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Subject: Re: agcount for 2TB, 4TB and 8TB drives
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From: Eric Sandeen <sandeen@sandeen.net>
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Date: Fri, 6 Oct 2017 17:21:47 -0500
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To: Dave Chinner <david@fromorbit.com>
Cc: "Darrick J. Wong" <darrick.wong@oracle.com>, Gandalf Corvotempesta <gandalf.corvotempesta@gmail.com>, linux-xfs@vger.kernel.org



On 10/6/17 5:20 PM, Dave Chinner wrote:
> On Fri, Oct 06, 2017 at 11:18:39AM -0500, Eric Sandeen wrote:
>> On 10/6/17 10:38 AM, Darrick J. Wong wrote:
>>> On Fri, Oct 06, 2017 at 10:46:20AM +0200, Gandalf Corvotempesta wrote:
>>> Semirelated question: for a solid state disk on a machine with high CPU
>>> counts do we prefer agcount == cpucount to take advantage of the
>>> high(er) iops and lack of seek time to increase parallelism?
>>>
>>> (Not that I've studied that in depth.)
>>
>> Interesting question.  :)  Maybe harder to answer for SSD black boxes?
> 
> Easy: switch to multidisk mode if /sys/block/<dev>/queue/rotational
> is zero after doing all the other checks. Then SSDs will get larger
> AG counts automatically.

The "hard part" was knowing just how much parallelism is actually inside
the black box.  But "multidisk mode" doesn't go too overboard, so yeah
that's probably fine.

-Eric
 
> Cheers,
> 
> Dave.
>