Re: [PATCH] nbd/server: Advertise MULTI_CONN for shared writable exports

[Eric Blake]

On Thu, Oct 28, 2021 at 04:37:36PM +0200, Kevin Wolf wrote:
> Am 27.08.2021 um 17:09 hat Eric Blake geschrieben:
> > According to the NBD spec, a server advertising
> > NBD_FLAG_CAN_MULTI_CONN promises that multiple client connections will
> > not see any cache inconsistencies: when properly separated by a single
> > flush, actions performed by one client will be visible to another
> > client, regardless of which client did the flush.  We satisfy these
> > conditions in qemu because our block layer serializes any overlapping
> > operations (see bdrv_find_conflicting_request and friends): no matter
> > which client performs a flush, parallel requests coming from distinct
> > NBD clients will still be well-ordered by the time they are passed on
> > to the underlying device, with no caching in qemu proper to allow
> > stale results to leak after a flush.
> > 
> > We don't want to advertise MULTI_CONN when we know that a second
> > client can connect (which is the default for qemu-nbd, but not for QMP
> > nbd-server-add),
> 
> Do you mean when a second client _can't_ connect?

Oops, yes. The default for qemu-nbd is a single client (you have to
request -e for more than one), so a second client can't connect; for
nbd-server-add it is unlimited clients [1].

> 
> > so it does require a QAPI addition.  But other than
> > that, the actual change to advertise the bit for writable servers is
> > fairly small.  The harder part of this patch is setting up an iotest
> > to demonstrate behavior of multiple NBD clients to a single server.
> > It might be possible with parallel qemu-io processes, but concisely
> > managing that in shell is painful.
> 
> I think it should be fairly straightforward in a Python test case.

Probably, but my python is rather weak for writing such a case
off-hand from scratch. Is there an existing test that you are aware of
that might be easy to copy-and-paste from?

> 
> Another option is using a single QEMU or QSD instance that has multiple
> -blockdev for the same NBD server. For the server these are multiple
> clients, even if all connnection come from a single process.
> 
> > I found it easier to do by relying
> > on the libnbd project's nbdsh, which means this test will be skipped
> > on platforms where that is not available.
> > 
> > Signed-off-by: Eric Blake <eblake@redhat.com>
> 
> > diff --git a/docs/interop/nbd.txt b/docs/interop/nbd.txt
> > index 10ce098a29bf..d03910f1e9eb 100644
> > --- a/docs/interop/nbd.txt
> > +++ b/docs/interop/nbd.txt
> > @@ -68,3 +68,4 @@ NBD_CMD_BLOCK_STATUS for "qemu:dirty-bitmap:", 
> > NBD_CMD_CACHE
> >  * 4.2: NBD_FLAG_CAN_MULTI_CONN for shareable read-only exports,
> >  NBD_CMD_FLAG_FAST_ZERO
> >  * 5.2: NBD_CMD_BLOCK_STATUS for "qemu:allocation-depth"
> > +* 6.2: NBD_FLAG_CAN_MULTI_CONN for shareable writable exports
> > diff --git a/docs/tools/qemu-nbd.rst b/docs/tools/qemu-nbd.rst
> > index 5643da26e982..81be32164a55 100644
> > --- a/docs/tools/qemu-nbd.rst
> > +++ b/docs/tools/qemu-nbd.rst
> > @@ -138,8 +138,7 @@ driver options if ``--image-opts`` is specified.
> >  .. option:: -e, --shared=NUM
> > 
> >    Allow up to *NUM* clients to share the device (default
> > -  ``1``), 0 for unlimited. Safe for readers, but for now,
> > -  consistency is not guaranteed between multiple writers.
> > +  ``1``), 0 for unlimited.
> > 
> >  .. option:: -t, --persistent
> 
> If qemu-nbd supports a maximum number of connections rather than just a
> bool...
> 
> > diff --git a/qapi/block-export.json b/qapi/block-export.json
> > index 0ed63442a819..b2085a9fdd4c 100644
> > --- a/qapi/block-export.json
> > +++ b/qapi/block-export.json
> > @@ -95,11 +95,15 @@
> >  #                    the metadata context name "qemu:allocation-depth" to
> >  #                    inspect allocation details. (since 5.2)
> >  #
> > +# @shared: True if the server should advertise that multiple clients may
> > +#          connect, default false. (since 6.2)
> > +#
> >  # Since: 5.2
> >  ##
> >  { 'struct': 'BlockExportOptionsNbd',
> >    'base': 'BlockExportOptionsNbdBase',
> > -  'data': { '*bitmaps': ['str'], '*allocation-depth': 'bool' } }
> > +  'data': { '*bitmaps': ['str'], '*allocation-depth': 'bool',
> > +             '*shared': 'bool' } }
> 
> ...wouldn't it be better to mirror this in the QAPI interface?

Yeah, now that you mention it.  (In fact, before I got to this part of
the email, at point [1] above I was trying to look in
block-export.json to see whether there is any way to use QMP to expose
less than unlimited clients, and couldn't find it - because it isn't
there)

> 
> I think eventually we want to add everything missing to the built-in NBD
> server and then change qemu-nbd to use it instead of managing the
> connections itself. So I'm not sure if diverging here is a good idea.

That argument alone makes it sound like it is worth respinning this
series to at least pick up on exposing max-clients through QMP, so
that qemu-nbd and QMP have the same knobs (even if those knobs have
different defaults).

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