This page describes a possible design for Cross-pool migration (which also works for within-pool migration with and without shared storage).

This is an alternative design to CrossPoolMigration

This design has the following features:

DRBD is used to replicate disks on demand
- this is storage-format agnostic: it doesn't require .vhd
the SM on one host can generate URIs which can be used by other SMs to bootstrap the disk mirroring process
a single codepath is used in xapi, making testing easier
- this replaces the existing migration API
a simple RESTful API makes the whole thing quite easy to test, prod etc.
the xapi pieces and the SM pieces can be developed and tested independently and integrated at the end

Pros/Cons of DRBD vs snapshot/copy

Pro: by separating storage replication from memory transfer we can switch easily to "libxl" without modifications. Otherwise we would need callbacks from libxl to synchronise the disk copy with the memory copy.
Pro: we don't need to write a snapshot/copy loop
Con: we do need to write something to manage a drbd instance, which may be on a shared host (eg vanilla Debian dom0)
Pro: migration downtime is expected to be lower, since replication cost is spread over each I/O request, rather than being bursty
Con: continuous replication might slow down some workloads more than snapshot/copy
Con: DRBD is linux only: it's not clear how this would work with (eg) a FreeBSD storage driver domain.

Component diagram

The following milestones are proposed:

Milestone number

1

2

3

4

The following tasks are proposed:

Task	Status
Create sample drbd.conf files	Completed
Implement a DRBD service in dom0	In progress
Implement xapi HTTP operations
Implement SMAPI VDI.replicate_to, VDI.get_replication_target
Implement xapi HTTP heartbeat
Implement XenAPI VM.migrate, VM.receive