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Date: Tue, 18 Jun 2013 11:11:21 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] block: add 'backing' option to drive_add
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To: Kevin Wolf <kwolf@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>, stefanha@redhat.com, qemu-devel@nongnu.org

Il 18/06/2013 10:56, Kevin Wolf ha scritto:
>> > 
>> > It is a huge swamp indeed.
>> > 
>> > Since we stop all block jobs on media change already, what about just
>> > adding a command "block-job-attach" or something like that which exposes
>> > the target of the job as a blockdev (presumably so that you can then add
>> > it to the NBD server)?
> This is backwards. You should really give the block job a target BDS to
> begin with, not a file name.

That's also ok...  We can get the backing file right for the target BDS
at the beginning of the backup job with a combination of
close_unused_images (so that it is "as if" block-backup had opened the
file with BDRV_O_NO_BACKING), bdrv_append and bdrv_swap (append the
target to the VM's BDS, swap the two).

So why do we need this new option to drive_add?  The "drive"
ImageCreationMode, for which Fam has a patch too, should do.

Paolo