An incremental backup backs up all `new things', that is all files and symbolic links in a set that were created or changed since the previous (full or incremental) backup.
This includes the files that were `installed' (e.g. extracted from a
tar archive) after the last backup. If you do an ls -l
on such
a file you usually don't get the date it was created (installed) on
your system, but the date it was last modified by the author of the
tar file. To see the `created/last modified' or `last status change'
date that is used for incremental backups, do a ls -cl
on the
file.
An incremental backup usually costs much less time than a full backup, but incremental backups do have their shortcomings.
An incremental backup does not record
Thus a restore of a full backup followed by a restore of (one or more) incremental backups will usually not reproduce an exact copy of the backed up filesystem: files that were moved (renamed) once may be restored under the old name rather than the new name, and files that were deleted after the full backup will be present again.
So if you do a drastic reorganization of your filesystem, it is best to make a new full backup.
Another problem with incremental backups is that files moved *between* directory sets may never get recorded in any incremental backup. This is possible because moving a file does not affect its `created/modified' date.
Consider the following scenario:
important
in a directory belonging
to the system
setuser
setimportant
file to the user
setsystem
setuser
setStep 2) and 4) did not back up important
, but step 5) didn't either
because important
was created/modified before, and not after, the
previous backup of user
.
If you want to move files or directories between directory sets, first `synchronize' the sets by making an (incremental) backup of both. After you have done that, you can safely move the files without worrying about the above scenario.
Of course, the same kind of synchronization needs to be done if you
move directory names from one directory set description (.set
)
file to another.
MSDOS filesystems do not record enough information to make the incremental backup process reliable. In general, you should always make tbackup full backups, never incremental backups, of files on an MSDOS filesystem.
The problem is that MSDOS filesystems do not keep `last status change' timestamps on files. When used on an MSDOS filesystem, an incremental backup will look at the regular file date to see if the file is new. This date is an unreliable indicator of `newness', however, because files which were installed (e.g. extracted from a tar archive) after the last backup usually have the timestamp set to the date of creation of the original file, not the date of installation. Such new files will thus be missed by the incremental backup.
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