Re: [PATCH v2 14/29] 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 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 <farosas@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: