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

[Vladimir Sementsov-Ogievskiy]

01.10.2019 7:28, Peter Krempa wrote:
> On Mon, Sep 30, 2019 at 20:09:28 -0400, 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.)
> 
> One possibility if there isn't an elegant qemu-based solution would be
> to add a migration feature libvirt could enable which would simply stop
> bitmaps from being copied and libvirt would do that in the synchronised
> phase of the migration explicitly.

Hmm. How it can do it?

I now the only way to migrate bitmaps without migration-capability: through
shared storage during migration downtime. But this means that downtime becomes
long if there are a lot of bitmap data and storage is not fast enough.

> 
> Libvirt might possibly need to do it anyways for inactive bitmaps if
> the automatic bitmap copying includes only active bitmaps.

All bitmaps are migrated.

> 
> I'm not sure though how that would combine with post-copy migration or
> what the impact on latency would be, but if you are migrating with
> storage I think performance will not be stelar anyways.

Just keep in mind that bitmap with default granularity (64k) for 16TB disk
is 32M, and we may have several such bitmaps. Storing/loading them
during migration downtime will of course influence this downtime

> 
>> 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.))
> 
> I think this should be okay. In libvirt I opted to forbid checkpoints
> which map to bitmap creation until blockdev will be supported where we
> manage node names ourselves.
> 

Hmm, I'm afraid that we'll have to fix this problem sooner.

-- 
Best regards,
Vladimir