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Re: [Qemu-devel] [PATCH v6 for 2.1 01/10] block: Auto-generate node_name
From: |
Jeff Cody |
Subject: |
Re: [Qemu-devel] [PATCH v6 for 2.1 01/10] block: Auto-generate node_names for each BDS entry |
Date: |
Thu, 19 Jun 2014 08:30:19 -0400 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
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 <address@hidden>
> > Signed-off-by: Jeff Cody <address@hidden>
> > ---
> > 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.
[Qemu-devel] [PATCH v6 for 2.1 02/10] block: add helper function to determine if a BDS is in a chain, Jeff Cody, 2014/06/17
[Qemu-devel] [PATCH v6 for 2.1 03/10] block: simplify bdrv_find_base() and bdrv_find_overlay(), Jeff Cody, 2014/06/17
[Qemu-devel] [PATCH v6 for 2.1 04/10] block: make 'top' argument to block-commit optional, Jeff Cody, 2014/06/17