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.
> +
> +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?
> +
> 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.
Which transports are seekable?
Out of curiosity: what happens if the transport isn't seekable?
> +#
> # 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:
