Differential Backup -> What's really going on?

[Markus2791]

Hello everyone....

When I read page 175 of the BeAdmin_en.pdf, it says:

After a differential backup, each subsequent differential backup backs up the same files as the previous differential backup. It also backs up as any new files or changed files dating back to the last full backup or incremental backup.

That statement is very clear.  I want to utilize it with a 1 TB file server.  Here is my strategy:

1. Do a full backup on the first Saturday of each month.

2. Do a differential backup on the remaining Saturdays.

3. From Monday to Friday, do an incremental backup at 6 PM.

I have created a backup job using "Modified date".

The full backup is successful, and the 5 incremental backups work as expected: only the newer and modified files are backed up.  (Several gigabytes of files are backed up.)

On "differential backup Saturday", only a few files are backed up, but nothing close what was is backed up over the course of the week in the incremental backups.

Doing further testing, I see that as soon as the incremental backup is done, any future differential backups will only backup files from the previous incremental backup.

But, the documentation cleanly states:

After a differential backup, each subsequent differential backup backs up the same files as the previous differential backup. It also backs up as any new files or changed files dating back to the last full backup or incremental backup.

The keywords here, are "...it also backs up as any new files...".  (Yes, there appears to be a gramatical error in the PDF.)

However, what I am seeing is that once the incremental is run, the files from the "previous differential" are NEVER re-copied.  No where does it mention "unless a incremental backup was run."

Is this the expected behaviour?  Should a restore be required, I only want to restore the last full backup, the last differential backup, and any incremental backups that take place.  The worst case scenario is that I would have to restore 7 backup sets.  (Full, a Saturday differential, and 5 incrementals.)

Any suggestions?

[Markus2791]

And just to add to this....

As mentioned previously, I am using "Modified Time" as the File/Folder setting.  On page 184, under "Modified Time", it says this:

When Backup Exec runs a full backup or incremental backup job, the time that the backup job starts is recorded in the Backup Exec Database. Backup Exec adds the time of the backup job to the Backup Exec Database only if the full backup job completes successfully

When you run an incremental backup job, Backup Exec records a new time in the Backup Exec Database. The database time is not updated for differential backup jobs.

That implies that the previous documenation "After a differential backup, each subsequent differential backup backs up the same files as the previous differential backup. It also backs up as any new files or changed files dating back to the last full backup or incremental backup." is incorrect.

Any comments for anyone?

Thanks!

[pkh]

You might want to read my article below before proceeding with your scheme.

https://vox.veritas.com/t5/Articles/The-Pros-And-Cons-Of-Various-Backup-Schemes/ta-p/810751

Unfortunately the images got lost when the article was ported from Symantec to Veritas.

[Colin_Weaver]

To try and explain this

Modified Time and Archive bit methods both use a backed up / not backed up markers in the same way (just one uses an attribuite of the file system itself and one uses a change journal)

If no backups have ever run, every file has it's marker set to a status of backup needed.

A full backup always resets the markers to backup not needed

As each individual file is modified the change also changes that file's marker to backup needed.

An incremental backup also always resets the markers to backup not needed

A differential backup leaves the markers on backup still needed.

When full backups run they don't bother looking at the status of the markers as they just backup everything.

Incremental and differential backups do check the status of the file markers to identify what needs to be backed up.

Hence

Full --> Incremental1 --> Incremental2 --> Incremental3 results in all the incrementals and full being needed for a restore and each incremental only contains files changes since last  full or incremental

Full --> Differential1 --> Differential2 --> Differential3 results in Differential3 and full being needed for a restore as each differential contains everything changed since the full and ignores previous differentials

Full -> Inc 1 --> Inc2 --> Diff1 --> Inc3 --> Inc4 is classified as bad practice as it makes things extremely complicated for understanding your media retention and what is needed for a restore (especially as most admins would set incrementals with the shortest retention times, then differentials then fulls)

1) Inc4 is actually still chained through all the incrementals to the fulls (not any differentials) and all incremental sets and the full would be needed for a restore. (but you may have overwritten Inc1 and Inc2 based on media retention timeframes)

2) Diff1 is not chained dfirectly to the full but also via Inc1 and Inc2 (which would make Diff1 act as an out of ongoing chain Inc3 and again you may have let Inc1 and Inc2 be overwritten) - so now Diff1, Inc1, Inc2 and full would be needed for a restore getting worse if a Diff2 had been run later than Inc4.

As more iterations of the sequence occur (until the next full runs anyway) you lose the restore benefits of a differential strategy only needing two backup sets (full and last differential) , you also increase the chance that a required incremental may have been overwritten (if tape) or that DLM won't have reclaimed storage space (if backing up to disk).

For these reasons it is bad practice to mix incremental and differential backups in the same job sequence and therefore NOT recommended.

[Markus2791]

Thanks for replying!  I have no doubt that what you are saying is taking place during the backup is true.

My complaint is, why does the documentation conflict with what is really happening?  The user's manual is telling me the wrong information:

"After a differential backup, each subsequent differential backup backs up the same files as the previous differential backup. It also backs up as any new files or changed files dating back to the last full backup or incremental backup."

Would you say that that statement is true or false?

Another misleading are is the user interface when I choose between differential and incremental.  It clearly states:  "Backup changed files since last full".

....and if it is "bad practice", then that should be mentioned in the documentation.  The user interface should be preventing me from doing it as well, or at least give me a warning.

I would just like it if the instructions matched the application.  It's not like this is brand new software, right?

[Colin_Weaver]

Thanks for pointing out the issue with the guide and console - I will pass the info on.