Re: [libvirt] RFC: network interface "tags" vs. portgroups

A couple releases ago (commit 7d5bf484, first appeared in 1.2.2) I modified the domain interface status xml to show what resources are actually in use for an interface, superseding the interface config in the cases where they conflict with each other. In particular, if there is an interface of type='network' that references a portgroup of the network in the <source> element, the interface status will not contain a <source> element showing the network and portgroup names, but instead the <source resulting from applying the config is shown. For example, given the following domain interface and network definitions: <interface type='network'> <source network='mynet' portgroup='xyzzy'/> ... </interface> <network> <name>mynet</name> <forward mode='bridge'/>\ <bridge name='br0'/> <portgroup name='xyzzy'> <bandwidth> <inbound average='1000' peak='5000' floor='200' burst='1024'/> <outbound average='128' peak='256' burst='256'/> </bandwidth> </portgroup> </network> the status that was previously displayed when the domain was running would be identical to the config above (except that it would also contain the tap device name and alias). But now the status will be this: <interface type='bridge'> <source bridge='br0'/> <bandwidth> <inbound average='1000' peak='5000' floor='200' burst='1024'/> <outbound average='128' peak='256' burst='256'/> </bandwidth> ... </interface> The advantage here is that a hook script for the domain will be able to see the bandwidth (and vlan and physical device, if any) that are actually being used by the domain's interface. Because the config and status both use the same elements/attributes, we can only show one or the other; the thinking was that normally the status will be what is desired, and anyone who really wants to look at the config should use the VIR_DOMAIN_XML_INACTIVE flag when calling virDomainGetXMLDesc(). As you would expect, a few months later (after the release of 1.2.4) someone on IRC checked in with a problem caused by this change - they had been using the portgroup name in the <source> element of the interface to determine what action to take during migration; they didn't even have any libvirt config stored in the portgroup, but were just using its name as a tag. Since the portgroup name is only a part of the <source> element when the interface is type='network', they now don't have a tag in the xml to use for their decision (and since they aren't explicitly calling virDomainGetXMLDesc() themselves, they can't simply get the INACTIVE xml to solve their problem). This use of a portgroup name struck me as potentially useful (although it is a slight hijacking of the original intent of portgroups), so I would like to restore that functionality. I came up with a couple different ways to solve the problem, and am looking for opinions before I spend any time on either. Solution 1: My initial thought was to just restore the portgroup name in the status XML; that could be done by moving the portgroup name out of the network-specific part of the object and into common data for all interface types (this way it could show up in the <source> element no matter what is the network type). However, once we've done that it becomes enticing to allow specification of a portgroup even in cases where the interface type != network; in those cases though, the portgroup would be *only* a tag to be used by external entities; this would lead to lax checking for existence of the specified portgroup, and may end up with people misspelling a portgroup name, then mistakenly believing that (e.g.) they had a bandwidth limit applied to a domain interface when none was in fact in effect. (alternately, we could allow it only if the interface *config* was for type='network', but that seems somehow logically broken, and you can bet that eventually someone would ask for us to allow it for all types) Solution 2: An alternate idea I had was to add a new <tag name='x'/> element to interfaces, networks, and portgroups. An interface could have multiple tags, and would assume the tags of its <network> when active. A <tag> would be purely for use by external entities - it would mean nothing to libvirt. For example, given this extreme example: <interface type='network'> <source network='mynet' portgroup='xyzzy'/> <tag name='wumpus'/> ... </interface> <network> <name>mynet</name> <tag name='twisty'/> <forward mode='bridge'/>\ <bridge name='br0'/> <portgroup name='xyzzy'> <tag name='xyzzytag'/> <bandwidth> <inbound average='1000' peak='5000' floor='200' burst='1024'/> <outbound average='128' peak='256' burst='256'/> </bandwidth> </portgroup> </network> <network> when the interface was active, its status xml would look like this: <interface type='bridge'> <tag name='wumpus'/> <tag name='twisty'/> <tag name='xyzzytag'/> <source bridge='br0'/> <bandwidth> <inbound average='1000' peak='5000' floor='200' burst='1024'/> <outbound average='128' peak='256' burst='256'/> </bandwidth> ... </interface> These tags would mean nothing to libvirt, but would be available for management applications to make decisions during domain start/stop/migration/etc. The advantages of solution 1 are that it is simpler, wouldn't require any new XML elements, and any existing code using the portgroup name in this manner would once again work (I don't know if anyone else uses it this way). The advantages of solution 2 are that it doesn't try to make a single element work for two different functions which may not always mesh with each other, and it allows for more than one tag per interface, so different parts of a management application may use the tag element without fear of breaking some other use. It may be a case of over-engineering though :-) So which should I do? -- libvir-list mailing list libvir-list(a)redhat.com https://www.redhat.com/mailman/listinfo/libvir-list