netdev - Re: Block device throttling [Re: Distributed storage.]

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Date:	Tue, 14 Aug 2007 04:13:10 -0700
From:	Daniel Phillips <phillips@...nq.net>
To:	Evgeniy Polyakov <johnpol@....mipt.ru>
Cc:	Jens Axboe <jens.axboe@...cle.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: Block device throttling [Re: Distributed storage.]

On Tuesday 14 August 2007 01:46, Evgeniy Polyakov wrote:
> On Mon, Aug 13, 2007 at 06:04:06AM -0700, Daniel Phillips 
(phillips@...nq.net) wrote:
> > Perhaps you never worried about the resources that the device
> > mapper mapping function allocates to handle each bio and so did not
> > consider this hole significant.  These resources can be
> > significant, as is the case with ddsnap.  It is essential to close
> > that window through with the virtual device's queue limit may be
> > violated.  Not doing so will allow deadlock.
>
> This is not a bug, this is special kind of calculation - total limit
> is number of physical devices multiplied by theirs limits. It was
> done _on purpose_ to allow different device to have different limits
> (for example in distributed storage project it is possible to have
> both remote and local node in the same device, but local device
> should not have _any_ limit at all, but network one should).
>
> Virtual device essentially has _no_ limit. And that as done on
> purpose.

And it will not solve the deadlock problem in general.  (Maybe it works 
for your virtual device, but I wonder...)  If the virtual device 
allocates memory during generic_make_request then the memory needs to 
be throttled.

Regards,

Daniel
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