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

[Stefan Hajnoczi <stefanha@gmail.com>]

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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.

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.

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?

Stefan

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