Q. What is Quick Storage Migration (QSM)?

A. System Center Virtual Machine Manager 2008 R2 adds support for a technology called Quick Storage Migration, which is designed to allow the storage of a virtual machine (VM) to be moved between storage systems while the VM is still running. This technology works with SAN-based storage systems, such as those accessed via iSCSI and Fiber Connect, and direct attached storage types. The technology is independent of the storage and storage protocols.

Imagine I have a VM running on direct attached storage (which includes the VHD files) and I want to move it to iSCSI–based storage. QSM technology allows the VM to be moved to the iSCSI storage with minimal downtime. This reduced downtime is facilitated through the use of existing technologies, differencing disks, and Background Intelligent Transfer.

You may remember how differencing disks work from this FAQ about Virtual Machine snapshots, but a quick summary is that when you take a snapshot of a VM the virtual hard disk (VHD) is set into a read mode and any further writes are written to a separate differencing virtual hard disk (AVHD) file. QSM makes use of this as follows:

  1. A storage migration is initiated through the SCVMM console via the "Migrate Storage" action, as shown here.

  2. You’re prompted for the new location of the VHD files and the configuration.
  3. A Hyper-V snapshot is taken of the running VM. When this happens, the VHD file becomes read-only and any further changes are written to a differencing disk (AVHD).
  4. The original VHD file can now be copied via BITS to the destination storage. You know it won't have its content changed because all change is being written to the AVHD differencing disk.
  5. Once the VHD has been transferred, the VM is placed into a saved state, at which point the differencing disk (AVHD) and the configuration and memory save file are copied to the target storage.
  6. The AVHD file is merged back into the VHD on the target storage.
  7. The VM is resumed from the saved state.

The storage migration can be combined with a move of the VM to a different host with very little change to the process. The only additional step is to export and import the VM. Everything else stays the same.

You’ll notice this copying of the storage and then a copy of the storage difference is very similar to the method Live Migration uses. Live Migration copies the memory over then copies over the changes to the memory since the original copy occurred. It just happens that storage is much slower to copy, and you can't do iterative copies of the storage changes. This final copying of the differencing disk and the saved state memory will dictate how long the VM will be unavailable, but it’s typically under a minute.

Up to 10 concurrent storage migrations are supported.

Related Reading:

Check out hundreds more useful Q&As like this in John Savill's FAQ for Windows. Also, watch instructional videos made by John at ITTV.net.
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