linux-kernel - Re: nbd, nbdkit, loopback mounts and memory management

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Message-ID: <20190215224126.GX12500@redhat.com>
Date:   Fri, 15 Feb 2019 22:41:26 +0000
From:   "Richard W.M. Jones" <rjones@...hat.com>
To:     Pavel Machek <pavel@....cz>
Cc:     kernel list <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...l.org>, Wouter Verhelst <w@...r.be>,
        nbd@...er.debian.org
Subject: Re: nbd, nbdkit, loopback mounts and memory management

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?

> 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.

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