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Re: [Qemu-block] [PATCH v8 03/26] block: Add BDS.backing_overridden

From: Kevin Wolf
Subject: Re: [Qemu-block] [PATCH v8 03/26] block: Add BDS.backing_overridden
Date: Thu, 22 Feb 2018 16:12:10 +0100
User-agent: Mutt/1.9.1 (2017-09-22)

Am 22.02.2018 um 15:55 hat Max Reitz geschrieben:
> On 2018-02-22 14:39, Kevin Wolf wrote:
> > Am 05.02.2018 um 16:18 hat Max Reitz geschrieben:
> >> If the backing file is overridden, this most probably does change the
> >> guest-visible data of a BDS. Therefore, we will need to consider this in
> >> bdrv_refresh_filename().
> >>
> >> Adding a new field to the BDS is not nice, but it is very simple and
> >> exactly keeps track of whether the backing file has been overridden.
> > 
> > ...as long as we manage to actually keep it up to date all the time.
> > 
> > First of all, what I'm missing here (or in fact in the comment in the
> > code) is a definition what "overridden" really means. "specified by the
> > user" is kind of vague: You consider the backing file relationship for
> > snapshot=on as user specified, even though the user wasn't explicit
> > about this. On the other hand, creating a live snapshot results in a
> > node that isn't user specified.
> > 
> > Isn't the real question to ask whether the default backing file (taken
> > from the image header) would result in the same tree? The answer to this
> > changes after more operations, like qmp_change_backing_file().
> With you so far.
> > Considering that there are so many ways to change the answer, I think
> > the simplest reliable option isn't a new BDS field that needs to updated
> > everywhere, but looking at the current value of bs->options and
> > bs->backing_file and see if they match.
> I don't see how that is simple.  First, bs->options does not necessarily
> reflect the "current options", those would be bs->full_open_options.
> And for generating that, we need a way to determine whether the backing
> file has been overridden or not, so whether we need to put the backing
> options into it or whether we do not.

For the purpose of this comparison, we need a set of options that
contains the backing file options unconditionally.

> (I am right that bs->backing_file is what the image header says, right?
> So we need to compare it against something that reflects the runtime state.)

I think so, yes.

> What I could see would be comparing bs->backing_file to
> bs->backing->bs->filename.  But this sounds very hacky to me.
> One thing the comes to mind is that it can break whenever
> bdrv_refresh_filename() is clever.  So you specify
> 'json:{"driver":"null-co"}' in the image header, and
> bdrv_refresh_filename() optimizes that to "null-co://".  Now the
> filenames differ even though it's still the original filename.  So this
> wouldn't work very well either.

On the other hand, the problem with your current approach is that it
results in a JSON filename even if you override the backing file and
specify the same file name as we already have in the image header.

In the future, libvirt is going to manually build the graph, so we will
always have the backing file overridden according to the logic in this
patch. I don't think we want to get JSON filenames for all libvirt
managed VMs, so can we realistically do without any kind of comparison?


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