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Date: Fri, 24 May 2024 09:51:50 +0200
From: Christoph Hellwig <hch@lst.de>
To: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
	Mike Snitzer <snitzer@kernel.org>, linux-block@vger.kernel.org,
	linux-scsi@vger.kernel.org
Subject: Re: [PATCH 1/2] sd: also set max_user_sectors when setting
 max_sectors
Message-ID: <20240524075150.GA18024@lst.de>
References: <20240523182618.602003-1-hch@lst.de> <20240523182618.602003-2-hch@lst.de> <yq1o78wjrsw.fsf@ca-mkp.ca.oracle.com>
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On Thu, May 23, 2024 at 02:53:40PM -0400, Martin K. Petersen wrote:
> 
> Christoph,
> 
> > sd can set a max_sectors value that is lower than the max_hw_sectors
> > limit based on the block limits VPD page.   While this is rather
> > unusual,
> 
> It's not particularly unusual. Virtually all arrays have a much smaller
> stripe or cache line size than what the average HBA can handle in one
> transfer. Using the device's preferred I/O size to configure max_sectors
> made a substantial difference performance-wise.

Well, in terms of Linux it is weird in that drivers weren't ever supposed
to set max_sectors directly but only provide max_hw_sectors (although
nbd also decreases it and rbd increases it in odd ways).  Especially
as we already have an opt_in limit for the optimal size.

I'll find a way to sort it out and build a grand unified and somewhat
coherent theory out of it..