Peter Xu <[email protected]> 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? > 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 prone to outdated counts but it's benign, please refer to the comment above 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 <[email protected]> > --- > 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. > +# 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". > +# 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? > +# > # Features: > # > # @unstable: Member @x-checkpoint-delay is experimental. > @@ -982,7 +995,8 @@ > '*multifd-compression': 'MultiFDCompression', > '*multifd-zlib-level': 'uint8', > '*multifd-zstd-level': 'uint8', > - '*block-bitmap-mapping': [ 'BitmapMigrationNodeAlias' ] } } > + '*block-bitmap-mapping': [ 'BitmapMigrationNodeAlias' ], > + '*switchover-hold': 'bool' } } > > ## > # @migrate-set-parameters: > @@ -1137,6 +1151,10 @@ > # 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) Likewise. > +# > # Features: > # > # @unstable: Member @x-checkpoint-delay is experimental. > @@ -1171,7 +1189,8 @@ > '*multifd-compression': 'MultiFDCompression', > '*multifd-zlib-level': 'uint8', > '*multifd-zstd-level': 'uint8', > - '*block-bitmap-mapping': [ 'BitmapMigrationNodeAlias' ] } } > + '*block-bitmap-mapping': [ 'BitmapMigrationNodeAlias' ], > + '*switchover-hold': 'bool' } } > > ## > # @query-migrate-parameters: [...]
