linux-kernel - Re: absurdly high "optimal_io

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Date:	Thu, 6 Nov 2014 13:14:18 -0600
From:	Chris Friesen <chris.friesen@...driver.com>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>
CC:	Jens Axboe <axboe@...nel.dk>, lkml <linux-kernel@...r.kernel.org>,
	<linux-scsi@...r.kernel.org>, Mike Snitzer <snitzer@...hat.com>
Subject: Re: absurdly high "optimal_io_size" on Seagate SAS disk

On 11/06/2014 12:12 PM, Martin K. Petersen wrote:
>>>>>> "Chris" == Chris Friesen <chris.friesen@...driver.com>
>>>>>> writes:
>
> Chris> That'd work, but is it the best way to go?  I mean, I found
> one Chris> report of a similar problem on an SSD (model number
> unknown).  In Chris> that case it was a near-UINT_MAX value as well.
>
> My concern is still the same. Namely that this particular drive
> happens to be returning UINT_MAX but it might as well be a value
> that's entirely random. Or even a value that is small and innocuous
> looking but completely wrong.
>
> Chris> The problem with the blacklist is that until someone patches
> it, Chris> the drive is broken.  And then it stays blacklisted even
> if the Chris> firmware gets fixed.
>
> Well, you can manually blacklist in /proc/scsi/device_info.
>
> Chris> I'm wondering if it might not be better to just ignore all
> values Chris> larger than X (where X is whatever we think is the
> largest Chris> conceivable reasonable value).
>
> The problem is that finding that is not easy and it too will be a
> moving target.


Do we need to be perfect, or just "good enough"?

For a RAID card I expect it would be related to chunk size or stripe
width or something...but even then I would expect to be able to cap it
at 100MB or so.  Or are there storage systems on really fast interfaces
that could legitimately want a hundred meg of data at a time?

On 11/06/2014 12:15 PM, Jens Axboe wrote:
> Didn't check, but assuming the value is the upper 24 bits of 32. If
> so, might not hurt to check for as 0xfffffe00 as an invalid value.

Yep, in all three wonky cases so far "optimal_io_size" ended up as 
4294966784, which is 0xfffffe00.  Does something mask out the lower bits?

Chris
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