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Virtual machine's live migration
Virtual machine's live migration allows you to move live running virtual machines from
one physical server to another. In short, from one RHEV hypervisor host to another, it is
also possible to migrate non-live virtual machines from one host to another in RHEV. Dur-
ing the course of live migration, the virtual machine remains powered on, and the applica-
tion hosted on it continues to serve the end user without any loss in network connectivity or
application access while relocating from one host to another.
Migration works by sending the state of the virtual machine memory and all its virtualized
devices to a destination host. In live migration, virtual machine memory pages are trans-
ferred to the destination host. RHEV monitors the guest memory page transfer from the
source to the destination host and also estimates the transfer speed. If the estimated period
of migration goes beyond the default configurable time of 10 minutes, RHEV suspends the
original guest machine on the source host, transfers the remaining data, and resumes the
same guest on the destination host.
In offline or cold migration, RHEV suspends the guest virtual machine before moving the
image of the virtual machine memory to the destination host. After a successful transfer of
the memory pages, the virtual machine is resumed back on the destination host.
Each live migration event is limited to a maximum transfer speed of 30 MBps, and the
number of concurrent migrations supported is also limited by default, and it's tunable in the
hypervisor vdsm configuration file. The time it takes to complete such a migration depends
on the network bandwidth and latency. If the network is experiencing heavy use or low
bandwidth, the migration will take much longer.
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