Re: [Qemu-devel] [PATCH 1/2] commit: Add top-node/base-node options

[Alberto Garcia <berto@igalia.com>]

On Mon 03 Sep 2018 05:03:11 PM CEST, Kevin Wolf wrote:
>> libvirt-3-format is read-write and all other node names are readonly in
>> the above example.
>> 
>> The same also happens when using filenames:
>> 
>> {"execute":"block-commit",
>>  "arguments" : {"device":"libvirt-3-format",
>>                 "job-id":"libvirt-3-format",
>>                 "top":"/var/lib/libvirt/images/rhel7.3.1483615252",
>>                 "base":"/var/lib/libvirt/images/rhel7.3.1483605924",
>>                 "auto-finalize":true,
>>                 "auto-dismiss":false},
>>  "id":"libvirt-13"}
>> 
>> {"id":"libvirt-13","error":{"class":"GenericError","desc":"Block node is read-only"}}
>
> I see what's happening here. So we have a graph like this:
>
>      guest device
>           |
>           v
>     overlay-format -------> backing-format
>     [read-only=off]         [read-only=on]
>           |                        |
>           v                        v
>     overlay-proto           backing-proto
>     [read-only=off]         [read-only=on]
>
> The difference between your -blockdev use and -drive is that you
> explicitly specify the read-only option for backing-proto (and you use
> a separate -blockdev option anyway), so it doesn't just inherit it
> from backing-format.

Are these format and protocol block devices opened with four separate
-blockdev parameters? Is that how libvirt does it?

Berto