Re: [Qemu-devel] [PATCH 1/1] block/gluster: add support for multiple gluster backup volfile servers

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, Sep 08, 2015 at 08:40:58PM +0530, Deepak Shetty wrote:
> On Tue, Sep 8, 2015 at 8:32 PM, Daniel P. Berrange <berrange@redhat.com>
> wrote:
> 
> > On Tue, Sep 08, 2015 at 06:34:09PM +0530, Prasanna Kumar Kalever wrote:
> > > This patch adds a way to specify multiple backup volfile servers to the
> > gluster
> > > block backend of QEMU with both tcp and rdma transport types.
> > >
> > > Problem:
> > >
> > > Currenly VM Image on gluster volume is specified like this:
> > >
> > > file=gluster[+tcp]://server1:24007/testvol/a.img
> > >
> > > Assuming we have have three servers in trustred pool with replica 3
> > volume
> > > in action and unfortunately server1 (mentioned in the command above)
> > went down
> > > for some reason, since the volume is replica 3 we now have other 2
> > servers
> > > active from which we can boot the VM.
> > >
> > > But currently there is no mechanism to pass the other 2 gluster server
> > > addresses to qemu.
> > >
> > > Solution:
> > >
> > > New way of specifying VM Image on gluster volume with backup volfile
> > servers:
> > >
> > > file=gluster[+transport-type]://server1:24007/testvol/a.img\
> > >      ?backup-volfile-servers=server2&backup-volfile-servers=server3
> >
> > Comparison with RBD syntax:
> >
> >   file=rbd:pool/image:auth_supported=none:\
> >     mon_host=mon1.example.org\:6321\;mon2.example.org\:6322\;\
> >     mon3.example.org\:6322,if=virtio,format=raw
> >
> > As Peter already mentioned, you're missing port numbers.
> >
> > It is slightly unpleasant to have different ways of specifying the first
> > vs second, third, etc hosts. I wonder if it would be nicer to keep all
> > the hostnames in the host part of the URI. eg
> >
> >
> >  file=gluster[+transport-type]://server1:24007,server2:3553,server3:2423/testvol/a.img\
> >       ?backup-volfile-servers=server2&backup-volfile-servers=server3
> >
> > Of course it ceases to be a wellformed URI at that point, so another option
> > would be to just allow the host part of the URI to be optional, and then
> > accept mutliple instances ofa  'server' arg, eg
> >
> >  file=gluster[+transport-type]:///testvol/a.img\
> >       ?server=server1:2424&server=server2:2423&sever=server3:34222
> >
> >
> Is it allowed to have this syntax and be a valid URI ? I admit i haven't
> looked at the
> URI rfc for a long time now, hence the Q. Also looking at rbd syntax, it
> looks
> to follow this model already is it ? Whats the difference between using ':'
> to
> separate key=value pairs Vs using '?" query syntax ? Should we look at
> having
> a uniform way of specifying URI be it rbd or gluster or sheepdog ... ? If
> yes
> what that uniform syntax be using ':" or '?" ?

Instead of trying to make a gluster:// URI that accommodates multiple
volfile servers, perhaps the block driver can take a list of URIs.
Something like:

  -drive driver=gluster,uri[0]=gluster[+transport-type]://server1:24007/testvol/a.img,
                        uri[1]=gluster[+transport-type]://server2:24008/testvol/a.img,
                        uri[2]=gluster[+transport-type]://server3:24009/testvol/a.img

This approach allows full flexibility.

I have CCed Kevin in case he has comments.

Stefan