Migration

2009-05-26T08:33:09Z

Themel: Removed file:// migration reference - no longer in current kvm.

=Migration=

== Introduction ==
KVM currently supports savevm/loadvm and offline or live migration
Migration commands are given when in qemu-monitor (Alt-Ctrl-2).
Upon successful completion, the migrated VM continues to run on the destination host.

==== Note ====
You can migrate a guest between an AMD host to an Intel host and back. Naturally, a 64-bit guest can only be migrated to a 64-bit host, but a 32-bit guest can be migrated at will.

There are some older Intel processors which don't support NX (or XD), which may cause problems in a cluster which includes NX-supporting hosts. To disable NX for a given guest, start it with such a parameter: -cpu qemu64,-nx

== Requirements ==
* The VM image is accessible on both source and destination hosts (located on a shared storage, e.g. using nfs).
* It is recommended an images-directory would be found on the same path on both hosts (for migrations of a copy-on-write image -- an image created on top of a base-image using "qemu-image create -b ...")
* The src and dst hosts must be on the same subnet (keeping guest's network when tap is used).
* Do not use -snapshot qemu command line option.
* For tcp: migration protocol
* the guest on the destination must be started the same way it was started on the source.

== highlights / merits ==
* Almost unnoticeable guest down time
* Guest is not involved (unique to KVM Live Migration [#1 1])
* Capability to tunnel VM state through an external program (unique to KVM Live Migration [#1 1])
* ssh/gzip/bzip2/gpg/your own
* Upon success guest continues to run on destination host, upon failure guest continues to run on source host (with one exception)
* Short and Simple
* Easy to enhance
* Hardware independence (almost).
* Support for migration of stopped (paused) VMs.
* Open

[[Anchor(1)]] 1 These features are unique to KVM Live Migration as far as I know. If you know of other hypervisor that support any of them please update this page or let me (Uri) know.

== User Interface ==
The user interface is through the qemu monitor (alt-ctrl-2 on the SDL window)

=== Management ===
migrate [-d] <URI>
migrate_cancel

The '-d' will let you query the status of the migration.

With no '-d' the monitor prompt returns when the migration completes.
URI can be one of 'exec:<command>' or tcp:<ip:port>

=== Status ===

info migrate

=== Migration Parameters ===
migrate_set_speed <speed> set bandwidth control parameters (max transfer rate per second)

== Example / HOWTO ==
A is the source host, B is the destination host:
===''TCP'' example:===
1. Start the VM on B with the exact same parameters as the VM on A, in migration-listen mode (-incoming tcp:0:4444 (or other PORT))
2. Start the migration (always on the source host):
A: migrate -d tcp:B:4444 (or other PORT)
3. Check the status (on A only):
info migrate

===''Offline'' example:===
1. unlimit bandwidth used for migration:
A: migrate_set_speed 1g
2. stop the guest:
A: stop
3. continue with either TCP or exec migration as described above.

== Problems / Todo ==
* TSC offset on the new host must be set in such a way that the guest sees a monotonically increasing TSC, otherwise the guest may hang indefinitely after migration.
* usbdevice tablet complains after migration.
* handle migration completion better (especially when network problems occur).
* More informative status.
* Migration does not work while CPU real-mode/protected mode are still changing.

== savevm/loadvm to an external state file (using pseudo-migration) ==

* To be supported directly by Migration Protocols, but until then...
* Save VM state into a compressed file
** Save
stop
migrate_set_speed 4095m
migrate "exec:gzip -c > STATEFILE.gz"

** Load
gzip -c -d STATEFILE.gz | <qemu-command-line> -incoming stdio
* Save VM State into an encrypted file (assumes KEY has already been generated)
** Save
stop
migrate_set_speed 4095m
migrate "exec:gpg -q -e -r KEY -o STATFILE.gpg"

* Load VM state from an encrypted file
gpg -q -d -r KEY STATEFILE.gpg | <qemu-command-line> -incoming stdio

== more exec: options ==

* Send encrypted VM state from host A to host B (Note: ssh is probably better, this is just for show)

** on host A
migrate "exec:gpg -q -e -r KEY | nc B 4444"

** on host B
nc -l 4444 | gpg -q -d -r KEY | <qemu-command-line> -incoming stdio

== Algorithm (the short version) ==
1. Setup
* Start guest on destination, connect, enable dirty page logging and more
2. Transfer Memory
* Guest continues to run
* Bandwidth limitation (controlled by the user)
* First transfer the whole memory
* Iteratively transfer all dirty pages (pages that were written to by the guest).
3. Stop the guest
* And sync VM image(s) (guest's hard drives).
4. Transfer State
* As fast as possible (no bandwidth limitation)
* All VM devices' state and dirty pages yet to be transferred
5. Continue the guest
* On destination upon success
** Broadcast "I'm over here" Ethernet packet to announce new location of NIC(s).
* On source upon failure (with one exception).

Instructions for kvm-13 and below: [[MigrationQemu0.8.2]].

KVM - User contributions [en]

Migration