Re: [Qemu-devel] [PATCH] qapi: add dirty-bitmaps to query-named-block-nodes result

[Eric Blake]

On 5/30/19 11:26 AM, John Snow wrote:
> 
> 
> On 5/30/19 10:39 AM, Vladimir Sementsov-Ogievskiy wrote:
>> Let's add a possibility to query dirty-bitmaps not only on root nodes.
>> It is useful when dealing both with snapshots and incremental backups.
>>

>> +++ b/block/qapi.c
>> @@ -78,6 +78,11 @@ BlockDeviceInfo *bdrv_block_device_info(BlockBackend *blk,
>>          info->backing_file = g_strdup(bs->backing_file);
>>      }
>>  
>> +    if (!QLIST_EMPTY(&bs->dirty_bitmaps)) {
>> +        info->has_dirty_bitmaps = true;
>> +        info->dirty_bitmaps = bdrv_query_dirty_bitmaps(bs);
>> +    }
>> +
>>      info->detect_zeroes = bs->detect_zeroes;
>>  
>>      if (blk && blk_get_public(blk)->throttle_group_member.throttle_state) {
>>
> 
> So query-block uses bdrv_query_info, which calls bdrv_block_device_info,
> so we'll duplicate the bitmap output when doing the old-fashioned block
> query, but that's probably harmless overall.

We already know that none of our existing query- interfaces are sane
(either too little information, or too much).  Duplication starts to
push an interface towards too much (it takes processor time to bundle up
the extra JSON, especially if the other end is not going to care if it
was present). I know Kevin still has somewhere on his to-do list the
implementation of a saner query- command for the information we really
want (about each block, without redundant information, and where we
don't repeat information in a nested manner, but where we also don't
omit information that would otherwise require multiple existing query-
to reconstruct).

> 
> We can continue to support the output in both places, or we could opt to
> deprecate the older interface; I think this is one of the last chances
> we'd get to do so before libvirt and wider adoption.
> 
> I think that's probably Eric's choice.

If you want to try to deprecate the old location, introspection at least
works to allow libvirt to know which place to look for it on a given
qemu. If you don't think deprecation is necessary, the duplication is
probably tolerable for now (as ideally we'd be deprecating ALL of our
not-quite-perfect query- block interfaces in favor of whatever sane
interface Kevin comes up with).

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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