linux-kernel - Re: [PATCH v3 2/2] dm unstripe: Add documentation for unstripe target

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Date:   Wed, 13 Dec 2017 16:58:30 -0700
From:   Scott Bauer <scott.bauer@...el.com>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     dm-devel@...hat.com, snitzer@...hat.com, agk@...hat.com,
        linux-kernel@...r.kernel.org, keith.busch@...el.com,
        jonathan.derrick@...el.com
Subject: Re: [PATCH v3 2/2] dm unstripe: Add documentation for unstripe target

On Wed, Dec 13, 2017 at 04:18:06PM -0800, Randy Dunlap wrote:
> Use ASCII characters ___ or ---,  not whatever those bottom block characters are.

Argh, sorry, I removed those for an internal review but forgot to remove them here.

> 
> > +
> > +    The purpose of this unstriping is to provide better QoS in noisy
> > +    neighbor environments. When two partitions are created on the
> > +    aggregate drive without this unstriping, reads on one partition
> > +    can affect writes on another partition. This is because the partitions
> > +    are striped across the two cores. When we unstripe this hardware RAID 0
> > +    and make partitions on each new exposed device the two partitions are now
> > +    physically separated.
> > +
> > +    With the module we were able to segregate a fio script that has read and
> > +    write jobs that are independent of each other. Compared to when we run
> > +    the test on a combined drive with partitions, we were able to get a 92%
> > +    reduction in five-9ths read latency using this device mapper target.
> 
> 	            5/9ths
> although I can't quite parse that sentence.

I'll reword it, thanks.