linux-kernel - Re: *Really* bad I/O latency with md raid5+dm-crypt+lvm

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Date:	Mon, 12 Oct 2009 19:37:56 +0200
From:	Mike Galbraith <efault@....de>
To:	Tomasz Chmielewski <mangoo@...g.org>
Cc:	linux-kernel@...r.kernel.org, pernegger@...il.com,
	arjan@...radead.org
Subject: Re: *Really* bad I/O latency with md raid5+dm-crypt+lvm

On Mon, 2009-10-12 at 16:48 +0200, Tomasz Chmielewski wrote:
> > Summary: I was hoping to use a layered storage setup, namely lvm on
> > dm-crypt on md raid5 for a new box I'm setting up, but that isn't
> > looking so good since a single heavyish writer will monopolise any and
> > all I/O on the "device". F. ex. while cp'ing a few GB of data from an
> > external disk to the array it takes ~10sec to run ls and ~2min to
> > start aptitude. Clueless attempts at a diagnosis below.
> 
> Did you try running strace to see where ls pauses?
> 
> Did you try running latencytop (and generally, top/htop while doing your 
> tests)?
> 
> 
> (...)
> 
> > Anyway, as soon as I copy something to the array or create a larger
> > (upwards of a few hundred MiB) tar archive the box becomes utterly
> > unresponsive until that job is finished. Even on the local console the
> > completion time for a simple ls or cat is of the order of tens of
> > seconds, just forget about launching emacs.
> > Now I know that people have been ranting about desktop responsiveness
> > for a while but that was very much an abstract thing for me until now.
> 
> I think the above (big latency when doing some bigger IO) is a general 
> Linux problem.

It would be interesting to test latest -rc.  Though it may prove to be
unrelated. the symptoms sound very much like a recent thread wrt writers
starving readers.

	-Mike

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