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

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Date:	Mon, 12 Oct 2009 16:48:18 +0200
From:	Tomasz Chmielewski <mangoo@...g.org>
To:	linux-kernel@...r.kernel.org
CC:	pernegger@...il.com, arjan@...radead.org
Subject: Re: *Really* bad I/O latency with md raid5+dm-crypt+lvm

> 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.

I see similar behaviour on quite powerful hardware, i.e. Core i7, 8 GB 
RAM, 2x HDD in a software RAID-1 array (no dm-crypt), when tarring 
something big, or writing dd if=/dev/zero of=/home/me/bigfile - doing ls 
in another terminal or just starting top can take up to a minute.

Quite interestingly, background RAID synchronization have almost no 
effect on latency.

(...)

> According to openssl speed aes-256-cbc the CPUs encryption speed is
> ~113 MiB/s (single core, est. for 512b blocks). Obviously the array is
> much faster than that. I can't find the benchmarks ATM but the numbers
> seemed plausible for 70 MiB/s (optimistic est. for sequential access)
> disks at the time.

You can find some dm-crypt benchmarks i.e. here:

http://blog.wpkg.org/2009/04/23/cipher-benchmark-for-dm-crypt-luks/

Obviously, they will not match your hardware.

Also note that dm-crypt is not "SMP-ready", so whatever hardware you 
have, it will only use once CPU - this may seriously limit the 
performance, depending on your usage and hardware.

-- 
Tomasz Chmielewski

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