Hi all,
I havent been on here for a long time so hello :).
I wanted to post something that worried and disturbed me slightly regarding using the archive bit to backup windows servers. It all looks great on the covers but there is an issue which I dont think most people are aware of.
When you run a full backup the Archive bit is cleared. What people dont realise is that the archive bit is cleared as the backup progresses. Alot of people believe that the "wait time before clearing archive bit" setting is the delay before the bit is cleared when the job completes. This isnt the case and i'll explain why.
If you had a very large file server that took 10+ hours to backup and you clear the archive bit at the end of the job. During those 10 hours someone could have made changes to files that got backed up 9 hours ago. When the archive bit gets cleared it would assume that the files are already backed up and clear the bit on all of them. The users changes are then not backed up in an incremental or cumilative incremental as the archive bit was cleared by the full backup. If the file was not modified again it wouldnt get backed up until the next full backup.
This is why the archive bit is cleared during the backup to stop clearing an Archive bit that could be a new modification to a file.
I too was unaware of the way NetBackup processed the archive bit and never really thought about it until we saw an issue.
We recently logged a case with Symantec (at the time) due to and issue where files had been missed in the backup. What happened was that a full backup had run and failed using the archive bit. We then ran a cumaltive incremental and it backed up fine. We then went to look for some files to restore and noticed that some were missing in the backup from the day before when the full backup had run and failed. Odd! We looked back at the files and discovered the archive bit had been reset but we didnt have them in any backups and the files were created the day before. It turns out that as the full backup had run it was clearing the archive bit for the files it had completed during the backup. As the backup failed it didnt get commited to the catalog but the archive bit had already been cleared. We then didnt get the files in a incremental backup as there was no archive bit set on the files for the incremental to pick up.
When we pushed the issue with Symantec (at the time) and they confirmed that NetBackup by design clears the archive bit as it processes a full backup honoring the wait time before it clears it for each file backed up. We were advised to fix the issue with the full backup which was causing the clearing of the archive bit or move to time stamp backups where this isnt an issue.
If you have a failed full backup using the archive bit you must get another full backup after it. If you think that an incremental will cover all your files from the last incremental with a failed full backup inbetween, it wont.
For me this begs the question is there any point in using the Archive bit? Am i missing something?
If a full fails when you use timestamp NetBackup looks for the last completed backup time stamp for an incremental and uses that timestamp. It doesnt matter if a full backup failed in between incrementals.
Does anyone have any thoughts on this?
