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Administrator Guide

Rick_DeBay
Level 5
From page 264, "About the differential backup method:"

"Differential backups include all files that have changed since the last full or incremental backup. The difference between differential and incremental backups is that incremental backups include only the files that have changed since the last full or incremental backup."

According to that, differential and incremental are exactly the same.  It should read:

"Differential backups include all files that have changed since the last full [strike the word incremental] backup.  Incremental backups include all files changed since the last full, differential, or incremental backup [add the word differential]."
11 REPLIES 11

AmolB
Moderator
Moderator
Employee Accredited Certified
In Incremental backup the archive bit is reset whereas in Differential backup the archive bit is not reset.

If you take a full backup on Monday and then incremental on subsequent days then Tuesday's incremental will back up 

only those  files which were modified on Tuesday. Wednesday's incremental will backup only those files which were modified on

Wednesday. Whereas Wednesday's differential will backup Tuesday's + Wednesday's modified files as the archive bit is not reset.

Ken_Putnam
Level 6

Rick,

 Can't believe that got past the review!   But your correction is not quite right

What they are missing is the info about the Archive Bit


"Differential backups include all files that have changed (have the Archive Bit set) since the last full or incremental backup and DO NOT reset the Archive Bit.  Incremental backups include all files changed since the last full or incremental backup and DO set the Archive bit]."

I've set the Support Flag on this thread, so hopefully a Symentec Employee will see it sooner rather than later

Rick_DeBay
Level 5
The archive bit has nothing to do with the definition of full, differential, or incremental backup.  That is just one implementation, which is specific to DOS based file systems.

Rick_DeBay
Level 5
In the context of the original post, that is not correct.  The definition of differential and incremental backup does not have any reference to archive bits.
Computer data backups existed long before the FAT file system :)

Ken_Putnam
Level 6
Well

with BackupExec, unless you specify modified time, DIFF and INCR by default DO use the Archive Bit

Ben_L_
Level 6
Employee
So after reading over this thread and thinking about it for a while, the statement is correct from the admin's guide, but it is a bit vague.
 
Regardless of if you are using Archive bit or Modified time, the differential will back up only changed files since the last archive bit reset or the modified time.  The Modified time of the last backup is stored in the database, so we will only look for files that have been changed since then.
 
Example: Full + Incrementals and Differentials
Monday Full - All files backed up, Archive Bit reset or Modified time set.
Tuesday - Incremental, only files changed since the Monday full.
Wednesday - Differential, only files changed since the Tuesday Incremental
Thursday - Differential, only files changed since the Tuesday Incremental
Friday -  Incremental, only files changed since the Tuesday Incremental
Saturday - Differential, Only files changed since the Friday Incremental
 
 
Example: Full + only incrementals
Monday Full - All files backed up, Archive Bit reset or Modified time set.
Tuesday - Incremental, only files changed since the Monday full.
Wednesday - Incremental, only files changed since the Tuesday Incremental
Thursday - Incremental, only files changed since the Wednesday Incremental
Friday - Incremental, only files changed since the Thursday Incremental
Saturday - Incremental, only files changed since the Friday Incremental
 
So what you see here is regardless of archive bit or modified time, the differential will backup multiple days’ worth of data since the last time either the archive bit or modified time has been set.
 
Also note that you really should not do both incrementals and differnetials in the same backup configuration.

Rick_DeBay
Level 5
"Wednesday - Differential, only files changed since the Tuesday Incremental"

If this is what BE actually does, it's wrong.  By definition a differential is ALL changes since the last full backup.
The correct implementation would be:

Tuesday - Incremental, only files changed since the Monday full
Wednesday - Differential, only files changed since the Monday full
Thursday - Differential, only files changed since the Monday full
Friday -  Incremental, only files changed since the Thursday differential
Saturday - Differential, Only files changed since the Monday full

Obviously if your implementation involves flipping archive bits, the correct behavior can't be achieved.  Using a file timestamp or even better a journal, it can if the time or journal entry of the last full and the last incremental are retained.

Ben_L_
Level 6
Employee
Per the admin guide as you already pointed out, The way Backup Exec defines a differential backup is "Differential backups include all files that have changed since the last full or incremental backup." 

So my example is correct.  The Definition of a differential backup can change from application to application,

Now if you were using ArcServ for example, there Definition of a differential is "Differential - Archive Bit—Files that have changed since the last full backup job are backed up. This backup method does not change the archive bit." (I pulled this from there admin guide Page 137).  This would work how you are expecting it to work.  

Rick_DeBay
Level 5
The definition of a differential backup DOES NOT change from application to application.  That's like saying "yeah, our calculator works, because we define 2+2 to equal 5."

I find it hard to believe that Veritas didn't understand what a differential backup was, so either the admin guide has an error or they had to cut corners during the implementation.

Your ArcServ example is again an a description of a particular implementation.  Anything depending on the MS-DOS archive bit can only perform one type of backup, as once the bit is flipped all previous state information is lost.

It's a common problem, especially in the software industry, to conflate a concept with the implementation of that concept.

Ben_L_
Level 6
Employee
The NTBackup the differential backup is "A differential backup copies files that have been created or changed since the last normal or incremental backup." This was taken from the help files for NTBackup. 

Arconis defines a differnetial backup as "A differential backup backs up only the files that changed since the last full back." Pulled from there website. 

So I will stand by my statement that it does change from application to application.

Regardless of how other applications do differential backups, I have explained how Backup Exec works in regards to differential backups.  If you would like to suggest a change please post your ideas to the ideas site. https://www-secure.symantec.com/connect/backup-and-archiving/ideas 

Colin_Weaver
Moderator
Moderator
Employee Accredited Certified

Going back to the origional query - the description / grammer in the admin admin guide is confusing rather than wrong.

If we assume Archive bit is being used, then a differential backup will only protect files modified since the last full or incremental because the archive bit is reset by both full and incremental backups. If you only use full and differential backups then the differential will go back to the last full because the archive bit is ONLY reset on the full. This would work the same way for any backup product that uses the archive bit, because of this we recommend that you never mix strategies of incremental and differential backups.

Note I don't know what would happen if you mix Incremental and Differential strategies using modified time which depends more on how the change tracking is implemented