linux-kernel - Re: [BUG] Regression introduced with "block: split bios to max possible length"

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Message-ID: <20160122145559.GA21984@localhost.localdomain>
Date:	Fri, 22 Jan 2016 14:56:00 +0000
From:	Keith Busch <keith.busch@...el.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jens Axboe <axboe@...com>,
	Stefan Haberland <sth@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-s390 <linux-s390@...r.kernel.org>,
	Sebastian Ott <sebott@...ux.vnet.ibm.com>
Subject: Re: [BUG] Regression introduced with "block: split bios to max
 possible length"

On Thu, Jan 21, 2016 at 08:15:37PM -0800, Linus Torvalds wrote:
> For the case of nvme, for example, I think the max sector number is so
> high that you'll never hit that anyway, and you'll only ever hit the
> chunk limit. No?

The device's max transfer and chunk size are not very large, both fixed
at 128KB. We can lose ~70% of potential throughput when IO isn't aligned,
and end users reported this when the block layer stopped splitting on
alignment for the NVMe drive.

So it's a big deal for this h/w, but now I feel awkward defending a
device specific feature for the generic block layer.

Anyway, the patch was developed with incorrect assumptions. I'd still
like to try again after reconciling the queue limit constraints, but
I defer to Jens for the near term.

Thanks!