Re: [Nbd] [PATCH 1/1] NBD: allow nbd to be used locally
- To: Wouter Verhelst <w@...112...>
- Cc: nbd-general@lists.sourceforge.net, Laurent Vivier <Laurent.Vivier@...154...>, Andrew Morton <akpm@...133...>, linux-kernel@...25...
- Subject: Re: [Nbd] [PATCH 1/1] NBD: allow nbd to be used locally
- From: Pavel Machek <pavel@...28...>
- Date: Mon, 24 Mar 2008 22:58:47 +0100
- Message-id: <20080324215846.GA4494@...155...>
- In-reply-to: <20080324203744.GA13603@...147...>
- References: <47D468F0.8000506@...124...> <20080310102917.GA27751@...155...> <47D56F4A.5070208@...124...> <20080311100904.GA807@...155...> <20080324203744.GA13603@...147...>
Hi!
> > >>> This patch allows a Network Block Device to be mounted locally
> > >>> (nbd-client to nbd-server over 127.0.0.1).
> > >>>
> > >>> It creates a kthread to avoid the deadlock described in NBD tools
> > >>> documentation. So, if nbd-client hangs waiting for pages, the kblockd
> > >>> thread can continue its work and free pages.
> > >>
> > >> What happens if your new kthread blocks on memory allocation?
> > >
> > > Well, we expect that. The reason for the new thread is so that it hangs,
> > > rather than kblockd hanging (which on a UP system brings all I/O to a
> > > halt). As long as kblockd can continue making progress, we eventually free
> > > up memory and then NBD can finish its requests, too.
> >
> > ....unless all the memory is in dirty buffers for nbd, and nbd server
> > is swapped out or something?
>
> Note that I'm not a kernel hacker, so might be terribly mistaken here...
>
> but I feel I should point out that this patch solves the issue that no
> two block devices can flush their dirty buffers at the same time.
> Without this patch, you can't write to a _filesystem_ on an NBD device
> if that's connected to a server on the localhost. You are correct that
> this does not solve the deadlock in swapping to NBD devices, but that's
> not the only existing deadlock issue in NBD to localhost...
Same issue with swapping probably exists with dirty block
writeout... swapoff -a, and filesystem becomes very similar to swap.
Pavel
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