Features/Migration/Troubleshooting

You're probably looking at this page because migration has just failed; sorry about that, but hopefully this page will give you some idea of how to figure out why and importantly what to include in any bug report.

Basics

QEMU's migration is like lifting the state out of one machine and dumping it into another - the (emulated) hardware of the two machines have to match pretty much exactly for this to succeed.

Note that QEMU supports migrating forward between QEMU versions but in general not backwards, although some distros support this on their packaged QEMU versions.

Machine types

QEMU's machine types (the parameter to -M or --machine) is a definition of the basic shape of the emulated machine; the closest analogy is to the model of motherboard in a system. Migration requires you to have the same machine type on the source and destination machines. Architectures tend to have a variety of machine types (e.g. on x86 there is the 'pc' and the 'q35' family) that correspond to different generations of system. In addition some architectures version the machine types - e.g. pc-i440fx-2.5, pc-i440fx-2.6. Newer QEMUs normally keep (most of) the older machine types around so that you can migrate. So for example, a 2.6 release of QEMU should be able to migrate from a 2.5 release using the pc-i440fx-2.5 or pc-i440fx-2.4 machine types; Note it's not heavily tested!

Note that some machine types are aliases; on x86 the 'pc' and 'q35' machine types are aliases to whatever the latest version is on that version of qemu, and thus migrating between two different qemu versions both started with machine type 'pc' often won't work - use the full machine type.

ROMs

The ROM images used on the two hosts should be approximately (within a page size) the same size; if the ROMs do not match in size the migration is normally refused; care should be taken when packaging or upgrading BIOS, net boot roms etc to ensure this constraint is met.

Devices

The devices on the source and destination VMs must be identical - although any host devices they depend on can be different; for example you can't migrate between a VM with an IDE controller and another that replaced it with a SATA controller; but you can migrate between a VM with an IDE controller connected to a local file and another VM with an IDE controller backed by an iSCSI LUN.

Ordering and addressing of devices

When adding a device using -device on the qemu command line, it's normally added to the next available slot on the bus unless an address is specified. It's best to specify the address explicitly to avoid the source and destination ending up with different allocations; e.g. use -device pci-ohci,addr=5 -device usb-mouse,port=1

Hotplugged devices

Hotplugging can't normally be performed during a migration, however it's fine to hot plug/unplug a device before migration starts as long as care is taken to ensure that the state of the destination VM is identical to the current state of the source VM prior to the start of migration. Particular care should be taken to specify the address/port of devices with hotplugged devices since the automatic allocation on the command line of the destination won't necessariyl reflect the history of hot plug/unplug events on the source.

Host devices

Host PCI devices that are passed through to the guest normally block migration. There are various attempts to fix this for special cases of network cards, but none of them are complete yet.

Block storage

To do: cache=none, all the different ways to migrate block