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Re: [Qemu-devel] [PATCH 11/34] block: Allow references for backing files

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH 11/34] block: Allow references for backing files
Date: Wed, 27 May 2015 14:31:18 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

Am 21.05.2015 um 07:47 hat Wen Congyang geschrieben:
> On 05/09/2015 01:21 AM, Kevin Wolf wrote:
> > For bs->file, using references to existing BDSes has been possible for a
> > while already. This patch enables the same for bs->backing_hd.
> 1. We reference to an existing BDSes, and some disk uses this blk. Do
> we allow this?

Currently yes. If it breaks, you get to keep both pieces.

As long as your guest device is read-only, it should just work. It would
be a very bad idea, though, to write to a backing file.

Op blockers should eventually prevent this from happening (Jeff, you may
want to take a note ;-))

> 2. bs->backing_hd->blk can be not NULL now? If we do an active commit
> to this backing file(use mirror job), we will call bdrv_swap() in
> mirror_exit(), and the function bdrv_swap() doesn't allow that
> new_bs->blk(here is bs->backing_hd) is not NULL.

You're right.

I can remove this patch from the series for now, but of course that
doesn't solve the problem. I'm not sure what to do about it. Making
bdrv_swap() work with BDSes that have BB attached is probably another
item in the list of "dynamic reconfiguration" problems.

Markus, any ideas?


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