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Commentary from a long term NetWorker consultant and Backup Theorist

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Does NetWorker scan the indices for changed files when it does a backup?

Posted by Preston on 2009-05-21

This is a fairly common question to see asked – does NetWorker, when a non-full backup is run, scan the existing client indices to determine what files have changed from previous backups?

The short answer is: no.

The more in-depth answer is that NetWorker will use one of a few different mechanisms for determining what files should be backed up in a non-full backup scenario, and none of those mechanisms involve scanning the client indices. These mechanisms are:

  • Check for files that have changed since a certain date. Whenever a non-full backup is run, the NetWorker server includes in the backup command the last savetime. Thus, all changed files can be quickly calculated from this.
  • Check for changes according to the change journal (Windows only).
  • Check for changes based on the archive bit (Windows only).

Personally, I really dislike the use of the archive bit. Too many programmers on Windows take liberty with this odious little setting, and it’s become so bastardised and unreliable that my very firm recommendation is you follow the instructions in the NetWorker administration guide to turn off use of the archive bit in incremental backups. (Hint: search for NSR_AVOID_ARCHIVE*).

So, there’s 3 ways that NetWorker can be expected to use to determine what files should be backed up in a non-full backup – and none of those mechanisms are achieved through an index scan.


* [Updated 2009-06-18]

Expanding on this more fully – on the backup server itself, establish an environment variable called NSR_AVOID_ARCHIVE and set it to any value other than “No”. I prefer to set it to “YES” or 1 so it’s entirely clear what the desired result is.

On Unix, places to set this is in the /etc/profile or the NetWorker startup script; however, the problem with setting it in the NetWorker startup script is that you have to remember to re-create that setting every time you upgrade NetWorker, since the startup script is fully replaced each time.

In Windows, set it as a system environment variable under the properties for the system itself. These variables are established before programs are started, meaning that NetWorker will be aware of them when it starts.

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