Is differential backup reliable?

[windwizard]

Hello!

I'm pondering the backup strategy in my organisation right now. I've got three Windows servers, including one SQL server and one IBM DDS-5 tape device (DAT72).
I've got also BackupExec 9.1 installed on one server.

I'm thinking of implementing differential backup scheme. However, I have never used it before, and therefore I have doubts regarding rekiability of this scheme.

Could you advise me on which differential backup to choose. Archive Bit, Modified Time or may be NTFS Change journal. What's is your opinion?

For instance, if I choose Archive Bit method should I avoid using different backup programs like MS Windows Backup?

Any considerations are welcomed.

[Keith_Langmead]

HI,

Well personally, I don't know anything about teh NTFS change journal, but from the other two options I'd pick the Archive bit method rather than modified time, purely because I don't like relying on nothing screwy happening to the system date and time on the server.

In reliability terms I can't think of any reason for concern about the reliability of differential backups. Given the choice I'd prefer to only use Full backups, but I'd rather have differential backups than incremental backups any day. Not just because it takes less time to restore the data is required, but also because you're not as reliant on a series of tapes not having problems, and if the worst does happen, and your most recent tape becomes faulty, you can still just move back to the previous one.

One thing to note however, if memory serves correctly, SQL data is always backed up as a Full backup, as Backup Exec can't delve into the database and pick out only the data which has changed, so you won't see any space saving from backing up that data using a differential backup.

[Deepali_Badave]

Hello,



Backup Exec 9.x provides an option to create and run incremental and differential jobs using the modified time on the files, as opposed to being limited to the archive bit. This feature adds flexibility to those running operating systems that do not have an archive bit, such as UNIX or Linux based operating systems.


In order for these backups to function correctly, a full backup using modified time must be run and all jobs must use the same selection list when created. The modified time of the files is kept in the Backup Exec database, and is associated with the selection list. If a full backup using modified time is created, and then a differential or incremental job using modified time is created with a different selection list, the jobs will back up the same amount of data.

In case of backup using archive bit :-
A Full Backup backs up all files on a system, and then resets the Archive Bit on all files that were backed up. A Differential Backup backs up only new and changed files on the system; it does not change the Archive Bit





DIFFERENTIAL - Changed Files. Includes all files that have
changed (based on the archive bit) since the last full backup, and
does not affect any media rotation scheme because the archive bit is not reset.


DIFFERENTIAL - Using modified time. Includes all files since
the last full backup, using the files’ last modified date and time stamp. Make sure that the same script or selection list is used for the differential backup that was used for the full backup.

Use the Windows Change Journal if available
Select this option if you want to use Windows’ NTFS Change Journal to determine which files have been modified since the last full backup. This option can only be used with NTFS volumes and only when the backup method selected is FULL - Back Up Files -
Allows incrementals and differentials using modified time,
DIFFERENTIAL - Using modified time or INCREMENTAL - Using modified time.


we hope this will help you.

NOTE : If we do not receive your intimation within two business days, this post would be 'assumed answered' and archived.

[Sheetal_Risbood]

As per our previous reply, marking the case as assumed answered and moving it to answered questions pool.

[Paul_Banda]

Hello,

Our situation is this: we have Backup Exec 9.1 running on a Windows 2000 server, which backs up our XServer. There are two shares: archive, and artserver. We currently have the Full backup set up for allowing the modified time setting, and it backs up (in this order) archive and artserver on a weekly basis (Saturday afternoon). The daily differentials are set up to only backup artserver.

What's happening now is that the daily differentials are backing up the entire artserver, regardless of the fact the full backup ran without errors or exceptions the weekend before. The script uses the same resources (except for the daily, which *doesn't* read from the archive share), uses the same user log in, and the same network device.

Is it then the case that the full backup database contains the archive file modification dates, *then* the artserver file modification dates so that when the daily differentials start, they look for the archive file date data, don't find it, and assume they need to do a full backup?