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

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Date:	Mon, 20 Jul 2015 16:44:26 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org, dmilburn@...hat.com
Subject: Re: [patch] Revert "block: remove artifical max_hw_sectors cap"

Jens Axboe <axboe@...nel.dk> writes:

> 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.

I'll request this information and get back to you.  Sorry, I should have
done more digging first, but this seemed somewhat urgent to me.

>> 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).

Sure, the hardware sucks.  I still don't like foisting silent data
corruption on users.  Besides, given that this patch went in without any
performance numbers attached, I'd say the risk/reward ratio right now is
in favor of the revert.

Cheers,
Jeff
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