Re: [PATCH v11 20/27] Support colo mode for qemu disk

[Wen Congyang <wency@cn.fujitsu.com>]

On 03/18/2016 01:18 AM, Ian Jackson wrote:
> Wen Congyang writes ("Re: [PATCH v11 20/27] Support colo mode for qemu disk"):
>> How does block replication work:
> 
> Thanks for this explanation, which is reallt helpful.
> 
> I would like to repeat back to you what I think I have understood:
> 
> Between resynchs, you allow each VM to run in parallel and to generate
> possibly-divergent disk writes.
> 
> So on host B you retain both the A and B disk writes.  They are
> stored as differences (qcow2) for performance reasons.
> 
> If A fails and it becomes necessary to resume the VM only on B, you
> use B's version of the VM (both disk and memory).
> 
> If B fails then A can use the A version (disk and memory).
> 
> If the two are still up, but they diverge in network traffic, you
> resynch the memory from A to B, and drop B's disk and replace it with
> a copy of A's.
> 
> Have I understood correctly ?

Yes.

> 
> 
> If so, what software, where, arranges for the management of the
> different qcow2 `layers' ?  Ie, what creates the layers; what resynchs
> them, etc. ?

active disk and hidden disk are seperate disk. The management application
can create an empty qcow disk before running COLO. These two disks are
empty disk, and have the same size with the secondary disk.

Thanks
Wen Congyang

> 
> The reason I started asking all these questions is because of these
> parameters in your disk config:
> 
>  +    ("colo_enable", libxl_defbool),
>  +    ("colo_restore_enable", libxl_defbool),
>  +    ("colo_host", string),
>  +    ("colo_port", string),
>  +    ("colo_export", string),
>  +    ("active_disk", string),
>  +    ("hidden_disk", string)
> 
> For COLO to work properly it is necessary that the `active_disk' and
> `hidden_disk' to relate in a specific way to the main disk: they must
> be related snapshots (qcow2, currently).
> 
> Would it be possible for these disk names to have formulaic,
> predicatable, names, so that they wouldn't need to be specified
> separately ?
> 
> Is there any value in being able to specify them separately ?
> 
> Ian.
> 
> 
> .
> 




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