Re: [RFC v2] migration: Add migrate-set-bitmap-node-mapping

[Max Reitz]

On 13.05.20 18:11, Eric Blake wrote:
> On 5/13/20 9:56 AM, Max Reitz wrote:
>> This command allows mapping block node names to aliases for the purpose
>> of block dirty bitmap migration.
>>
>> This way, management tools can use different node names on the source
>> and destination and pass the mapping of how bitmaps are to be
>> transferred to qemu (on the source, the destination, or even both with
>> arbitrary aliases in the migration stream).
>>
>> Suggested-by: Vladimir Sementsov-Ogievskiy <address@hidden>
>> Signed-off-by: Max Reitz <address@hidden>
>> ---
> 
>> @@ -713,6 +731,44 @@ static bool dirty_bitmap_has_postcopy(void *opaque)
>>       return true;
>>   }
>>   +void
>> qmp_migrate_set_bitmap_node_mapping(MigrationBlockNodeMappingList
>> *mapping,
>> +                                         Error **errp)
>> +{
>> +    QDict *in_mapping = qdict_new();
>> +    QDict *out_mapping = qdict_new();
>> +
>> +    for (; mapping; mapping = mapping->next) {
>> +        MigrationBlockNodeMapping *entry = mapping->value;
>> +
>> +        if (qdict_haskey(out_mapping, entry->node_name)) {
>> +            error_setg(errp, "Cannot map node name '%s' twice",
>> +                       entry->node_name);
>> +            goto fail;
>> +        }
> 
> Can we call this command more than once?  Is it cumulative (call it once
> to set mapping for "a", second time to also set mapping for "b"), or
> should it reset (second call wipes out all mappings from first call, any
> mappings that must exist must be passed in the final call)?

I tried to make it clear in the documentation:

> +# @mapping: The mapping; must be one-to-one, but not necessarily
> +#           complete.  Any mapping not given will be reset to the
> +#           default (i.e. the identity mapping).

So everything that isn’t set in the second call is reset.  I thought
about what you proposed (because I guess that’s the most intuitive
idea), but after consideration I didn’t see why we’d need different
behavior, so it would only serve to make the code more complicated.

Max

> The idea makes sense, and the interface seems usable.  It's nice that
> either source, destination, or both sides of migration can use it (which
> helps in upgrade vs. downgrade scenarios).

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