Re: [PATCH v4 11/34] migration/ram: Introduce 'fixed-ram' migration capability

[Markus Armbruster <armbru@redhat.com>]

Fabiano Rosas <farosas@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 <farosas@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?

Apart from that, QAPI schema
Acked-by: Markus Armbruster <armbru@redhat.com>