PRE-COPY
Copy while it runs
Memory pages are copied in rounds while the machine keeps running, re-sending pages that changed. Then a short stop-and-copy sends the rest.
Compare methodsHOW LIVE MIGRATION WORKS
Move it
while it runs.
A virtual machine can change servers mid-sentence.
Compare methods
COPYthen switch
Live migration copies a running machine's memory to another host while it keeps working, then pauses it just long enough to send the last changes. Here is how the main methods work and what they trade off.
At a glance
- Copy memory while it runs
- Pause only for the last pages
- Keep connections open
- Hardware maintenance without the outage
60 ms
Downtime moving an online game server
The 2005 paper reports a virtual machine running a game server migrating with 60 ms of downtime, and one running a web benchmark with 210 ms of unavailability.
LIVE PRE-COPY · NSDI 2005
32 s
Downtime for 512 MB at 128 Mbit/s
Naive stop-and-copy of a 512 MB virtual machine would need 32, 16 and 8 seconds of downtime at 128, 256 and 512 Mbit/s respectively, the paper calculates.
STOP-AND-COPY · NSDI 2005
TWO NUMBERS WORTH KNOWING
A pause you
might not notice.
One figure from the live migration paper,
one for the naive approach it replaced.
The paper found that even in its worst case, a single pre-copy round cut downtime by a factor of four compared with plain stop-and-copy.
A QUICK METHOD CHECK
Which way
is it moving?
Pick a migration method to see how it works. The bar shows how disruptive it tends to be for the running machine.
Based on Clark et al. (NSDI 2005) and the QEMU migration documentation. Real downtime depends on memory size, workload and network.
THE SHORT ANSWER
Copy first,
pause last.
Memory is the big thing to move.
The methods differ in when they move it.
THE NETWORK
A full copy of RAM in transit
The migration stream contains a complete copy of the guest's RAM, so libvirt advises keeping migration traffic separate and using encryption such as TLS.
Security and storage
WHY BOTHER
Free up the old host
The 2005 paper notes that once migration completes, the original host can be taken down for maintenance, with clients staying connected.
See the numbers
THE STAGES OF A MOVE
Six steps,
one short pause.
The 2005 paper breaks a live migration into stages. Only one of them stops the machine.
Reserve the destination
The target host confirms it has the resources and reserves a container. If it cannot, the machine simply keeps running where it is.
Pre-copy in rounds
All memory is sent first, then only the pages that changed during the previous round.
Stop and copy
The machine is suspended, and the remaining pages and processor state are sent across.
Commit and activate
The destination confirms it has a consistent copy, the source lets go, and the machine resumes on its new host.
SECURITY AND STORAGE
Memory moves,
disks may not.
Because the migration stream includes a complete copy of the guest's RAM, libvirt warns that snooping on it could expose sensitive data, and recommends separating migration traffic from guest traffic and using TLS where the hypervisor supports it.
Libvirt by default expects disk images to be shared between the hosts, through a network filesystem or remote block storage. Offline migration is different again: it moves the machine's inactive definition, leaving it defined but inactive on the destination.
Questions
Questions people ask
What is live migration?
Moving a running virtual machine from one physical host to another while it keeps working. QEMU describes it as letting the guest continue running while its state is transferred, stopping it only for the last part.
How long is a virtual machine paused during live migration?
It depends on many factors. QEMU says the unresponsive time is typically in the low hundreds of milliseconds, and the 2005 paper reported downtimes as low as 60 ms.
What is the difference between pre-copy and post-copy?
Pre-copy sends memory before switching hosts. Post-copy starts the machine on the new host first and fetches missing memory pages on demand, which bounds the time but means a failure of either side loses the guest.
Do network connections survive a live migration?
The 2005 paper says that because the whole virtual machine moves, including its protocol state, open connections can be maintained, so clients do not need to reconnect.
Is migration traffic sensitive?
Yes. Libvirt notes the stream contains a complete copy of the guest's RAM and recommends separating migration traffic and using encryption.
Sources
Where the facts come from
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