linux-ext4 - Re: [dm-devel] do not disable ext4 discards on first discard failure? [was: Re: dm snapshot: ignore discards issued to the snapshot-origin target]

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Date:	Mon, 02 May 2011 12:58:34 -0400
From:	"Martin K. Petersen" <martin.petersen@...cle.com>
To:	Lukas Czerner <lczerner@...hat.com>
Cc:	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Christoph Hellwig <hch@...radead.org>,
	device-mapper development <dm-devel@...hat.com>,
	Alasdair G Kergon <agk@...hat.com>, sandeen@...hat.com,
	Mike Snitzer <snitzer@...hat.com>, DarkNovaNick@...il.com,
	linux-lvm@...hat.com, linux-ext4@...r.kernel.org
Subject: Re: [dm-devel] do not disable ext4 discards on first discard failure? [was: Re: dm snapshot: ignore discards issued to the snapshot-origin target]

>>>>> "Lukas" == Lukas Czerner <lczerner@...hat.com> writes:

Lukas,

Lukas> [root@...m ~]# lvcreate -L 3500M vg_test
Lukas>   Logical volume "lvol0" created

Ok, so here's what I think is going on. You're creating a linear target
which happens to fit inside the first PV.

Here's two devices. One that supports discard_zeroes_data=1 (8:17) and
one that doesn't (8:49).

# dmsetup table 
foo-bar: 0 1032192 striped 2 32 8:17 384 8:49 384
foo-baz: 0 106496 linear 8:17 516480

# grep . /sys/block/d*/queue/discard_z*
/sys/block/dm-0/queue/discard_zeroes_data:0
/sys/block/dm-1/queue/discard_zeroes_data:1

LV foo/bar (dm-0) is striped, straddling two devices with incompatible
values. Hence 0.

LV foo/baz (dm-1) is linear and fits inside the first PV. Thus it has
discard_zeroes_data=1.

-- 
Martin K. Petersen	Oracle Linux Engineering
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