On Wed, Feb 21, 2024 at 10:24:05AM -0300, Fabiano Rosas wrote:
> 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 reasons why we'd want
> >> such behavior are:
> >>
> >> - 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 O_DIRECT-enabled save/restore of the
> >> migration stream as the pages are ensured to be written at aligned
> >> offsets.
> >>
> >> - It allows the usage of multifd so we can write RAM pages to the
> >> migration file in parallel.
> >>
> >> For now, enabling the capability has no effect. The next couple of
> >> patches implement the core functionality.
> >>
> >> Signed-off-by: Fabiano Rosas <faro...@suse.de>
> >
> > [...]
> >
> >> diff --git a/qapi/migration.json b/qapi/migration.json
> >> index 5a565d9b8d..3fce5fe53e 100644
> >> --- a/qapi/migration.json
> >> +++ b/qapi/migration.json
> >> @@ -531,6 +531,10 @@
> >> # and can result in more stable read performance. Requires KVM
> >> # with accelerator property "dirty-ring-size" set. (Since 8.1)
> >> #
> >> +# @fixed-ram: Migrate using fixed offsets in the migration file for
> >> +# each RAM page. Requires a migration URI that supports seeking,
> >> +# such as a file. (since 9.0)
> >> +#
> >> # Features:
> >> #
> >> # @deprecated: Member @block is deprecated. Use blockdev-mirror with
> >> @@ -555,7 +559,7 @@
> >> { 'name': 'x-ignore-shared', 'features': [ 'unstable' ] },
> >> 'validate-uuid', 'background-snapshot',
> >> 'zero-copy-send', 'postcopy-preempt', 'switchover-ack',
> >> - 'dirty-limit'] }
> >> + 'dirty-limit', 'fixed-ram'] }
> >>
> >> ##
> >> # @MigrationCapabilityStatus:
> >
> > Can we find a better name than @fixed-ram? @fixed-ram-offsets?
> > @use-seek?
>
> I have no idea how we came to fixed-ram. The archives don't provide any
> clarification. I find it confusing at first glance as well.
>
> A little brainstorming on how fixed-ram is different from exiting
> migration:
>
> Fixed-ram:
> uses a file, like the 'file:' migration;
>
> needs a seeking medium, such as a file;
>
> migrates ram by placing a page always in the same offset in the
> file, contrary to normal migration which streams the page changes
> continuously;
>
> ensures a migration file of size bounded to VM RAM size, contrary to
> normal 'file:' migration which creates a file with unbounded size;
>
> enables multi-threaded RAM migration, even though we only use it when
> multifd is enabled;
>
> uses scatter-gatter APIs (pwritev, preadv);
>
> So a few options:
>
> (disconsidering use-seek, it might be even more generic/vague)
>
> - fixed-ram-offsets
> - non-streaming (or streaming: false)
> - ram-scatter-gather (ram-sg)
> - parallel-ram (even with the slight inaccuracy that we sometimes do it
> single-threaded)
I could add 'mapped-ram', as an alternative to 'fixed-ram'.
The key distinguishing & motivating feature here is that
RAM regions are mapped directly to file regions, instead
of just being streamed at arbitrary points.
With regards,
Daniel
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