Date: Tue, 11 Aug 2015 13:47:17 -0400
From: Mike Snitzer <snitzer@redhat.com>
To: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: Ming Lin <mlin@kernel.org>, axboe@fb.com,
        Christoph Hellwig <hch@infradead.org>,
        device-mapper development <dm-devel@redhat.com>,
        Ming Lei <ming.lei@canonical.com>, Christoph Hellwig <hch@lst.de>,
        Alasdair Kergon <agk@redhat.com>,
        Lars Ellenberg <drbd-dev@lists.linbit.com>,
        Philip Kelleher <pjk1939@linux.vnet.ibm.com>,
        Kent Overstreet <kent.overstreet@gmail.com>,
        Nitin Gupta <ngupta@vflare.org>, Ming Lin <ming.l@ssi.samsung.com>,
        Oleg Drokin <oleg.drokin@intel.com>, Al Viro <viro@zeniv.linux.org.uk>,
        Jens Axboe <axboe@kernel.dk>,
        Andreas Dilger <andreas.dilger@intel.com>,
        Geoff Levand <geoff@infradead.org>, Jiri Kosina <jkosina@suse.cz>,
        lkml <linux-kernel@vger.kernel.org>, Jim Paris <jim@jtan.com>,
        Minchan Kim <minchan@kernel.org>, Dongsu Park <dpark@posteo.net>,
        drbd-user@lists.linbit.com, Joe Thornber <ejt@redhat.com>
Subject: Re: [PATCH v5 01/11] block: make generic_make_request handle
 arbitrarily sized bios
Message-ID: <20150811174717.GA30439@redhat.com>
References: <20150809064128.GB2436@infradead.org>
 <1439103347.7880.3.camel@hasee>
 <20150809070128.GA25739@infradead.org>
 <1439104707.7880.8.camel@hasee>
 <20150810150216.GA22719@redhat.com>
 <yq1si7rgllc.fsf@sermon.lab.mkp.net>
 <1439230696.26458.6.camel@ssi>
 <yq1bneeh9df.fsf@sermon.lab.mkp.net>
 <20150811024154.GA26351@redhat.com>
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On Tue, Aug 11 2015 at  1:36pm -0400,
Martin K. Petersen <martin.petersen@oracle.com> wrote:

> >>>>> "Mike" == Mike Snitzer <snitzer@redhat.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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