Features/PostCopyLiveMigration: Difference between revisions

Revision as of 10:06, 30 September 2014

summary

post-copy based live migration

owner

Name: Dave Gilbert
Email: dgilbert@redhat.com

description

This is yet another live migration mechanism for QEMU/KVM, which implements the migration technique known as "postcopy" or "lazy" migration. Just after the "migrate" command is invoked, the execution host of a VM is instantaneously switched to a destination host.

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.

TODO

future enhancement

optimization

@@ Line 13: / Line 13: @@
 == design ==
-The migration procedure looks like
+As much as possible the design attempts to build reusable components that other features can reuse.
-* start migration: stop the guest VM on the source and send the machine states except guest RAM to the destination
-* resume the guest VM on the destination without guest RAM contents
-* Hook guest access to pages, and pull page contents from the source. This continues until all the pages are pulled to the destination
+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.
 == TODO ==