linux-kernel - Re: [PATCH v5 01/11] block: make generic_make

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Message-ID: <20150811174717.GA30439@redhat.com>
Date:	Tue, 11 Aug 2015 13:47:17 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>
Cc:	Ming Lin <mlin@...nel.org>, axboe@...com,
	Christoph Hellwig <hch@...radead.org>,
	device-mapper development <dm-devel@...hat.com>,
	Ming Lei <ming.lei@...onical.com>,
	Christoph Hellwig <hch@....de>,
	Alasdair Kergon <agk@...hat.com>,
	Lars Ellenberg <drbd-dev@...ts.linbit.com>,
	Philip Kelleher <pjk1939@...ux.vnet.ibm.com>,
	Kent Overstreet <kent.overstreet@...il.com>,
	Nitin Gupta <ngupta@...are.org>,
	Ming Lin <ming.l@....samsung.com>,
	Oleg Drokin <oleg.drokin@...el.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Jens Axboe <axboe@...nel.dk>,
	Andreas Dilger <andreas.dilger@...el.com>,
	Geoff Levand <geoff@...radead.org>,
	Jiri Kosina <jkosina@...e.cz>,
	lkml <linux-kernel@...r.kernel.org>, Jim Paris <jim@...n.com>,
	Minchan Kim <minchan@...nel.org>,
	Dongsu Park <dpark@...teo.net>, drbd-user@...ts.linbit.com,
	Joe Thornber <ejt@...hat.com>
Subject: Re: [PATCH v5 01/11] block: make generic_make_request handle
 arbitrarily sized bios

On Tue, Aug 11 2015 at  1:36pm -0400,
Martin K. Petersen <martin.petersen@...cle.com> wrote:

> >>>>> "Mike" == Mike Snitzer <snitzer@...hat.com> writes:
> 
> Mike> DM-thinp processes discards internally before it passes them down
> Mike> (if configured to do so).  If a discard is smaller than the
> Mike> granularity of a thinp block (whose size is configurable) or if
> Mike> the start and end of the discard's extent is misaligned (relative
> Mike> to the thinp blocks mapped to the logical extent) then the discard
> Mike> won't actually discard partial thinp blocks.
> 
> That's fine. You can throw away anything you don't like as long as
> discard_zeroes_data=0.

Correct, dm-thinp sets discard_zeroes_data=0

> But I don't understand why having an artificial cap at 2GB fixes
> things. Other than making it less likely for you to receive a runt by
> virtue of being aligned to a power of two.

That is the benefit.  And when coupled with the new default max_discard
of 64K (pending change from Jens for 4.3) this 2GB upper limit really
isn't such a big deal.  Unless I'm missing something...
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