Re: [RFC PATCH v1 10/26] migration/ram: Introduce 'fixed-ram' migration stream capability

[Markus Armbruster]

Fabiano Rosas <farosas@suse.de> writes:

> From: Nikolay Borisov <nborisov@suse.com>
>
> Implement 'fixed-ram' feature. The core of the feature is to ensure that
> each ram page of the migration stream 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 DIO-enabled save/restore of the
>    migration stream as the pages are ensured to be written at aligned
>    offsets.
>
> The feature requires changing the stream format. First, a bitmap is
> introduced which tracks which pages have been written (i.e are
> dirtied) during migration and subsequently it's being written in the
> resulting file, again at a fixed location for every ramblock. Zero
> pages are ignored as they'd be zero in the destination migration as
> well. With the changed format data would look like the following:
>
> |name len|name|used_len|pc*|bitmap_size|pages_offset|bitmap|pages|
>
> * pc - refers to the page_size/mr->addr members, so newly added members
> begin from "bitmap_size".
>
> This layout is initialized during ram_save_setup so instead of having a
> sequential stream of pages that follow the ramblock headers the dirty
> pages for a ramblock follow its header. Since all pages have a fixed
> location RAM_SAVE_FLAG_EOS is no longer generated on every migration
> iteration but there is effectively a single RAM_SAVE_FLAG_EOS right at
> the end.
>
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> Signed-off-by: Fabiano Rosas <farosas@suse.de>

[...]

> diff --git a/qapi/migration.json b/qapi/migration.json
> index c84fa10e86..22eea58ce3 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -485,7 +485,7 @@
>  ##
>  { 'enum': 'MigrationCapability',
>    'data': ['xbzrle', 'rdma-pin-all', 'auto-converge', 'zero-blocks',
> -           'compress', 'events', 'postcopy-ram',
> +           'compress', 'events', 'postcopy-ram', 'fixed-ram',
>             { 'name': 'x-colo', 'features': [ 'unstable' ] },
>             'release-ram',
>             'block', 'return-path', 'pause-before-switchover', 'multifd',

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