[libvirt] [PATCH 1/1] qemu: add support for multiple gluster hosts

[Peter Krempa]

On Mon, Oct 05, 2015 at 18:59:54 +0530, Deepak C Shetty wrote:
> 

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> 
> On 10/05/2015 04:31 PM, Peter Krempa wrote:
> > On Mon, Oct 05, 2015 at 15:30:42 +0530, Prasanna Kumar Kalever wrote:

[snip]

> > So while all the above looks pretty okay at this point (except for
> > clearing the image type) this patch is incomplete.
> >
> > Since libvirt is loading the backing chain to be able to traverse it and
> > set correct permissions on individual images. This equals to two
> > additional parts of the code that need modification:
> >
> > 1) virStorageFileBackendGlusterInit has the same "src->nhosts != 1"
> > condition and is not prepared to work with multiple protocols
> 
> IIUC this is part of the storage pool feature of libvirt and that can be 
> handled
> as part of a separate patch. Using gluster as a block backend for 
> qemudoesn't
> enforce using gluster as a file backend for libvirt storage pool, they 
> both are
> separate, IMHO

No, the part of the storage driver I'm refering to is used in the
backing chain detection. So even if you don't use the libvirt's gluster
storage pool, the code I'm refering to will be invoked.

It can be a separate patch, but it needs to go in before this patch
lands.

> 
> >
> > 2) I presume (I didn't test it yet) that the qemu patch that adds this
> > stuff will result into using the "json:{" protocol string in the
> > 'backing_store' field once you do a snapshot of a gluster disk that uses
> > this new syntax. If that happens the VM will not be able to start in
> > libvirt any more, as libvirt does not have a 'json:{' protocol parser to
> > parse the backing store.
> 
> We tried to take a disk-only external snapshot with libvirtd running with
> this patch and I was unable to see the "json:{..." format for backing file.
> 
> I think we understand what you are saying, but we are unable to reproduce
> the problem ( of having the json way in backing file). This is what we 
> did ....
> 
> * Domain HFM running with CentOs7.qcow2 as a network disk type w/ 
> protocol=gluster
> 
> 0 :) dhcp1-86 ~/deepakcs $ virsh list --all
>   Id    Name                           State
> ----------------------------------------------------
>   4     HFM                            running
> 
> 
> * Create a qcow2 file for use with external snapshot
> 
> qemu-img create -f qcow2 -b gluster://10.70.1.86/sample/CentOs7.qcow2 
> gluster://10.70.1.86/sample/newsnap.qcow2

If you pre-create backing files to use with --reuse-external, you have
to use the json syntax here to express the backing file.

> 
> * Create a domainsnapshot XML (snap.xml)
> 
> <domainsnapshot>
>      <disks>
>          <disk name="vda" snapshot="external" type="network">
>              <source protocol="gluster" name="sample/newsnap.qcow2">
>                  <host name="10.70.1.86" port="24007"/>
>                  <host name="10.70.1.88" port="24007"/>
>                  <host name="10.70.1.87" port="24007"/>
>              </source>
>          </disk>
>      </disks>
> </domainsnapshot>

You are doing it the other way around. I was speaking of doing a VM
started with a multi-server gluster volume and then doing a snapshot
wherever you like. This should result in a snapshot that has json
definition of the backing file.

> 
> * Run virsh snap cmd (error in bold)

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> 
> 0 :) dhcp1-86 ~/deepakcs $ virsh snapshot-create HFM --xmlfile 
> ./snap.xml --disk-only --reuse-external
> *error: internal error: missing storage backend for network files using 
> gluster protocol*

Erm, how are you attempting to test this if you don't even have libvirt
with gluster support installed?

> 
> * Run virsh snap cmd w/ domain is off (error in bold)
> 
> 0 :) dhcp1-86 ~/deepakcs $  virsh list --all
>   Id    Name                           State
> ----------------------------------------------------
>   -     HFM                            shut off
> 
> 0 :) dhcp1-86 ~/deepakcs $ virsh snapshot-create HFM --xmlfile 
> ./snap.xml --disk-only --reuse-external
> *error: internal error: external inactive snapshots are not supported on 
> 'network' disks using 'gluster' protocol*

This is expected, inactive external snapshots have only a very limited 
set of supported configurations. Networked storage is not there.

> >
> > So in order to accept this patch, the helpers in
> > src/util/virstoragefile.c (virStorageSourceParseBackingURI) will need to
> > be made modular so that they can accept a driver specific callback, that
> > will then parse the 'json:{' source definitions. (We can't just hardcode
> > it there, since src/util "should" not contain any qemu specific code.)
> 
> Questions:
>      1) Why does snapshot doesn't work, is it bcos the support for json 
> is not present in the backup parse code ?
>         The error msg seen above, isn't clear

Well the error message says that you have libvirt without gluster
support and thus it refuses to do a snapshot on a gluster disk.

> 
>      2) IIUC, you mean that the backing file will be in the json format 
> something like ...
>         backing file: json"{...}" in `qemu-img info --backing-chain` 
> output ?

Exactly.

> 
>        How to create such a scenario, so that we can "see" the problem 
> happening and hence debug/fix it :)

Start a VM with the multi-host gluster support (as of by this patch)
and create an external snapshot even on local disk, no need to go to
gluster again.

Peter

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