Re: nbd, nbdkit, loopback mounts and memory management

["Richard W.M. Jones" <rjones@redhat.com>]

On Fri, Feb 15, 2019 at 10:41:26PM +0000, Richard W.M. Jones wrote:
> On Fri, Feb 15, 2019 at 08:19:54PM +0100, Pavel Machek wrote:
> > Hi!
> > 
> > I watched fosdem talk about
> > nbdkit... https://www.youtube.com/watch?v=9E5A608xJG0 . Nice. But word
> > of warning: I'm not sure using it read-write on localhost is safe.
> > 
> > In particular, user application could create a lot of dirty data
> > quickly. If there's not enough memory for nbdkit (or nbd-client or
> > nbd-server), you might get a deadlock.
> 
> Thanks for the kind words about the talk.  I've added Wouter Verhelst
> & the NBD mailing list to CC.  Although I did the talk because the
> subject is interesting, how I actually use nbdkit / NBD is to talk to
> qemu and that's where I have most experience and where we (Red Hat)
> use it in production systems.
> 
> However in January I spent a lot of time exercising the NBD loop-mount
> + nbdkit case using fio in order to find contention / bottlenecks in
> our use of threads and locks.  I didn't notice any particular problems
> then, but it's possible my testing wasn't thorough enough.  Or that
> fio only creates small numbers of dirty pages (because of locality in
> its access patterns I guess?)
> 
> When you say it's not safe, what could happen?  What would we observe
> if it was going wrong?

Reading more carefully I see you said we'd observe a deadlock.  I
didn't see that, but again my testing of this wouldn't have been very
thorough.  When I have some time I'll try creating / spooling huge
files into an NBD loop mount to see if I can cause a deadlock.

Thanks, Rich.

> > Also note that nbd.txt in Documentation/blockdev/ points to
> > sourceforge; it should probably point to
> > https://github.com/NetworkBlockDevice/nbd ?
> 
> Wouter should be able to say what the correct link should be.
> 
> Rich.
> 
> -- 
> Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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