perspective. It is also not possible to perform a storage migration for a Shared VHDX.
The ability to move the storage of a virtual machine at any time without impacting the
availability of the virtual machine is vital in two key scenarios:
The organization acquires new storage, such as a new SAN, or is migrating to a new
SMB 3.–based appliance and needs to move virtual machines with no downtime as
part of a planned migration effort.
The storage in the environment was not planned out as well as hoped and now has
either run out of space or can’t keep up with the IOPS requirements. Virtual
machines need to be moved as a matter of urgency. In my experience, this is the
most common scenario. It is important to realize that performing a storage
migration puts a large additional load on the storage, because every block has to be
read and written to. Therefore, if you are having a storage performance problem,
the problem will be worse during a storage migration.The mechanics behind the Windows Server 2012 storage migration are simple, but
they provide the most optimal migration process. Remember that the virtual machine
is not moving between hosts; it’s only the storage moving from a source location to a
target location.
Storage migration uses a one-pass copy of virtual hard drives that works as follows:
1 . The storage migration is initiated from the GUI or PowerShell.
2 . The copy of the source virtual hard disks, smart paging file, snapshots, and
configuration files to the target location is initiated.
3 . At the same time as the copy initiates, all writes are performed on the source and
target virtual hard disks through a mirroring process in the virtual storage stack.
4 . Once the copy of the virtual hard disks is complete, the virtual machine is switched
to use the virtual hard disks on the target location (the target is up-to-date because
all writes have been mirrored to it while the copy was in progress).
5 . The virtual hard disks and configuration files are deleted from the source.The storage migration process is managed by the VMMS process in the parent
partition, but the heavy lifting of the actual storage migration is performed by the
virtual machine’s worker process and the storage virtualization service provider (VSP)
in the parent partition. The mechanism for the copy of the storage is just a regular,
unbuffered copy operation plus the additional I/O on the target for the mirroring of
writes occurring during the copy. However, in reality the additional I/O for the
ongoing writes is negligible compared to the main unbuffered file copy. The path used
is whatever path exists to the target, which means that if it’s SAN, it will use
iSCSI/Fibre Channel, and if it’s SMB, it will use whichever network adapter or
network adapters have a path to the share. Any underlying storage technologies that
optimize performance are fully utilized. This means that if you are copying over SMB
(from or to) and you are using NIC Teaming, SMB direct, or SMB Multichannel, then