As shown in my last post, I recently added some storage to our SAN, and will be moving existing VHD files from our Hyper-V cluster to this new storage.
The unique thing about this is that these VHD’s contain data that is being served with Microsoft DFS and replicated with DFSR. Hopefully word is spreading about DFSR data stores requirements for backup, which include some specific requirements for backup, especially when it comes to snapshots due to the multi-master database DFSR uses. Because of this information, I was a little concerned about the Quick Storage Migration (QSM) and so I started digging.
I eventually came across this blog post that went into detail about how the QSM works; It mentions that it does a snapshot of the VM and creates differencing disks, and eventually the snapshot is merged and VM restarted from that saved state. At this point I was concerned about my DFS data and so I sent an email to the wonderful and always helpful AskDS blog seeking clarification.
Here’s the response from Ned Pyle:
My presumption is that this is safe because – from what I can glean – this feature never appears to roll back time to an earlier state as part of its differencing process.
He then went the extra mile and contacted internal Microsoft peers closely related to the QSM feature, who responded:
That’s correct, we don’t revert the machine state to an earlier time. A differencing disk is created to keep track of the changes while the parent vhd is being copied. Once the VHD is copied, the differencing disk is merged into the parent.
Based on that I performed a QSM of my 1.6 TB VHD yesterday. It took 12.5 hours to complete, but in the end it was fully successful, with no negative repercussions.
Something interesting to note, is that I had to manually move a different 350GB vhd file to my new storage first, instead of a QSM since I was out of space on the original storage to create the avhd differencing disk. I shut down the VM, transferred the VHD (took about an hour), and then re-pathed it within the VM settings and turned the VM back on.
Following this I received a DFSR error # 2212 that “The DFS Replication service has detected an unexpected shutdown on volume F:”. I’m not sure why this occurred, and I only did the one transfer so I can’t verify that it wasn’t related to some other operation or bad shutdown.