[Libvir] Proposal: Block device and network stats

With the very timely question that has been raised about block and network device stats, I'm posting my proposed interface. I almost have this working for the Xen case on my test machine. The idea of this interface is to be optimised for the following case: * stats need to be fetched frequently (eg. once per second) * the layout of devices doesn't change often (ie. adding or removing devices from domains is very infrequent) * most domains have only a single block and single network device With the above assumptions, the idea is that you would use the API like this: (1) When the program starts up or (infrequently) is notified of a change or when a new domain appears, the program calls virDomainGetXMLDesc and parses out the //domain/devices/interface and //domain/devices/disk fields to get the list of network interfaces and block devices. So for each domain, you'll have two lists like this: dom[1]["blockdevs"] = ["xvda"] dom[1]["interfaces"] = ["virbr0"] (2) Frequently (eg. once per second) the program calls (in the above case): virDomainBlockStats (dom1, "xvda", &bstats, sizeof bstats); virDomainInterfaceStats (dom1, "virbr0", &istats, sizeof istats); (3) Since stats are cumulative the program must do its own subtraction and calculation in order to display throughput per second. The implementation goes directly from the name "xvda" to the backend device (/sys/devices/xen-backend/- [type]-[domid]-[major:minor]/statistics) for block devices, and slightly more complicatedly to a particular line in /proc/net/dev for network interfaces. (Note in the Xen case, the name "virbr0" is little more than a placeholder to mean "the zeroth interface for domain d"). In particular the current implementation doesn't cache anything. This should all work fine in the Linux / Xen case. libxenstat gives us sample code that we can copy for the Solaris / Xen case, but it would need testing from someone with access to such a machine. I don't think qemu supports stats at all. Initially we won't support stats for tap devices because there needs to be an upstream patch to support this (http://lists.xensource.com/archives/html/xen-changelog/2007-02/msg00278.html). The extra size parameter will allow us to extend the stats structures in future, maintaining binary backwards compatibility with existing clients. Rich. -- Emerging Technologies, Red Hat - http://et.redhat.com/~rjones/ Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SL4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 03798903