Features/PostCopyLiveMigration

summary

post-copy based live migration

owner

• Name: Dave Gilbert
• Email: dgilbert@redhat.com

description

A postcopy implementation that allows migration of guests that have large page change rates (relative to the avialable bandwidth) to be migrated in a finite time.

design

As much as possible the design attempts to build reusable components that other features can reuse.

This postcopy implementation uses the Linux 'userfault' kernel mechanism from Andrea Arcangeli; it's not specific to Postcopy and is designed to allow use with all of the standard kernel features (like transparent huge pages, KSM etc).

Mixed pre/post copy is built into the design from the start; a command is sent to switch modes after the migration has been stated (as long as postcopy mode has been enabled first by a capability)

Major components

• 'command' section type for sending migration commands that don't directly reflect guest state; this is used to send messages that move through different phases of postcopy and is expandable for use by others.
• 'return path' a method for the destination to send messages back to the source; used for postcopy page requests, and allows the destination to signal failure back to the source
• 'sent map' a bitmap on the source populated with the set of all pages that have already been transmitted
• 'postcopy pagemap inbound (PMI)' a map on the destination holding the state of each page, whether it's been requested from the source and whether it has been received.

future enhancement

• optimization - rate limit the background page transmission to reduce the impact on the latency of postcopy page requests.

links

• GitHub repo
• This work is part of the ORBIT project
• Previous postcopy implementation as part of Yabusame