linux-kernel - Re: [ANNOUNCE] Ramback: faster than a speeding bullet

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Message-ID: <170fa0d20803120741n1f9f89ban27b18b4b671d48dd@mail.gmail.com>
Date:	Wed, 12 Mar 2008 10:41:30 -0400
From:	"Mike Snitzer" <snitzer@...il.com>
To:	"Daniel Phillips" <phillips@...nq.net>
Cc:	"Willy Tarreau" <w@....eu>, "Chris Friesen" <cfriesen@...tel.com>,
	"Lars Marowsky-Bree" <lmb@...e.de>,
	"Alan Cox" <alan@...rguk.ukuu.org.uk>,
	"Grzegorz Kulewski" <kangur@...com.net>,
	linux-kernel@...r.kernel.org, nbd-general@...ts.sourceforge.net
Subject: Re: [ANNOUNCE] Ramback: faster than a speeding bullet

On 3/12/08, Daniel Phillips <phillips@...nq.net> wrote:
> On Tuesday 11 March 2008 13:53, Willy Tarreau wrote:
>  > BTW, I would say that IMHO nothing here makes RAID impossible to use :-)
>  > Just wire 2 of these beasts to a central server with 10 Gbps NICs and
>  > you have a nice server :-)
>
>
> Right.  See ddraid.  It is in the pipeline, but everything takes time.
>  We also need to reroll NBD complete with deadlock fixes before I feel
>  good about that.

You've eluded to NBD needing deadlock fixes quite a few times in the
past.  I've even had some discussions with you on where you see NBD
lacking (userspace nbd-server doesn't lock memory or set PF_MEMALLOC,
etc).  But I've lost track of what changes you have in mind for NBD.
Are you talking about a complete re-write or do you have specific
patches that will salvage the existing NBD client and/or server?  Has
this work already been done and you just need to dust it off?

As an aside, using a kernel with the new per bdi dirty page accounting
I've not been able to hit any deadlock scenarios with NBD.  Am I not
trying hard enough?  Or are they now mythical?  If real, do you have a
reproducible scenario that will cause NBD to deadlock?

I'm not interested in swap over NBD (e.g. network memory reserves?)
because in practice I've found that the VM doesn't allow non-swap NBD
use-cases to actually need that "netvm" sophistication... any other
workload that deadlocks NBD would interesting.

thanks,
Mike
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