linux-kernel - Re: [patch] Revert "block: remove artifical max_hw

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Date:	Mon, 20 Jul 2015 14:24:34 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Jeff Moyer <jmoyer@...hat.com>,
	Christoph Hellwig <hch@...radead.org>
CC:	linux-kernel@...r.kernel.org, dmilburn@...hat.com
Subject: Re: [patch] Revert "block: remove artifical max_hw_sectors cap"

On 07/20/2015 01:17 PM, Jeff Moyer wrote:
>
> <resent with Jens' email address fixed>
>
> Hi,
>
> This reverts commit 34b48db66e08, which caused significant iozone
> performance regressions and uncovered a silent data corruption
> bug in at least one disk.
>
> For SAN storage, we've seen initial write and re-write performance drop
> 25-50% across all I/O sizes.  On locally attached storage, we've seen
> regressions of 40% for all I/O types, but only for I/O sizes larger than
> 1MB.

Do we have any understanding of where this regression is coming from? 
Even just basic info like iostats from a run would be useful.

> In addition to the performance issues, we've also seen data corruption
> on one disk/hba combination.  See
>    http://marc.info/?l=linux-ide&m=143680539400526&w=2

That's just sucky hardware... That said, it is indeed one of the risks. 
We had basically the same transition from 255 as max sectors, since we 
depended on ATA treating 0 == 256 sectors (as per spec).

-- 
Jens Axboe

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