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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