linux-kernel - Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

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Date:   Fri, 9 Sep 2016 17:37:21 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Josef Bacik <jbacik@...com>, Wouter Verhelst <w@...r.be>
Cc:     linux-block@...r.kernel.org, mpa@...gutronix.de,
        kernel-team@...com, nbd-general@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

On 09/09/2016 05:00 PM, Josef Bacik wrote:
>> Right. Alternatively, you could perhaps make it so that the lost
>> connection is removed, unack'd requests on that connection are resent,
>> and the session moves on with one less connection (unless the lost
>> connection is the last one, in which case we die as before). That might
>> be too much work and not worth it though.
>
> Yeah I wasn't sure if we could just randomly remove hw queue's in blk mq
> while the device is still up.  If that is in fact easy to do then I'm in
> favor of trucking along with less connections than we originally had,
> otherwise I think it'll be too big of a pain.  Also some users (Facebook
> in this case) would rather the whole thing fail so we can figure out
> what went wrong rather than suddenly going at a degraded performance.

We can do that online. We do that for CPU hotplug/unplug events, and we
also expose that functionality to drivers through
blk_mq_update_nr_hw_queues(). So yes, should be trivial to support from
nbd.

-- 
Jens Axboe