUX.
e.g.[root@bobafett tmp]# stat kj_notes.txt
Size: 3742 Blocks: 8 IO Block: 4096 regular file
Device: 302h/770d Inode: 1815141 Links: 2
Access: (0000/----------) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2007-02-09 10:46:16.000000000 +0000
Modify: 2006-04-11 17:34:52.000000000 +0100
Change: 2006-08-11 10:23:45.000000000 +0100
A file is included in an incremental backup on Windows if its last modified time is more recent than the last recorded backup or the Archive bit has been set and Archive Bits are NOT ignored in the backup job.
On a Linux, Mac or Unix system, a file has several time based attributes:
Last Access Time
Last Modified Time
Last Changed Time.
The second two of these may appear to be the same, however, there are differences:
The Last Modified time is when the file was last modified by a write or touch command. In UNIX this is defined as:
Time when data was last modified. Changed by the fol-
lowing functions: creat(), mknod(), pipe(), utime(),
The Last changed time includes changes which effect the attributes AND the contents of the file:
Time when file status was last changed. Changed by the
following functions: chmod(), chown(), creat(),
link(2), mknod(), pipe(), unlink(2), utime(), and
On Windows a file has the following time based attributes:
For an incremental backup, the Modified time is used to identify files to be included in the backup, however, the Archive bit is also taken into account unless the "ignore archive bit" is selected on the Backup Options tab.
If the archive bit is set - this file will be backed up in an incremental backup if the "ignore archive bit" is NOT selected, even if the last modified time has not changed.
The archive bit will be cleared by the backup.
Troubleshooting Incremental Backups.
In the first instance you can see how many files have been backed up in a full and incremental backup by looking at the context information in the log message "Backup Completed":
Job Message 2007/04/18 10:20:57 173 Data Plugin nv_test Job completed
Backup tally for job : 1 directories, 4 files
If you compare these and think there are more than should be included in an incremental, you will need to identify which files have been included. In order to do this - ensure that you have a minimum filesystem APM version of 7.0, then on the Backup Options tab, create a "log of files backed up".
This will generate a list of all the files that were backed up in this run of the job.
Having identified the files that have been backed up, use the UNIX "stat" command on the files to identify when they were last changed. For Windows use the properties information of the file, but note that if the archive bit had been set, you will not see this indicated on the file after the backup.
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