Re: bitmap migration bug with -drive while block mirror runs

[Max Reitz <mreitz@redhat.com>]

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On 01.10.19 17:09, Kevin Wolf wrote:
> Am 01.10.2019 um 16:34 hat Max Reitz geschrieben:
>> On 01.10.19 16:27, Vladimir Sementsov-Ogievskiy wrote:
>>> 01.10.2019 17:13, Max Reitz wrote:
>>>> On 01.10.19 16:00, Vladimir Sementsov-Ogievskiy wrote:
>>>>> 01.10.2019 3:09, John Snow wrote:
>>>>>> Hi folks, I identified a problem with the migration code that Red Hat QE
>>>>>> found and thought you'd like to see it:
>>>>>>
>>>>>> https://bugzilla.redhat.com/show_bug.cgi?id=1652424#c20
>>>>>>
>>>>>> Very, very briefly: drive-mirror inserts a filter node that changes what
>>>>>> bdrv_get_device_or_node_name() returns, which causes a migration problem.
>>>>>>
>>>>>>
>>>>>> Ignorant question #1: Can we multi-parent the filter node and
>>>>>> source-node? It looks like at the moment both consider their only parent
>>>>>> to be the block-job and don't have a link back to their parents otherwise.
>>>>>>
>>>>>>
>>>>>> Otherwise: I have a lot of cloudy ideas on how to solve this, but
>>>>>> ultimately what we want is to be able to find the "addressable" name for
>>>>>> the node the bitmap is attached to, which would be the name of the first
>>>>>> ancestor node that isn't a filter. (OR, the name of the block-backend
>>>>>> above that node.)
>>>>>
>>>>> Not the name of ancestor node, it will break mapping: it must be name of the
>>>>> node itself or name of parent (may be through several filters) block-backend
>>>>>
>>>>>>
>>>>>> A simple way to do this might be a "child_unfiltered" BdrvChild role
>>>>>> that simply bypasses the filter that was inserted and serves no real
>>>>>> purpose other than to allow the child to have a parent link and find who
>>>>>> it's """real""" parent is.
>>>>>>
>>>>>> Because of flushing, reopen, sync, drain &c &c &c I'm not sure how
>>>>>> feasible this quick idea might be, though.
>>>>>>
>>>>>>
>>>>>> - Corollary fix #1: call error_setg if the bitmap node name that's about
>>>>>> to go over the wire is an autogenerated node: this is never correct!
>>>>>>
>>>>>> (Why not? because the target is incapable of matching the node-name
>>>>>> because they are randomly generated AND you cannot specify node-names
>>>>>> with # prefixes as they are especially reserved!
>>>>>>
>>>>>> (This raises a related problem: if you explicitly add bitmaps to nodes
>>>>>> with autogenerated names, you will be unable to migrate them.))
>>>>>>
>>>>>> --js
>>>>>>
>>>>>
>>>>> What about the following:
>>>>>
>>>>> diff --git a/block.c b/block.c
>>>>> index 5944124845..6739c19be9 100644
>>>>> --- a/block.c
>>>>> +++ b/block.c
>>>>> @@ -1009,8 +1009,20 @@ static void bdrv_inherited_options(int *child_flags, QDict *child_options,
>>>>>        *child_flags = flags;
>>>>>    }
>>>>>
>>>>> +static const char *bdrv_child_get_name(BdrvChild *child)
>>>>> +{
>>>>> +    BlockDriverState *parent = child->opaque;
>>>>> +
>>>>> +    if (parent->drv && parent->drv->is_filter) {
>>>>> +        return bdrv_get_parent_name(parent);
>>>>> +    }
>>>>> +
>>>>> +    return NULL;
>>>>> +}
>>>>> +
>>>>
>>>> Why would we skip filters explicitly added by the user?
>>>>
>>>
>>> Why not? Otherwise migration of bitmaps will not work: we may have different set
>>> of filters on source and destination, and we still should map nodes with bitmaps
>>> automatically.
>>
>> Why would we have a different set of explicitly added filters on source
>> and destination and allow them to be automatically changed during
>> migration?  Shouldn’t users only change them pre or post migration?
> 
> We never made a requirement that the backend must be the same on the
> source and the destination. Basically, migration copies the state of
> frontends and the user is responsible for having these frontends created
> and connected to the right backends on the destination.
> 
> Using different paths on the destination is a very obvious requirement
> for block devices. It's less obvious for the graph structure, but I
> don't see a reason why it couldn't change on migration. Say we were
> using local storage on the source, but now we did storage migration to
> some network storage, access to which should be throttled.

I don’t quite see why we couldn’t add such filters before or after
migration.  And it was my impression that bitmap migration was a problem
now precisely because it is bound to the graph structure.

But anyway.  I’ll gladly remove myself from this discussion because I
don’t know much about migration and actually I’d prefer to keep it that
way.  (Sorry.)

Max


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