linux-kernel - Re: [PATCH 3/3] blkdev: honor discard_granularity in blkdev_issue

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Date:	Mon, 02 May 2011 12:10:41 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>
Cc:	Lukas Czerner <lczerner@...hat.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Dmitry Monakhov <dmonakhov@...nvz.org>,
	Jens Axboe <jaxboe@...ionio.com>
Subject: Re: [PATCH 3/3] blkdev: honor discard_granularity in blkdev_issue_discard()

"Martin K. Petersen" <martin.petersen@...cle.com> writes:

>>>>>> "Lukas" == Lukas Czerner <lczerner@...hat.com> writes:
>
> Lukas> As Jeff Moyer pointed out we do not honor discard granularity
> Lukas> while submitting REQ_DISCARD bios of size smaller than
> Lukas> max_discard_sectors.  That fact might have unwanted consequences
> Lukas> of device ignoring the request, or even worse if device firmware
> Lukas> is buggy.
>
> We've discussed this before and the consensus was not to do it. The
> granularity is a hint, not a hard limit like max_discard_sectors.
>
> We want the reporting to be comprehensive throughout the block layer. If
> we start aligning to the granularity at the top we lose information for
> stacked devices below with a finer granularity.
>
> So if we were to align to the granularity we'd want to do it at the
> bottom of the stack when we issue the command to the device. We've had a
> few proposed patches to did that but so far we've only found one device
> where it made a difference. And that case didn't justify adding a quirk.

Hm, I wonder where it makes sense to document this.  Maybe
Documentation/block/queue-sysfs.txt, but I admit I wouldn't have looked
there while reviewing this code.

Cheers,
Jeff
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