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actively pushes the remaining memory pages of the VM to the target - an activity known as pre-paging. At the target, if the VM tries to access a page that has not yet been transferred, it generates a page-fault. These faults, known as network faults, are trapped at the target and redirected to the source, which responds with the faulted page. Too many network faults can degrade performance of applications running inside the VM. Hence pre-paging can dynamically adapt the page transmission order to network faults by actively pushing pages in the vicinity of the last fault. An ideal pre-paging scheme would mask large majority of network faults, although its performance depends upon the
252:(VM) or application between different physical machines without disconnecting the client or application. Memory, storage, and network connectivity of the virtual machine are transferred from the original guest machine to the destination. The time between stopping the VM or application on the source and resuming it on destination is called 'downtime'. When the downtime of a VM during live migration is small enough that it is not noticeable by the end user, it is called a 'seamless' live migration.
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copies all the memory pages from source to destination while the VM is still running on the source. If some memory pages change (become 'dirty') during the pre-copy phase, they will be copied again and again over several 'pre-copy rounds'. Usually the pre-copy phase ends when the number of dirtied
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Post-copy VM migration is initiated by suspending the VM at the source. With the VM suspended, a minimal subset of the execution state of the VM (CPU state, registers and, optionally, non-pageable memory) is transferred to the target. The VM is then resumed at the target. Concurrently, the source
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Post-copy sends each page exactly once over the network whereas pre-copy can transfer the same page multiple times if the page is dirtied repeatedly at the source during migration. On the other hand, pre-copy retains an up-to-date state of the VM at the source during migration, whereas during
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After the pre-copy phase, the VM will be paused on the source host, the remaining dirty pages will be copied to the destination, and the VM will be resumed at the destination. The downtime due to this phase can range from a few milliseconds to seconds depending on the number of dirty pages
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pages remaining becomes small enough to yield a short stop-and-copy phase. However, if a VM keeps dirtying memory faster than can be re-copied to the destination, then pre-copy phase will end after a set time limit or maximum number of pre-copy rounds to begin the next stop-and-copy phase.
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post-copy, the VM's state is split across the source and the destination. If the destination fails during live migration, pre-copy can recover the VM, whereas post-copy cannot.
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Two techniques for moving the virtual machine's memory state from the source to the destination are pre-copy memory migration and post-copy memory migration.
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transferred during downtime. VMs that dirty a lot of memory during the pre-copy phase tend to have a larger downtime.
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549:"Chapter 4. KVM Live Migration Red Hat Enterprise Linux 6"
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666:Microsoft: Step by Step Guide for live migration
347:since February 2007, with different techniques.
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509:Post-copy live migration of virtual machines
248:, refers to the process of moving a running
402:Cloud Platforms with live migration support
50:Learn how and when to remove these messages
429:Systems providing software live migration
227:Learn how and when to remove this message
209:Learn how and when to remove this message
147:Learn how and when to remove this message
629:"Qemu/KVM Virtual Machines - Proxmox VE"
651:OpenVZ checkpointing and live migration
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85:adding citations to reliable sources
563:"KVM-15 release [LWN.net]"
494:Live migration of virtual machines
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646:HOWTO Article about Xen migration
615:"VirtualBox Manual - Teleporting"
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39:or discuss these issues on the
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423:Oracle Cloud Infrastructure
396:Proxmox Virtual Environment
189:the claims made and adding
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521:"Xen project Mailing List"
356:Oracle VM Server for SPARC
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291:Post-copy memory migration
264:Pre-copy memory migration
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370:POWER Hypervisor (PHYP)
300:of the VM's workload.
273:In pre-copy phase, the
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366:Red Hat Virtualisation
330:Parallels Cloud Server
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413:Google Cloud Platform
298:memory access pattern
593:. Sams. 2012-09-16.
81:improve this article
445:Single-system image
335:Workload Partitions
282:Stop-and-copy phase
174:possibly contains
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535:"Migration - KVM"
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79:Please help
74:verification
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497:. NSDI'05.
478:References
391:VirtualBox
275:Hypervisor
183:improve it
107:newspapers
36:improve it
314:Virtuozzo
245:migration
187:verifying
42:talk page
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451:See also
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