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I would like to understand how Windows Home Server (v1) treats previously-backed-up disk drives, after the Operating System on C:\ has been re-installed, in order to work out whether my Home Server will run out of disk space when I back up a re-installed client PC.

The setup is:

PC (Windows 7)

  • C:\ drive
    • with 53 GB in use
    • just wiped and re-installed over this weekend
  • D:\ drive
    • with 60 GB of user data
    • was backed up by WHS machine 2 days ago
    • the files are mostly unchanged
    • but the file ownership of every file has changed, due to now being owned by the new user account on the new Windows 7 installation
  • I:\ drive
    • with 700 GB of user data
    • the entire 700 GB was backed up by WHS machine 2 days ago
    • again, data is unchanged, but the file ownership of every file has changed, here too

Windows Home Server (v1)

  • "Server Storage" reports that it has 593 GB free

My question is: is the WHS Backup system going to:

  1. See the D:\ and I:\ drives as totally new (either because it doesn't recognise identical files, or because the ownership has changed), and take a fresh copy of their 760 GB, thereby totally filling up
  2. Recognise that it has backed up the contents of these drives before, and only backup the 50 GB or so on the C drive, thereby giving me a few more months of backup capacity before I need to add more storage space?

I would particularly welcome a reply from anyone who has done this process before, and has actual experimental evidence of the answer!

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Update: I've added a new 2TB drive to the server's storage pool, and started a WHS backup of the re-installed machine... So I'll report back here when I know the answer! – Clare Macrae Mar 5 '12 at 21:46
up vote 2 down vote accepted

Amazingly, the answer is "yes, Windows Home Server does recognise previously-backed-up drives, after a Windows 7 OS re-install", even after file ownership has changed.

Note that these files were all still in their original locations, on the original internal RAID disk drive pair.

That is, I didn't use "Windows Easy Transfer" to move the files. I can imagine the results might be different if I had, because of the way that Microsoft's Home Computer Backup Technical Brief describes how the Home Server backup is implemented:

The home computer backup solution in Windows Home Server has a single-instance store at the cluster level. Clusters are typically collections of data stored on the hard drive, 4 kilobytes (KB) in size. Every backup is a full backup, but the home server only stores each unique cluster once. This creates the restore-time convenience of full backups (you do not have to repeat history) with the backup time performance of incremental backups.

Here's what I did...

Yesterday I got a new 2TB drive fitted, added to the server's storage pool, and started a WHS backup of the re-installed machine...

The backup took many hours to run, and shows all drives as successfully backed up.

Here are before-and-after screenshots showing the free space on the Home Server:

before-and-after screenshots showing the free space on the Home Serve

So, the free space had dropped by 0.01 TB (10 GB, plus-or-minus-a-few).

Based on the volumes of data described in my question above, I believe that the Home Server must have recognised the content on D:\ and I:\ as already backed up.

(Looking for possible alternative explanations, I wondered whether the Home Server had somehow cleared off a load of old backups from the old machine, but it didn't - they are still there. And here's a screenshot from Disk Management for Windows Home Server, showing how the usage of the existing drive has changed over the last few days.

screenshot from Disk Management for Windows Home Server

(The drop on 4th March was the standard Sunday-night pruning of a few old backups. And because this drive still has enough space, WHS hasn't started filling up the new 2TB drive yet).

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I think what you're seeing is the result of WHS single-instance storage at the block level -- it hashes each block of storage and keeps track if it has already backed that up before to save time and space. Cool plots by the way! – jacobsee Sep 18 '12 at 16:24
@jacobsee Yes, I agree - if you look at the section quoted near the top of the answer, it even uses that exact phrase "single-instance store at the cluster level". – Clare Macrae Sep 19 '12 at 15:19

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