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Re: [Qemu-devel] [PATCH 5/5] block: extend block-commit to accept a stri

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH 5/5] block: extend block-commit to accept a string for the backing file
Date: Thu, 15 May 2014 12:52:25 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0

On 05/15/2014 12:22 PM, Jeff Cody wrote:
>> Hmm - how will this be discoverable by libvirt? Maybe when libvirt is
>> doing the 'qemu -m none' probing, it can hotplug a device pointing to
>> /dev/null (libvirt _already_ does that to test if add-fd works), and
>> intentionally omit a node name.  If libvirt then queries the device, and
>> sees that the __qemu##000NNNN node-name was auto-assigned, then it can
>> be assumed that this qemu is new enough to provide node-names for ALL
>> operations (but that means this series is incomplete unless we add
>> node-name support to all remaining block commands, such as block-stream,
>> drive-mirror, and drive-backup).  This part is where I wonder if patch
>> 1/5 should be rebased to be last in the series.
> Ah... I had originally planned on submitting separate patches for each
> of the block jobs, to make reviewing easier.  But your idea on how
> libvirt can discover this is a good one, and would mandate changing
> those commands all in one series to be effective.  So this series will
> grow by a few patches.  :)
> If libvirt is going to use the autogenerated string format for
> decisions, we should also document the string format in the QAPI docs.

Adding a new command is much easier to probe for (a single
query-commands, which we are already using) than requiring a sequence of
operations (hotplug, then query to see if a name was assigned), and
probably even more direct (if the standalone command exists, then so
does the integrated use of setting backing names).

>> In short, this new command option allows the equivalent of 'qemu-img
>> rebase -u' on a live image.  Definitely a needed functionality.
> Would it be useful to have a stand-alone QMP command to change the
> backing-file, as well?  As this stands, it will only change the
> backing file if you are also merging data down the chain. 
> If you want/need the ability to do a true 'qemu-img rebase -u' on any
> given image without other chain modification, that needs a new
> command.

Yes, I think that's probably a wise idea to provide a dedicated command
for just changing the name recorded in a backing file.

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

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