[Veritas-bu] NT/2000 backup strategy and two class management
AhrensJ at psi.ca
Wed Oct 10 09:53:15 CDT 2001
> -----Original Message-----
> From: Rob Worman [mailto:rob at worman.org]
> Sent: October 5, 2001 08:46
> To: veritas-bu at mailman.eng.auburn.edu
> Subject: Re: [Veritas-bu] cumulative backups
> nope. An incremental/cumulative schedule defined in class1
> only does its file comparison based on other backups by
> class1. It's as if the other class doesn't exist.
Some of the reading I've done on NetBackup when used on NT is that NetBackup
does in fact use the archive bits on NT/2000. This can cause problems
depending on use (ie: two classes doing full backups, two classes managing
incremental backups, where one of them is not cumulative incrememtal,
I may be wrong, I am just going on what I read for the NT/2000 option. Bring
up the Client Properties on a Windows system. By default on the 'Windows
Client' tab, 'Perform incrementals based on archive bit' is turned on.
Turning off this option has other (not so pleasant) ramifications but will
get around the archive bit and two different classes problem on Windows.
By default, a full backup will backup up an entire file system and clear all
the archive bits. When a file is modified, the system is supposed to set the
archive bit again. NetBackup (and pretty much all other Windows based backup
schemes) use the archive bit to know which files to backup for an
incremental. Now, if you do a cumulative incremental, the archive bit is
left on after backup. If you do a differential incremental, the archive bit
is cleared again. A file with a cleared archive bit will not be backed up
until the next full (unless something sets the archive bit once again).
The danger here should be obvious. If two different classes are performing
backups of a Windows systems, and each class is playing with the archive
bits on the file system, the integrity and consistency of both sets of
backup images are suspect for anything except full backups (unless nothing
except full and cumulative incremental backups are done, you cannot use
Turning off the 'use archive bit' option causes NetBackup to begin looking
at the last modified date to decide if a file should be backed up. This gets
around the archive bit messiness, but if someone resets a modified date, or
moves a file (modified date won't be updated I don't think) or restores a
file from anywhere that keeps the old modified date, NetBackup will not back
up those files again until the next full backup (it thinks it's already on
tape). However, if you are going to have two different classes (or two
different programs, but that's another story altogether) doing backups of a
Windows system, this is your only option to maintain bakup integrity (if
differential incremental backups are involved).
On Unix, this problem is non existant. I beleive NetBackup uses ctime
comparisons, which is much more reliable that last modified time or archive
bit (which don't exist on any Unix I've seen anyway) settings.
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