Hard Links to Files (NTFS volumes only)

A hard link is a directory entry for a file. Every file can be considered to have at least one hard link. On NTFS volumes, each file can have multiple hard links; therefore, a single file can appear in many directories (or even in the same directory with different names). The actual file is indicated by a Volume Serial Number (VSN) and a File Index which is unique on the volume. Collectively, the VSN and File Index are referred to as the file ID.

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 file ID 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. The three files are all hard linked to the same data.
2. Next, the original copies of L2 and L3 are backed up to tape, then 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 file ID as L1. Instead, they are assigned a new file ID 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 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. Since the restore can complete only if it can link to L2, L2 is automatically restored by a secondary restore request to the NetBackup server that has the data. If you restore L2 by itself, there is no problem.