Xapi Storage Migration

The Xapi Storage Migration (XSM) also known as “Storage Motion” allows

  • a running VM to be migrated within a pool, between different hosts and different storage simultaneously;
  • a running VM to be migrated to another pool;
  • a disk attached to a running VM to be moved to another SR.

The following diagram shows how XSM works at a high level:

Xapi Storage Migration Xapi Storage Migration

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.