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Since Windows Server 2008 R2 (up to and including Server 2019, as far as I know), Windows Server Backup performs automatic management of full and incremental backup:

Automatic management of full and incremental backups. You no longer need to manage full and incremental backups. Instead, Windows Server Backup will, by default, create an incremental backup that behaves like a full backup. You can recover any item from a single backup, but the backup will only occupy space needed for an incremental backup. In addition, Windows Server Backup does not require user intervention to periodically delete older backups to free up disk space for newer backups—older backups are deleted automatically.

This sounds like a nice feature.

We, however, use two backup disks: One is always attached to the server for daily backups, and one is always in off-site storage. Every week we switch those disks, to ensure that we always have an off-site server backup that is as most a week old.

How do those incremental backups work with alternating disks?

Obviously, this would be fine (Option 1):

Day  1: Full backup on Disk A.
Day  2: Incremental backup (w.r.t. Day 1 backup) on Disk A.
Day  3: Incremental backup (w.r.t. Day 1+2 backup) on Disk A.
        ...
Day  8: Full backup on Disk B.
Day  9: Incremental backup (w.r.t. Day 8 backup) on Disk B.
Day 10: Incremental backup (w.r.t. Day 8+9 backup) on Disk B.
        ...
Day 15: Incremental backup (w.r.t. Day 1-7 backup) on Disk A.
Day 16: Incremental backup (w.r.t. Day 1-7+15 backup) on Disk A.

But this would not be fine (Option 2):

Day  1: Full backup on Disk A.
Day  2: Incremental backup (w.r.t. Day 1 backup) on Disk A.
Day  3: Incremental backup (w.r.t. Day 1-2 backup) on Disk A.
        ...
Day  8: Incremental backup (w.r.t. Day 1-7 backup) on Disk B.
Day  9: Incremental backup (w.r.t. Day 1-8 backup) on Disk B.
Day 10: Incremental backup (w.r.t. Day 1-9 backup) on Disk B.
        ...
Day 15: Incremental backup (w.r.t. Day 1-14 backup) on Disk A.
Day 16: Incremental backup (w.r.t. Day 1-15 backup) on Disk A.

because it would require both disks to restore (and, thus, defeat the purpose of having two disks).

Does Windows Server Backup use Option 1 or Option 2? And were is this documented?

(To clarify: The question is the previous paragraph in bold. It's not "how do I add a second disk to my backup set", nor is it "how do incremental backups work in general".)

  • I think option 1 is the way wbadmin handles this, because the previous backup isn't found on the target disk's backup repository, it will create a full one. Don't have any documentation, though. I'd just try it if I were you. Edit: found a thread in which it's discussed. For 2008R2 you had to add the second backup disk via command line, which can now be now done using the GUI, too. – Lenniey Mar 13 at 10:26
  • Also check this SF thread, don't know if it's an exact duplicate. – Lenniey Mar 13 at 10:31
  • @Lenniey: Thanks, I've seen that thread. Fortunately, adding a second disk is much easier now. I also think that it's option 1 (because otherwise the fact that it "behaves like a full backup" would not be satisfied), but I'd just like to make sure - backups are too important to rely on gut feelings. Testing is hard, because Windows Server Backup sometimes does full backups on its own (even if it could make an incremental backup), so I can't deduce future behavior from looking at previous backups. – Heinzi Mar 13 at 10:42
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"Behaves like a full backup" does not mean a full backup. It is still based on the incremental backup system, it's just something optimized for improved recovery like Veeam did long time ago. You need the previous points.

If you alternate disks you will have to do something to keep have all the incremental points on both disks.

To solve your problem you will have to configure 2 separate jobs and schedule them to run when you know a specific disk is online. Example: schedule job 1 for disk 1 on weeks 1,3,5, etc. and jub 2 for disk 2 on weeks 2,4,6, etc. The interval can be the one you want, it does not matter as long as the backup finds the proper disk in place.

For a detailed procedure see the official guide here.

  • So you are claiming that Windows Server Backup uses Option 2? Do you have any source for that? – Heinzi Mar 13 at 12:22
  • I'm not claiming anything not knowing what configuration you use, but it's safe to say you need previous incremental points for a valid restoration of data and that will not work on a default 'next'-like configuration in the case of 2 disks. Check the guide I provided. – Overmind Mar 13 at 12:49
  • Yes, I am fully aware of that. Unfortunately, that was not the question. The question was whether Windows Server Backup uses Option 1 or Option 2, and your answer seems to imply the latter. If that was not your intention, I suggest that you mention in your answer that this is not an answer to the question but just a comment about how incremental backups work in general, to prevent others from making the same mistake I did when reading your answer. :-) – Heinzi Mar 13 at 13:15
  • Both 1 and 2 scenarios depend on your initial configuration. By default, it's case 2. – Overmind Mar 13 at 13:37
  • 1
    OK, do you have any source for that claim ("By default, it's case 2")? And how would I "reconfigure" Windows Server Backup to use Option 1? – Heinzi Mar 13 at 14:02

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