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Re: [Qemu-block] [PATCH for-2.10] block: Skip implicit nodes in query-bl

From: Eric Blake
Subject: Re: [Qemu-block] [PATCH for-2.10] block: Skip implicit nodes in query-block/blockstats
Date: Wed, 19 Jul 2017 07:45:06 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1

On 07/19/2017 03:44 AM, Kevin Wolf wrote:
> Commits 0db832f and 6cdbceb introduced the automatic insertion of filter
> nodes above the top layer of mirror and commit block jobs. The
> assumption made there was that since libvirt doesn't do node-level
> management of the block layer yet, it shouldn't be affected by added
> nodes.

Those both landed in 2.9.  So this is a regression we're trying to fix.

> This is true as far as commands issued by libvirt are concerned. It only
> uses BlockBackend names to address nodes, so any operations it performs
> still operate on the root of the tree as intended.
> However, the assumption breaks down when you consider query commands,
> which return data for the wrong node now. These commands also return
> information on some child nodes (bs->file and/or bs->backing), which
> libvirt does make use of, and which refer to the wrong nodes, too.
> One of the consequences is that oVirt gets wrong information about the
> image size and stops the VM in response as long as a mirror or commit
> job is running:
> https://bugzilla.redhat.com/show_bug.cgi?id=1470634
> This patch fixes the problem by hiding the implict nodes created
> automatically by the mirror and commit block jobs in the output of
> query-block and BlockBackend-based query-blockstats as long as the user
> doesn't indicate that they are aware of those nodes by providing a node
> name for them in the QMP command to start the block job.

The approach makes sense to me.  I see from the rest of this thread that
you'll need a v2 to get the implementation of the approach correct, but
I think you're on the right track.

> The node-based commands query-named-block-nodes and query-blockstats
> with query-nodes=true still show all nodes, including implicit ones.
> This ensures that users that are capable of node-level management can
> still access the full information; users that only know BlockBackends
> won't use these commands.
> Cc: address@hidden
> Signed-off-by: Kevin Wolf <address@hidden>
> ---

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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