Markus Armbruster <arm...@redhat.com> writes: > Fabiano Rosas <faro...@suse.de> writes: > >> Add a new migration capability 'fixed-ram'. >> >> The core of the feature is to ensure that each ram page has a specific >> offset in the resulting migration stream. The reason why we'd want >> such behavior are two fold: >> >> - When doing a 'fixed-ram' migration the resulting file will have a >> bounded size, since pages which are dirtied multiple times will >> always go to a fixed location in the file, rather than constantly >> being added to a sequential stream. This eliminates cases where a vm >> with, say, 1G of ram can result in a migration file that's 10s of >> GBs, provided that the workload constantly redirties memory. >> >> - It paves the way to implement DIRECT_IO-enabled save/restore of the >> migration stream as the pages are ensured to be written at aligned >> offsets. >> >> For now, enabling the capability has no effect. The next couple of >> patches implement the core funcionality. >> >> Signed-off-by: Fabiano Rosas <faro...@suse.de> >> --- >> docs/devel/migration.rst | 14 ++++++++++++++ >> migration/options.c | 37 +++++++++++++++++++++++++++++++++++++ >> migration/options.h | 1 + >> migration/savevm.c | 1 + >> qapi/migration.json | 5 ++++- >> 5 files changed, 57 insertions(+), 1 deletion(-) >> >> diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst >> index c3e1400c0c..6f898b5dbd 100644 >> --- a/docs/devel/migration.rst >> +++ b/docs/devel/migration.rst >> @@ -566,6 +566,20 @@ Others (especially either older devices or system >> devices which for >> some reason don't have a bus concept) make use of the ``instance id`` >> for otherwise identically named devices. >> >> +Fixed-ram format >> +---------------- >> + >> +When the ``fixed-ram`` capability is enabled, a slightly different >> +stream format is used for the RAM section. Instead of having a >> +sequential stream of pages that follow the RAMBlock headers, the dirty >> +pages for a RAMBlock follow its header. This ensures that each RAM >> +page has a fixed offset in the resulting migration stream. > > This requires the migration stream to be seekable, as documented in the > QAPI schema below. I think it's worth documenting here, as well. >
Ok. >> + >> +The ``fixed-ram`` capaility can be enabled in both source and >> +destination with: >> + >> + ``migrate_set_capability fixed-ram on`` > > Effect of enabling on the destination? > > What happens when we enable it only on one end? > qemu-system-x86_64: Capability fixed-ram is off, but received capability is on qemu-system-x86_64: load of migration failed: Invalid argument So I guess that *can* be enabled up there should become a *must*. >> + >> Return path >> ----------- >> > > [...] > >> diff --git a/qapi/migration.json b/qapi/migration.json >> index 74f12adc0e..1317dd32ab 100644 >> --- a/qapi/migration.json >> +++ b/qapi/migration.json >> @@ -527,6 +527,9 @@ >> # VM before migration for an optimal migration performance. >> # Enabled by default. (since 8.1) >> # >> +# @fixed-ram: Migrate using fixed offsets for each RAM page. Requires > > Two spaces between sentences for consistency, please. > >> +# a seekable transport such as a file. (since 8.1) > > What is a migration transport? migration.json doesn't define the term. > The medium that transports the migration. We are about to define some terms at the QAPI series: [PATCH v15 00/14] migration: Modify 'migrate' and 'migrate-incoming' QAPI commands for migration https://lore.kernel.org/r/20231023182053.8711-1-faro...@suse.de > Which transports are seekable? > The ones that implement QIO_CHANNEL_FEATURE_SEEKABLE. Currently only QIOChannelFile. > Out of curiosity: what happens if the transport isn't seekable? > We fail the migration. At migration_channels_and_uri_compatible(): if (migration_needs_seekable_channel() && !uri_supports_seeking(uri)) { error_setg(errp, "Migration requires seekable transport (e.g. file)"); compatible = false; } >> +# >> # Features: >> # >> # @unstable: Members @x-colo and @x-ignore-shared are experimental. >> @@ -543,7 +546,7 @@ >> { 'name': 'x-ignore-shared', 'features': [ 'unstable' ] }, >> 'validate-uuid', 'background-snapshot', >> 'zero-copy-send', 'postcopy-preempt', 'switchover-ack', >> - 'dirty-limit', 'auto-pause'] } >> + 'dirty-limit', 'auto-pause', 'fixed-ram'] } >> >> ## >> # @MigrationCapabilityStatus:
