On Thu, Mar 24, 2016 at 08:07:17PM +0100, Max Reitz wrote: > As I responded to: > - http://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg04464.html > - http://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg05680.html > > I think a general solution for querying the block node tree would be > nice (I don't think we actually have one). The single patch in this > series implements such a command. > > However, this is an RFC because I'm not sure whether we really want this > and thus I didn't want to write the necessary tests for something we may > be going to discard anyway. > > There are two reasons why I fear we may not want this: > > The first is that the node graph is more or less something internal to > qemu. Its actual structure may (and most probably will) change over > time. We do want to be able to let the user or management application > manage the graph in fine detail, but these modifications are something > that can be emulated by legacy handling code later if we decide they are > no longer in line with the internal graph that we'd like to have. > > However, if we emit the full graph with a command such as introduced > here, we can hardly change its outside appearance just to please legacy > applications. The output will change if the internal representation > changes. > > I don't personally think this is too bad as long as we clearly state > this in the command's description: That qemu is free to implicitly > create intermediate nodes the user did not explicitly specify, and that > the set of nodes thus created may change over time. > > No, I didn't specify this in this version, but it's an RFC, after all. > :-)
I'm also concerned about exposing the graph since QEMU may implement features with internal block filters (I/O throttling, backup block job to get rid of write notifiers, etc). These nodes come and go depending on high-level commands issued by the guest *and* are dependent on the QEMU version. What is the purpose of this command - I didn't see that in your cover letter? Does it still serve its purpose if we warn the user that the graph structure can contain little surprises :)?
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