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Restoring differential backups

Created: 25 Jul 2010 • Updated: 23 Feb 2013 | 8 comments
This issue has been solved. See solution.

Hi every one.
I want to know that wich technic is standard about restoring differential backups.

1- Restore a full backup and when it finishes, restore the differential backup on the same directory as the full backup.
2- Check both of them (Full and differential backups of a file) when i want to choose the restore selection list.
3- I don't know, if there is any other way please some body tell me

Thanks in advanced.

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sazz.'s picture

I would like to tell first what is full backup and differential bacup

Full backup - Does the complete backup and reset the ARCHIVE bit

Differential - Run the backup after full backup or incremental backup but doesn't reset archive bit.

Example.

If you are running a full on Sunday and from monday to friday you are running the differential then on Monday it will backup the file that got changed after the full backup. On tuesday it will backup Monday + Tuesday (because differential doesn't reset the archive bit). On wednesday it will backup Monday + tuesday + wednesday and so on.

If your system crashes on Thursday or if you want to restore then you have to restore the full bacup of Sunday and then the last differential which was on Wednesday. The wednesday differential will consist of Monday and tuesday.

If you want to restore the backup then you have to restore first the full backup and then restore the differential backup on top of it.

m.amir's picture

Hi Sazz and thanks for the answer. But as i said i want to know the standard technic for restoring such backups. If i check both full and diff backup of a file and restore them at the same time in one restore job, what would happen? Is this true or not?
Thanks

sazz.'s picture

If you want to restore the complete system(in case the drive crash or system crash ) then you have to restore first the complete full and then the differential.

If you just want to restore a single file or couple of files and folders then you can just do the restore from the differential backup and no need to restore the full backup in that case.

Running the full and differential at the same time might result into incorrect data and defeat the purpose of full and differential. You can create 2 jobs at the same time with 1 full selected and 1 differential and run the full first and then the differential or schedule it. But if you run the full and differential in one selection list then it might do the adverse effect.

m.amir's picture

Realy! There is no need to restore a full backup of a file and only the differential restore in enough?! How is this possible? Does backupexec handle it or what?

sazz.'s picture

I will give you an example.

Lets see you have a file name m.amir.txt and did the full backup on sunday. That file is been backed up in the full backup and will see in the restore of full backup. When you do the differential backup on Monday and if there are no changes for that file m.amir.txt then it will not show it in the differential restore. Now let's see you changed something on that file on wednesday  morning. When you do the differental backup on wednesday night that file will backed up because the change has been made to that file and you will see that file in the wednesday differential restore. Now make sure it does the complete backup of the file if you think it just does the backup what changed thats not the way it works.

So if you want to restore that file from wednesday backup then definately you will use the differential media used on wednesday and if you want the old data then restore from full backup.

If still doesn't explain then please try to do some backup and restore as it will be very simple and will explain the things very clearly.

m.amir's picture

I agree. I can see that we are both saying one thing. The differential backups are containing the chenges from the last full backup up to now. And if i restore a differential backup, the restored files would be that changed files from the last full backup. OK. But do we need to restore just the differential backups without their  full backups? If yes when?

pkh's picture

You will need to restore the full backup when you are restoring a folder or a drive.  Take a simple example, a folder has 10 files and you do a full backup of it on Sunday.  On Monday, 2 files, say FileA and FileB, was modified.  These 2 files will be backed up by your Monday's differential backup.  On Tuesday, this folder was accidentally deleted.  When you restore the folder you need to first restore it from the full backup and then Monday's differential backup.  If you don't restore from the full backup, only the 2 modified files, FileA and FileB, will be restored.  Conversely, if you restore from the full backup, but don't restore from the differential backup, the modification done to FileA and FileB on Monday will not be reflected in the final result.

If you are restoring just FileA, you can just restore it from the differential.  This is the same as restoring it from the full backup and then restoring it from the differential backup.

SOLUTION
m.amir's picture

Thanks pkh. That's exactly what i was thinking. What about  restoring them. I mean you probably have seen that  you can restore the differential backup in the same time as full backup in Backup Exec 12.5. You can check those two (The differential and the Full backup of a file or folder) in a same time and restore them. I have tested this way and saw that it seems the restored files are fine. Just like when you restore the full backup at the first and then restore the diff backup on the same directory as the full backup. But i want to know is this a standard way to restore them or not? there is no any explanation of this on the backup exec documents unfortunately. But it is possible in the backup exec! If it works fine, why we don't use it. This is the simple and faster way than restoring two backups after each other. Right?