Assistance with Diff Disk

Nov 22, 2009 at 3:13 PM

First of all I have to say excellent job on this project and thank you for sharing!

I'm using version 0.7 with 2 machines.  Windows 7 and Server 2008 R2 both 64  bit.

I am having a little trouble understanding how I should be using/accessing the Differencing Disk.

I have been able to create 2 VHD's. A parent and a Diff.  If I attach the parent vhd using the Disk Management, copy files, disconnect and then attach the Diff vhd I see the exact same files that I had copied onto the parent as expected.  While I have the parent vhd online however, I cannot access the Diff disk as it says either it or the parent is being used by another process.  Conceptually, is the diff disk tracking changes to the parent (as the name implies) and is there anyway to view its contents while the parent is online?  What is standard practice for dealing with diff disks?  If I have to take the parent offline to view the diff disk it seems inherently dangerous as the parent drive is no longer available.




Nov 22, 2009 at 8:51 PM

Sadly, diff disks don't work how you expect.  Once you've created a differencing disk, you shouldn't then modify the parent disk.  If you make modifications in both the parent and the diff disk, chances are you'll end up with a corrupt file system.

The closest you could get is to create two differencing disks from the parent, you can then safely use both, because any changes will be written into the separate diff disks, rather than changing the parent disk.



Nov 22, 2009 at 9:28 PM
Edited Nov 22, 2009 at 9:31 PM

Hmm, okay it doesnt sound like what I want then.  Basically this is what I am wanting to do:

I have a file server I'm building with raid 0 SSD drives running SanFly iSCSI Target software.  It's primary purpose is to serve a high number of very small Xml files very quickly to a web server.

I would like to create "diff" backups every two hours or so.  I have already written a disk monitoring component in .NET that can listen for changes and then create my own diff VHD's.

Only problem is that I have not figured out the most efficient way to to do this since I have been unable to attach the Virtual Disk exposed via iSCSI on this machine as a Read Only drive so I can listen for changes.

In testing, I have to do this in a round about way by sharing the iSCSI drive on the web server and then configure my monitoring software, which runs on the file server, to monitor \\web server\share.  It works but is not clean.

Is there anyway my file server can mount a VHD in read-only mode that is already being exposed via iSCSI?