linux-kernel - Re: [PATCH v2 2/2] dm unstripe: Add documentation for unstripe target

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Date:   Tue, 12 Dec 2017 07:45:56 -0700
From:   Keith Busch <keith.busch@...el.com>
To:     Nikolay Borisov <n.borisov.lkml@...il.com>
Cc:     Scott Bauer <scott.bauer@...el.com>, dm-devel@...hat.com,
        snitzer@...hat.com, agk@...hat.com, linux-kernel@...r.kernel.org,
        jonathan.derrick@...el.com
Subject: Re: [PATCH v2 2/2] dm unstripe: Add documentation for unstripe target

On Tue, Dec 12, 2017 at 01:35:13PM +0200, Nikolay Borisov wrote:
> On 11.12.2017 18:00, Scott Bauer wrote:
> > +    As an example:
> > +
> > +    Intel NVMe drives contain two cores on the physical device.
> > +    Each core of the drive has segregated access to its LBA range.
> > +    The current LBA model has a RAID 0 128k stripe across the two cores:
> > +
> > +       Core 0:                Core 1:
> > +      __________            __________
> > +      | LBA 511|            | LBA 768|
> > +      | LBA 0  |            | LBA 256|
> > +      ⎻⎻⎻⎻⎻⎻⎻⎻⎻⎻            ⎻⎻⎻⎻⎻⎻⎻⎻⎻⎻
> 
> If it's 128k stripe shouldn't it be LBAs 0/256 on core0 and LBAs 128/511
> on core1?

Ah, this device's makers call the "stripe" size what should be called
"chunk". This device has a 128k chunk per core with two cores, so the
full stripe is 256k. The above should have core 0 owning LBA 512 rather
than 511 (assuming 512b LBA format).