Re: [Qemu-devel] Merging the quorum block driver

[Kevin Wolf]

Am 19.09.2013 um 15:38 hat Benoît Canet geschrieben:
> Le Thursday 19 Sep 2013 à 15:21:46 (+0200), Kevin Wolf a écrit :
> > Am 19.09.2013 um 14:55 hat Benoît Canet geschrieben:
> > > Le Thursday 19 Sep 2013 à 10:57:50 (+0200), Kevin Wolf a écrit :
> > > > Am 19.09.2013 um 10:26 hat Stefan Hajnoczi geschrieben:
> > > > > On Wed, Sep 18, 2013 at 05:05:27PM +0200, Benoît Canet wrote:
> > > > > > Le Tuesday 17 Sep 2013 à 14:44:13 (+0200), Stefan Hajnoczi a écrit :
> > > > > > > Hi Benoit,
> > > > > > > Kevin and I had a chance to chat face-to-face and we discussed 
> > > > > > > what
> > > > > > > concrete changes are necessary to merge quorum (without solving 
> > > > > > > all the
> > > > > > > other block layers problems at once).
> > > > > > > 
> > > > > > > I think quorum could be merged relatively quickly (and without 
> > > > > > > massive
> > > > > > > BlockFilter investments) by changing the following:
> > > > > > > 
> > > > > > > 1. Defining children on the command-line
> > > > > > > 
> > > > > > > Existing "filter" drivers use the protocol filename to embed their
> > > > > > > children, for example the blkverify driver.  This is a big hack 
> > > > > > > because
> > > > > > > we have no proper syntax or escaping for the embedded drive 
> > > > > > > definitions
> > > > > > > in the file= option.
> > > > > > > 
> > > > > > > This was one of the main arguments against merging quorum.  Now 
> > > > > > > that
> > > > > > > Kevin has implemented driver-specific open options (see
> > > > > > > block/qcow2.c:qcow2_runtime_opts), it is possible for the quorum 
> > > > > > > driver
> > > > > > > to open children specific on the command-line:
> > > > > > > 
> > > > > > >   -drive if=none,file.driver=quorum,format=raw,\
> > > > > > >          file.children0.file=/nfs1/test.qcow2,\
> > > > > > >          file.children1.file=/nfs2/test.qcow2,\
> > > > > > >          file.children2.file=/nfs3/test.qcow2
> > > > 
> > > > By the way, I think he concrete syntax might have to be a bit different
> > > > so it can be mapped to QAPI for blockdev-add. I think we'll want to use
> > > > a JSON list for the children; a mapping to QDict and to the command line
> > > > is yet to be implemented I guess.
> > > 
> > > Hi,
> > > 
> > > Just to make sure I'll code the right thing; is the following correct ?
> > > 
> > > -drive if=none,file.driver=quorum,format=raw,\
> > >        file.children=[/nfs1/test.qcow2, /nfs2/test.qcow2, 
> > > /nfs3/test.qcow2], \
> > >        file.vote_threshold=2
> > 
> > I was thinking more along the lines of:
> > 
> >     -drive if=none,file.driver=quorum,format=raw,\
> >            file.children[0].file.filename=/nfs1/test.qcow2, \
> >            file.children[1].file.filename=/nfs2/test.qcow2, \
> >            file.children[1].file.cache.direct=on, \
> >            file.children[2].file.driver=nbd, \
> >            file.children[2].file.host=localhost, \
> >            file.vote_threshold=2
> > 
> > The exact syntax doesn't matter all that much as long as it can be
> > mapped to QAPI, but as you can see this allows specifying detailed
> > options even for the children. I think this is a requirement.
> 
> So does it mean that on top of your blockdev branch qemu_opts_to_qdict should 
> do
> a little parsing of opt->name to regroup options of the same children 
> together ?
> 
> e.g: converting the incoming qoption into the qdict equivalent of:
> dict = { "driver": "quorum",
>          "children[0]": "file.filename=/nfs1/test.qcow2",
>          "children[1]": "file.filename=/nfs2/test.qcow2,file.cache.direct=on",
>          "children[2]": "file.driver=nbd,file.host=localhost",
>          "vote_threshold": 2 }

No, the qdict is supposed to be completely flat. I think there are two
parts to it:

1. Extend qdict_flatten() so that it converts QLists into an array
   representation. The resulting qdict should look pretty much like the
   command line:

    dict = { "driver": "quorum",
             "children[0].file.filename": "/nfs1/test.qcow2",
             "children[1].file.filename": "/nfs2/test.qcow2",
             "children[1].file.cache.direct": "on",
             "children[2].file.driver": "nbd",
             "children[2].file.host": "localhost",
             "vote_threshold": 2 }

2. The option parsing code in quorum must find a way to walk this
   flattened children array. Basically it will end up checking if
   the keys start with a "children[%d]", and if so, use
   extract_subqdict() to get the full array element back.

Kevin