Kvm Live Migration; Live Migration Requirements - Red Hat ENTERPRISE LINUX 5 - VIRTUALIZATION GUIDE Manual

Chapter 20.

KVM live migration

This chapter covers migrating guests running on a KVM hypervisor to another KVM host.
Migration
is name for the process of moving a virtualized guest from one host to another. Migration is a
key feature of virtualization as software is completely separated from hardware. Migration is useful for:
• Load balancing - guests can be moved to hosts with lower usage when a host becomes overloaded.
• Hardware failover - when hardware devices on the host start to fail, guests can be safely relocated
so the host can be powered down and repaired.
• Energy saving - guests can be redistributed to other hosts and host systems powered off to save
energy and cut costs in low usage periods.
• Geographic migration - guests can be moved to another location for lower latency or in serious
circumstances.
Migrations can be performed live or offline. To migrate guests the storage must be shared. Migration
works by sending the guests memory to the destination host. The shared storage stores the guest's
default file system. The file system image is not sent over the network from the source host to the
destination host.
An offline migration suspends the guest then moves an image of the guests memory to the destination
host. The guest is resumed on the destination host and the memory the guest used on the source host
is freed.
The time an offline migration takes depends network bandwidth and latency. A guest with 2GB of
memory should take an average of ten or so seconds on a 1 Gbit Ethernet link.
A live migration keeps the guest running on the source host and begins moving the memory without
stopping the guest. All modified memory pages are monitored for changes and sent to the destination
while the image is sent. The memory is updated with the changed pages. The process continues
until the amount of pause time allowed for the guest equals the predicted time for the final few pages
to be transfer. KVM estimates the time remaining and attempts to transfer the maximum amount
of page files from the source to the destination until KVM predicts the amount of remaining pages
can be transferred during a very brief time while the virtualized guest is paused. The registers are
loaded on the new host and the guest is then resumed on the destination host. If the guest cannot
be merged (which happens when guests are under extreme loads) the guest is paused and then an
offline migration is started instead.
The time an offline migration takes depends network bandwidth and latency. If the network is in heavy
use or a low bandwidth the migration will take much longer.

20.1. Live migration requirements

Migrating guests requires the following:
Migration requirements
• A virtualized guest installed on shared networked storage using one of the following protocols:
• Fibre Channel
• iSCSI
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