Re: [Qemu-devel] [PATCH v0 1/7] migration: add background snapshot capability

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Denis Plotnikov (dplotnikov@virtuozzo.com) wrote:
> The capability is used for the background vmstate saving
> using the migration infrastructure.
> Background vmstate saving means that the majority of vmstate
> (RAM) is saved in the background when VM's vCPUS are running.
> This helps to reduce the VM downtime on VM snapshotting.
> 
> Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>

(As normal the 'since' will need bumping)

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> ---
>  migration/migration.c | 35 +++++++++++++++++++++++++++++++++++
>  migration/migration.h |  1 +
>  qapi/migration.json   |  6 +++++-
>  3 files changed, 41 insertions(+), 1 deletion(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index d780601f0c..87096d23ef 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -697,6 +697,12 @@ static bool migrate_caps_check(bool *cap_list,
>              return false;
>          }
>  
> +        if (cap_list[MIGRATION_CAPABILITY_BACKGROUND_SNAPSHOT]) {
> +            error_setg(errp, "Postcopy is not compatible "
> +                        "with background snapshot");
> +            return false;
> +        }
> +
>          /* This check is reasonably expensive, so only when it's being
>           * set the first time, also it's only the destination that needs
>           * special support.
> @@ -711,6 +717,26 @@ static bool migrate_caps_check(bool *cap_list,
>          }
>      }
>  
> +    if (cap_list[MIGRATION_CAPABILITY_BACKGROUND_SNAPSHOT]) {
> +        if (cap_list[MIGRATION_CAPABILITY_RELEASE_RAM]) {
> +            error_setg(errp, "Background snapshot is not compatible "
> +                        "with release ram capability");
> +            return false;
> +        }
> +
> +        if (cap_list[MIGRATION_CAPABILITY_COMPRESS]) {
> +            error_setg(errp, "Background snapshot is not "
> +                        "currently compatible with compression");
> +            return false;
> +        }
> +
> +        if (cap_list[MIGRATION_CAPABILITY_XBZRLE]) {
> +            error_setg(errp, "Background snapshot is not "
> +                        "currently compatible with XBZLRE");
> +            return false;
> +        }
> +    }
> +
>      return true;
>  }
>  
> @@ -1635,6 +1661,15 @@ bool migrate_use_block_incremental(void)
>      return s->parameters.block_incremental;
>  }
>  
> +bool migrate_background_snapshot(void)
> +{
> +    MigrationState *s;
> +
> +    s = migrate_get_current();
> +
> +    return s->enabled_capabilities[MIGRATION_CAPABILITY_BACKGROUND_SNAPSHOT];
> +}
> +
>  /* migration thread support */
>  /*
>   * Something bad happened to the RP stream, mark an error
> diff --git a/migration/migration.h b/migration/migration.h
> index 663415fe48..21babbc008 100644
> --- a/migration/migration.h
> +++ b/migration/migration.h
> @@ -201,6 +201,7 @@ int migrate_compress_level(void);
>  int migrate_compress_threads(void);
>  int migrate_decompress_threads(void);
>  bool migrate_use_events(void);
> +bool migrate_background_snapshot(void);
>  
>  /* Sending on the return path - generic and then for each message type */
>  void migrate_send_rp_shut(MigrationIncomingState *mis,
> diff --git a/qapi/migration.json b/qapi/migration.json
> index 03f57c9616..eb5f52b7ff 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -352,12 +352,16 @@
>  #
>  # @x-multifd: Use more than one fd for migration (since 2.11)
>  #
> +# @background-snapshot: Using migration infrastructure makes VM snapshot
> +#         saving its RAM in background. This reduces  VM downtime. (since 2.12)
> +#
>  # Since: 1.2
>  ##
>  { 'enum': 'MigrationCapability',
>    'data': ['xbzrle', 'rdma-pin-all', 'auto-converge', 'zero-blocks',
>             'compress', 'events', 'postcopy-ram', 'x-colo', 'release-ram',
> -           'block', 'return-path', 'pause-before-switchover', 'x-multifd' ] }
> +           'block', 'return-path', 'pause-before-switchover', 'x-multifd',
> +           'background-snapshot' ] }
>  
>  ##
>  # @MigrationCapabilityStatus:
> -- 
> 2.17.0
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK