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

[Kevin Wolf <kwolf@redhat.com>]

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.

You can use my blockdev development branch if you want to play with the
QAPI part: git://repo.or.cz/qemu/kevin.git blockdev

> > Hello Stefan,
> > 
> > I started writing the code.
> > 
> > How would the quorum treshold be specified with this command line style ?
> 
> Kevin can correct me but I think:
> 
>   -drive ...,file.vote_threshold=2
> 
> BlockDrivers can have custom options.  The voting threshold is a good
> example.
> 
> quorum_open() can check for "vote_threshold" in its options qdict.

Yup, that's the right way.

Kevin