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Live Migration of Virtual Machines
Presented by:
Edward Armstrong
University of Guelph
Overview
Problem Description
 Solutions
 Strengths and Weaknesses
 Results

Migrating an OS - Complexity
Trivial
• Same machine.
• Hibernating a laptop.
Moderate
• Same Hardware, different
Machine.
• Changing cluster nodes.
Complex
• Different hardware.
Migrating an OS – Timescale
Reboot
Hibernation
Suspend
Live
• Requires only long term storage to be migrated.
• Loses all process state information.
• Driver reloading solves hardware problems.
• Typically performed on the same machine.
• Requires long term storage.
• Loses external connection information, ie. network status.
• Hibernation in short term storage (RAM).
• Typically used to maintain a low power state.
• Processes are not implicitly frozen.
• Differences in hardware create problems.
• Solved by using virtual machines.
Considerations
Both machines must be active at the same
time.
 Migration of active live services.
 Total migration time.
 Resource contention.

Migrating memory
Push
Pull
Pause
Memory - Pure stop and copy.
Push
Pull

Pros
◦ Simplicity.
◦ Consistency.
Pause

Cons
◦ Downtime proportional to memory.
◦ Unacceptable for live services.
Source Machine
Target Machine
Memory - On demand migration.
Push
Pull

Pros
◦ Shorter downtime.
◦ Consistency.
Pause

Cons
◦ Longer migration time.
Page fault request
Send page
Source Machine
Target Machine
Memory - Pre copy migration.
Push
Pull

Pros
◦ Copy low fault pages quickly.
◦ Works well for live processes.
Pause

Cons
◦ Large number of faults for busy
memory.
Iterative push
Live page fault
Source Machine
Target Machine
Network and disk resources.
Unique to an OS instance.
 Ordering of resources are nondeterminsitic.
 Need to maintain open network
connections.

Resolving network connections with
an ARP* response.
Source Machine
No Address
LAN
Packets
Target Machine
192.168.0.102
*ARP: Address Resolution Protocol
Resolving network connections with
an ARP* response.
Unsolicited ARP reply
Source Machine
No Address
LAN
Packets
Target Machine
192.168.0.102
*ARP: Address Resolution Protocol
Resolving network connections with
an ARP* response.
Source Machine
192.168.0.102
Packets
LAN
Target Machine
No Address
*ARP: Address Resolution Protocol
Network resources.

Pros
◦ Handled by external devices.
◦ Similar scheme can be used to migrate disk
services (not covered in paper).

Cons
◦ Small amount of packet loss.
◦ Requires a LAN with unsolicited ARP
responses enabled.
Design Overview
PreMigration
Reservation
Pre-Copy
Activate
Commit
Stop and
Copy
Select a new target machine
Source Machine
Target Machine
Design Overview
PreMigration
Reservation
Pre-Copy
Activate
Commit
Stop and
Copy
Source Machine
Confirm available resources on target.
Failure means VM continues to run on source.
Target Machine
Design Overview
PreMigration
Reservation
Pre-Copy
Activate
Commit
Stop and
Copy
Source Machine
Transfer memory.
Retransmit memory used during transfer.
Target Machine
Design Overview
PreMigration
Reservation
Pre-Copy
Activate
Commit
Stop and
Copy
Source Machine
Halt source to redirect network traffic
and transfer CPU state.
Target Machine
Design Overview
PreMigration
Reservation
Pre-Copy
Activate
Commit
Stop and
Copy
Source Machine
Verify transfer complete.
Disable source.
Target Machine
Design Overview
PreMigration
Reservation
Pre-Copy
Activate
Commit
Stop and
Copy
Activate target machine
Source Machine
Target Machine
Summary of Design
At all times there is at least one
consistent image available.
 Minimized down time.
 Not a fast process overall.
 Requires tuning.
 Requires certain hardware.

Performance – Dirty Pages.
8 second granularity, used to decide which pages make for good pre-migration.
Performance
Performance
1- 4 pre-copy iterations
Performance
1- 4 pre-copy iterations
Performance
1- 4 pre-copy iterations
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