In the Hyper-V world, lots of us talk about Live Migration. That is where a virtual machine (VM) is moved from one host to another with no perceived downtime. The caveat is that both hosts need be configured to use the same shared storage (such as a SAN or external drive array).
What if you want to move a virtual machine to a host which is not part of the same shared storage cluster? In the early days of Hyper-V, this process would look something like this (not in actual order):
- The VM would be put into a saved state
- The virtual hard disk (VHD) data would be copied to the new server along with any attached ISO images
- The saved state data would be copied to the new location
- The VM object is created on the destination, and then the VM is changed back to a running state.
Total time for transition (depending on VHD sizes): 40 mins to several hours
Now with SP1 for Windows 2008 R2, we have Quick Migration. While it is not as seamless as a Live Migration, Quick Migration allows the VM to stay running during the bulk of the transition. The new process looks something like this (not in actual order):
- Using VSS technology, Hyper-V creates a point-in-time snapshot of the VM. From this point until the end of the transition, the VM will be working off of a new differencing disk.
- VHDs and associated ISO images are copied to the destination host. The VM is still running normally during this part.
- The VM object is created on the destination server with all settings from previous host. The VM is still running normally during this part.
- Remaining data (active memory and the new differencing disk) are copied to the new host.
Total down time for transition: as little as 2 minutes for a VM with 4 GB of memory!!