The Xapi Storage Migration (XSM) also known as “Storage Motion” allows
The following diagram shows how XSM works at a high level:
The slowest part of a storage migration is migrating the storage, since virtual
disks can be very large. Xapi starts by taking a snapshot and copying that to
the destination as a background task. Before the datapath connecting the VM
to the disk is re-established, xapi tells
tapdisk to start mirroring all
writes to a remote
tapdisk over NBD. From this point on all VM disk writes
are written to both the old and the new disk.
When the background snapshot copy is complete, xapi can migrate the VM memory
across. Once the VM memory image has been received, the destination VM is
complete and the original can be safely destroyed.