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Backing up snapshots incrementally
Hi!
Does anyone know how Networker prevents dataloss when backing up a snapshot (VSS or PowerSnap) incrementally or differentially?
Take the sequence:
- time T : Do a snapshot
- time T+1 : Modify file A
- time T+2 : Back up the snapshot from T
Now the backup will be stamped with the timestamp T+2, even if it really contains the data from T. This will make the next incremental or differential backup miss the change in File A and not back it up
Other backup products have problems with this. How is Networker handling the situation?
Best
Dag
Message was edited by: Dag (The web interface messed up my text completely :-( )
CarlosRojas
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January 24th, 2013 06:00
Hi DAG,
Your statement is not really accurate.
Please note that snapshot based backups are PIT (Point In Time) backups, which means that the timestamp for the backup will be the time when the requestor asks for the snapshot to be created.
During the snapshot creation there will NOT be any I/O operations, as the writer (depending on the data being backed up) will freeze the I/O so, the data actually being backed up will have a timestamp the same as the snapshot creation, and NW is aware of this so no, the data changed during the snapshot being rolled over is not being backed up, but will be backed up during either the next incremental or next full backups.
I hope this clarifies the point, otherwise please let me know and I'll try to provide you with a better explanation.
Thank you.
Carlos.
CarlosRojas
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January 23rd, 2013 08:00
Hi Dag,
Not sure if I really understand your question however, talking about NMM for example, you need to know that Incremental Persistent Snapshots are not supported.
If you run a File System full backup, and then try to perform an incremental backup retaining the snapshot, that snapshot will be automatically promoted to full.
Also note that PowerSnap will check the snapshot policy set for that client/group and will act accordingly so as I said, any persistent snapshot will be promoted to full if the backup level is set to incremental.
Thank you.
Carlos.
Dag1_3de7fe
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January 23rd, 2013 23:00
Thanks Carlos.
But if you use NMM, do a file fystem backup, set the level to incremental you get an incremental. Even if you do a snapshot only for that purpose you still have a time period between the snapshot and the actual backup. Any file changed during that time interval Networker will believe has been backed up as the timestamp of the backup will be newer than the ctime of the file. Still the version of the file in the snapshot is of course the one in the snapshot, which is the unchanged one.
What am I missing here?
Best
Dag
Dag1_3de7fe
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January 24th, 2013 09:00
Ok. Thanks Carlos!
I just wanted a confirmation that this was considered!