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

[Deepak C Shetty <deepakcs@redhat.com>]

On 09/09/2015 03:16 PM, Stefan Hajnoczi wrote:
> 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

Whats the adv of this Vs extending existing gluster URI ?
This actually sounds like a lot more typing, and making qemu cmd line a 
lot more lengthier :)

Is -drive file=gluster://.... same as -drive driver=gluster, 
uri[0]=gluster..... ?

Also this would mean we need to change libvirt support for gluster based 
network disk completely
and also maintain the old code for backward compat, right ?

thanx,
deepak

>
> This approach allows full flexibility.
>
> I have CCed Kevin in case he has comments.
>
> Stefan