Re: [Qemu-devel] [PATCH v6 for 2.1 01/10] block: Auto-generate node_names for each BDS entry

[Jeff Cody <jcody@redhat.com>]

On Thu, Jun 19, 2014 at 04:55:02PM +0800, Stefan Hajnoczi wrote:
> On Tue, Jun 17, 2014 at 05:53:49PM -0400, Jeff Cody wrote:
> > Currently, node_name is only filled in when done so explicitly by the
> > user.  If no node_name is specified, then the node name field is not
> > populated.
> > 
> > If node_names are automatically generated when not specified, that means
> > that all block job operations can be done by reference to the unique
> > node_name field.  This eliminates ambiguity in resolving filenames
> > (relative filenames, or file descriptors, symlinks, mounts, etc..) that
> > qemu currently needs to deal with.
> > 
> > If a node name is specified, then it will not be automatically
> > generated for that BDS entry.
> > 
> > If it is automatically generated, it will be prefaced with "__qemu##",
> > followed by 8 characters of a unique number, followed by 8 random
> > ASCII characters in the range of 'A-Z'.  Some sample generated node-name
> > strings:
> >     __qemu##00000000IAIYNXXR
> >     __qemu##00000002METXTRBQ
> >     __qemu##00000001FMBORDWG
> > 
> > The prefix is to aid in identifying it as a qemu-generated name, the
> > numeric portion is to guarantee uniqueness in a given qemu session, and
> > the random characters are to further avoid any accidental collisions
> > with user-specified node-names.
> > 
> > Reviewed-by: Eric Blake <eblake@redhat.com>
> > Signed-off-by: Jeff Cody <jcody@redhat.com>
> > ---
> >  block.c | 16 +++++++++++++++-
> >  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> Who is this feature for?
> 
> Human users: they'll need to  read through query-named-block-nodes
> output to find the nodes they care about.  This is pretty cumbersome and
> not human-friendly.
> 

Currently, that is how a human user would find the node-names.  That
doesn't mean there might not be a new interface later on, that is more
human friendly.

And while a human parsing query-named-block-nodes isn't fun, I think
it is easier than a human assigning node-names to a graph, so it is
more human-friendly than the current system.

> Management tools: parsing query-named-block-nodes isn't trivial since
> the output can vary between QEMU versions (e.g. when we move I/O
> throttling to a block driver node there will be new internal nodes).
> Tools doing this should really use blockdev-add instead and assign their
> own node names.

Libvirt (and OpenStack) is already testing with these patches, and my
impression from Eric is that parsing the output of
query-named-block-nodes was less work than assigning node-names in
libvirt.

Can you expand a bit on moving i/o throttle to a block, and creating
new internal nodes?

The generated node-names have the same life cycle as a specified
node-name; anything that would invalidate a generated node-name would
invalidate a specified node-name as well.

And if a QMP command is issued that would cause new nodes to be
assigned, I believe libvirt is aware that they need to perform a
query-named-block-nodes again.

> 
> It seems like neither type of user will get much mileage out of this
> feature.  Is it really necessary or did I miss a use case?
>

Strictly speaking, it isn't required.  But it makes sense for QEMU to
assign node-names to any unassigned node-names, because it does make
life easier for both humans and management software, and QEMU is the
only one that can always ensure that every BDS has a node-name.

It is also nice for QEMU; we can now in future versions assume that
every BDS will always have a node-name, regardless if it has been
assigned by the user or not.

And the usage of the node-names is strictly optional by the human or
management software user; neither is required to use the generated
node-names, and are feel to specify their own node-name.  A user
specified node-name will prevent an auto-generated one from being
assigned for that specific BDS.