Re: [RFC PATCH v3 23/30] migration: Add direct-io parameter

[Markus Armbruster]

Fabiano Rosas <farosas@suse.de> writes:

> Add the direct-io migration parameter that tells the migration code to
> use O_DIRECT when opening the migration stream file whenever possible.

Why is that useful?

> This is currently only used with fixed-ram migration for the multifd
> channels that transfer the RAM pages. Those channels only transfer the
> pages and are guaranteed to perform aligned writes.
>
> However the parameter could be made to affect other types of
> file-based migrations in the future.
>
> Signed-off-by: Fabiano Rosas <farosas@suse.de>

[...]

> diff --git a/qapi/migration.json b/qapi/migration.json
> index 3b93e13743..1d38619842 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -878,6 +878,9 @@
>  # @mode: Migration mode. See description in @MigMode. Default is 'normal'.
>  #        (Since 8.2)
>  #
> +# @direct-io: Open migration files with O_DIRECT when possible. This
> +#     requires that the 'fixed-ram' capability is enabled. (since 9.0)

Two spaces between sentences for consistency, please.

> +#
>  # Features:
>  #
>  # @deprecated: Member @block-incremental is deprecated.  Use
> @@ -911,7 +914,8 @@
>             'block-bitmap-mapping',
>             { 'name': 'x-vcpu-dirty-limit-period', 'features': ['unstable'] },
>             'vcpu-dirty-limit',
> -           'mode'] }
> +           'mode',
> +           'direct-io'] }
>  
>  ##
>  # @MigrateSetParameters:
> @@ -1066,6 +1070,9 @@
>  # @mode: Migration mode. See description in @MigMode. Default is 'normal'.
>  #        (Since 8.2)
>  #
> +# @direct-io: Open migration files with O_DIRECT when possible. This
> +#     requires that the 'fixed-ram' capability is enabled. (since 9.0)
> +#
>  # Features:
>  #
>  # @deprecated: Member @block-incremental is deprecated.  Use
> @@ -1119,7 +1126,8 @@
>              '*x-vcpu-dirty-limit-period': { 'type': 'uint64',
>                                              'features': [ 'unstable' ] },
>              '*vcpu-dirty-limit': 'uint64',
> -            '*mode': 'MigMode'} }
> +            '*mode': 'MigMode',
> +            '*direct-io': 'bool' } }
>  
>  ##
>  # @migrate-set-parameters:
> @@ -1294,6 +1302,9 @@
>  # @mode: Migration mode. See description in @MigMode. Default is 'normal'.
>  #        (Since 8.2)
>  #
> +# @direct-io: Open migration files with O_DIRECT when possible. This
> +#     requires that the 'fixed-ram' capability is enabled. (since 9.0)
> +#
>  # Features:
>  #
>  # @deprecated: Member @block-incremental is deprecated.  Use
> @@ -1344,7 +1355,8 @@
>              '*x-vcpu-dirty-limit-period': { 'type': 'uint64',
>                                              'features': [ 'unstable' ] },
>              '*vcpu-dirty-limit': 'uint64',
> -            '*mode': 'MigMode'} }
> +            '*mode': 'MigMode',
> +            '*direct-io': 'bool' } }
>  
>  ##
>  # @query-migrate-parameters:

[...]