Hi, Markus, On Thu, Jul 06, 2023 at 03:38:13PM +0200, Markus Armbruster wrote: > Peter Xu <pet...@redhat.com> writes: > > > Add a new migration parameter switchover-hold which can block src qemu > > migration from switching over to dest from running. > > > > One can set this flag to true so src qemu will keep iterating the VM data, > > not switching over to dest even if it can. > > > > It means now live migration works somehow like COLO; we keep syncing data > > from src to dst without stopping. > > Out of curiosity: does it share code with COLO?
Nop, it is still purely the migration path. > > > When the user is ready for the switchover, one can set the parameter from > > true->false. That'll contain a implicit kick to migration thread to be > > alive and re-evaluate the switchover decision. > > > > This can be used in two cases so far in my mind: > > > > (1) One can use this parameter to start pre-heating migration (but not > > really migrating, so a migrate-cancel will cancel the preheat). When > > the user wants to really migrate, just clear the flag. It'll in most > > cases migrate immediately because most pages are already synced. > > > > (2) Can also be used as a clean way to do qtest, in many of the precopy > > tests we have requirement to run after 1 iteration without completing > > the precopy migration. Before that we have either set bandwidth to > > ridiculous low value, or tricks on detecting guest memory change over > > some adhoc guest memory position. Now we can simply set this flag > > then we know precopy won't complete and will just keep going. > > > > Here we leveraged a sem to make sure migration thread won't busy spin on a > > physical cpu, meanwhile provide a timedwait() of 10ms so it can still try > > its best to sync with dest QEMU from time to time. Note that the sem is > > prone to outdated counts but it's benign, please refer to the comment above > > the semaphore definition for more information. > > > > Signed-off-by: Peter Xu <pet...@redhat.com> > > --- > > qapi/migration.json | 25 ++++++++++-- > > migration/migration.h | 17 +++++++++ > > migration/migration-hmp-cmds.c | 7 ++++ > > migration/migration.c | 69 ++++++++++++++++++++++++++++++++-- > > migration/options.c | 17 +++++++++ > > 5 files changed, 128 insertions(+), 7 deletions(-) > > > > diff --git a/qapi/migration.json b/qapi/migration.json > > index 47dfef0278..c050081555 100644 > > --- a/qapi/migration.json > > +++ b/qapi/migration.json > > @@ -789,6 +789,15 @@ > > # Nodes are mapped to their block device name if there is one, and > > # to their node name otherwise. (Since 5.2) > > # > > +# @switchover-hold: Whether we should hold-off precopy switchover from > > +# src to dest QEMU, even if we can finish migration in the > > Spell out "source" and "destination", please. > > Recommend to spell it out in the commit message, too. Will do. > > > +# downtime specified. By default off, so precopy migration will > > +# complete as soon as possible. One can set it to explicitly keep > > +# iterating during precopy migration until set the flag to false > > +# again to kick off the final switchover. Note, this does not > > "until the flag is set to false again" > > or > > "until the flag is cleared". Will do. > > > +# affect postcopy switchover, because the user can control that > > +# using "migrate-start-postcopy" command explicitly. (Since 8.1) > > +# > > # Features: > > # > > # @unstable: Member @x-checkpoint-delay is experimental. > > @@ -810,7 +819,7 @@ > > 'xbzrle-cache-size', 'max-postcopy-bandwidth', > > 'max-cpu-throttle', 'multifd-compression', > > 'multifd-zlib-level' ,'multifd-zstd-level', > > - 'block-bitmap-mapping' ] } > > + 'block-bitmap-mapping', 'switchover-hold' ] } > > > > ## > > # @MigrateSetParameters: > > @@ -945,6 +954,10 @@ > > # Nodes are mapped to their block device name if there is one, and > > # to their node name otherwise. (Since 5.2) > > # > > +# @switchover-hold: Whether we should hold-off precopy switchover from > > +# src to dest QEMU. For more details, please refer to > > +# MigrationParameter entry of the same field. (Since 8.1) > > We normally duplicate the documentation. This would be the first > instance where we reference instead. Do we want that? Personally I'd hope to try avoid any form of duplication if possible. We have some structure duplications that I cannot avoid (not until someone, including myself, can dedup it..), so I tried to not duplicate exactly same thing in the documents at least, instead to put such a reference. But I agree only referencing other docs in this flag is special. We may want to do at least the same across all the parameters. I have no strong opinion either, so if we want to keep full descriptions in multiple places I'll do that in the new spin. It's more QAPI stuff IMHO; I'll just follow your advise. Thanks for taking a look! -- Peter Xu
