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Re: [Qemu-devel] [PATCH 1/5] block: Auto-generate node_names for each BD

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH 1/5] block: Auto-generate node_names for each BDS entry
Date: Fri, 16 May 2014 06:47:49 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0

On 05/16/2014 05:35 AM, Jeff Cody wrote:

> How does libvirt assign node-names to all the backing images in a
> qcow2 chain, for example?

We have the ability to do both command-line and QMP hotplug addition of
drives where you can set backing.file.node-name (or is it
backing.node-name?) for any element in a backing chain; libvirt just
needs to be taught to use it.

>> I'm not totally sure whether automatically generated node-names are
>> a good idea, but I can see how they are useful with human monitor users
>> which may not specify a node-name everywhere (I've used device_add
>> without an ID often enough, only to find that I can't remove the device
>> any more). We should just make sure that they are really only used by
>> human users.
> I don't understand.  What would be the downsides of having an
> automatic guaranteed unique id assigned to each BDS?  And why
> restrict that to human users only?

I'm not seeing downsides.

> If you are worried about node-names potentially being undesired by the
> user / management layer for some reason, how about this: we add a
> drive option, to either enable or disable automatic node-name
> generation for a particular drive?

No, this is silly.  If it defaults to off, then libvirt has to be taught
to enable it - but if you are going to modify libvirt, you might as well
modify it to set node names itself rather than to enable the option for
automatic node names.  If it defaults to on, no one has an incentive to
disable it.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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