Hard Links to Files

A hard link differs from a symbolic link in that it is not a pointer to another file, but is actually two directory entries pointing to the same inode number.

During a backup, if the backup selection list includes hard-linked files, the data is backed up only once, using the first file name reference found in the directory structure. If a second or subsequent file name reference is found, it is backed up as a link to the name of the first file. This means you get only one backup copy of the data, regardless of whether you include one or multiple hard links. You can include any of the paths that are hard links to the data in order to back up the data.

During a restore, if all of the hard-link references are restored, the hard-linked files still point to the same inode as the other files to which they are linked. However, if you do not restore all the hard links, you can encounter anomalies as shown in the following examples.

Example 1

Assume there are three hard links named L1, L2, and L3 that are pointing to the same data.

1. During a backup of L2 and L3, L2 is encountered first and backed up, then L3 is backed up as a link to L2.
2. Next, the original copies of L2 and L3 are both deleted, leaving only L1 on the disk.
3. During a subsequent restore, you restore L2 and L3. The restored files, however, do not point to the same inode as L1. Instead, they are assigned a new inode number and the data is written to a new place on the disk. The data in the new location is an exact copy of what is in L1. The inode duplication occurs because the backup does not associate L2 and L3 with L1.

Example 2

Assume in example 1, that you attempt to restore only L3. Here, NetBackup cannot link L3 to L2 because L2 does not exist. The restore therefore fails and you see an error message in the progress log. If you restore L2 by itself, there is no problem.