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Live migration
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Live migration
Live migration, also called migration, refers to the process of moving a running virtual machine (VM) between different physical machines in a manner that the VM and applications running within the VM are mostly unaffected. Memory, storage, and network connectivity of the virtual machine are transferred from the original host machine to the destination. The time between stopping the VM or application on the source and resuming it on destination is called "downtime" or "blackout".
Live migration requires the entire VM's state to be transferred from the source host to the target host. This state includes the state of any components of the VM, for example, the register contents of the virtual CPUs (vCPUs). The state of the VM is not stable until the VM is paused. For example, until the vCPUs have been paused, the state in the virtual registers may be changing.
The simplest way to implement live migration is to pause the VM, serialize and send the state to the target host, and resume the VM on the target host. Serializing and sending certain state, like the state of the VM's main memory or locally-attached storage devices, can be slow.
If the entire contents of the VM's main memory must be copied while the VM is paused, the VM will have to remain paused for an extended period of time, depending on the size of the VM's main memory. There are several techniques for reducing blackout for large-memory VMs.
Pre-copy memory migration requires the hypervisor to track which pages the guest is writing to. With this ability, the following strategy is possible:
This is the basic pre-copy strategy. Other optimizations can be applied, like not re-sending pages that will likely be dirtied again quickly.
The guest is usually able to write to memory faster than it can be copied to the target, which means that pre-copy might never converge to a small dirty set. If the hypervisor does not throttle writes to guest memory, blackout time may remain large, even with pre-copy memory migration.
Post-copy memory migration is another strategy for copying the VM's main memory outside of blackout. It can be used with pre-copy memory migration, as it can limit the amount of time the VM must stay paused during memory migration even if pre-copy doesn't converge on a small set of dirty pages.
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Live migration
Live migration, also called migration, refers to the process of moving a running virtual machine (VM) between different physical machines in a manner that the VM and applications running within the VM are mostly unaffected. Memory, storage, and network connectivity of the virtual machine are transferred from the original host machine to the destination. The time between stopping the VM or application on the source and resuming it on destination is called "downtime" or "blackout".
Live migration requires the entire VM's state to be transferred from the source host to the target host. This state includes the state of any components of the VM, for example, the register contents of the virtual CPUs (vCPUs). The state of the VM is not stable until the VM is paused. For example, until the vCPUs have been paused, the state in the virtual registers may be changing.
The simplest way to implement live migration is to pause the VM, serialize and send the state to the target host, and resume the VM on the target host. Serializing and sending certain state, like the state of the VM's main memory or locally-attached storage devices, can be slow.
If the entire contents of the VM's main memory must be copied while the VM is paused, the VM will have to remain paused for an extended period of time, depending on the size of the VM's main memory. There are several techniques for reducing blackout for large-memory VMs.
Pre-copy memory migration requires the hypervisor to track which pages the guest is writing to. With this ability, the following strategy is possible:
This is the basic pre-copy strategy. Other optimizations can be applied, like not re-sending pages that will likely be dirtied again quickly.
The guest is usually able to write to memory faster than it can be copied to the target, which means that pre-copy might never converge to a small dirty set. If the hypervisor does not throttle writes to guest memory, blackout time may remain large, even with pre-copy memory migration.
Post-copy memory migration is another strategy for copying the VM's main memory outside of blackout. It can be used with pre-copy memory migration, as it can limit the amount of time the VM must stay paused during memory migration even if pre-copy doesn't converge on a small set of dirty pages.