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From: =?utf-8?Q?Tomas_Dalebj=C3=B6rk?= <tomas.dalebjork@gmail.com>
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Date: Mon, 7 Sep 2020 19:37:41 +0200
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Subject: Re: [linux-lvm] exposing snapshot block device
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To: Zdenek Kabelac <zkabelac@redhat.com>
Cc: LVM general discussion and development <linux-lvm@redhat.com>

thanks 

ok
 vgsplit/merge instead
and after that lvconvert-s

yes, I am aware of the issues with corruption
but if the cow device has all data, than no corruption will happen, right?

if COW has a copy of all blocks
than a lvconvert —merge, or mount of the snapshot volume will be without issues

right?

regards Tomas

Sent from my iPhone

> On 7 Sep 2020, at 18:42, Zdenek Kabelac <zkabelac@redhat.com> wrote:
> 
> Dne 07. 09. 20 v 18:34 Tomas Dalebjörk napsal(a):
>> thanks for feedback
>> so if I understand this correctly
>> # fallocate -l 100M /tmp/pv1
>> # fallocate -l 100M /tmp/pv2
>> # fallocate -l 100M /tmp/pv3
>> # losetup —find —show /tmp/pv1
>> # losetup —find —show /tmp/pv2
>> # losetup —find —show /tmp/pv3
>> # vgcreate vg0 /dev/loop0
>> # lvcreate -n lv0 -l 1 vg0
>> # vgextend vg0 /dev/loop1
>> # lvcreate -s -l 1 -n lvsnap /dev/loop1
>> # vgchange -a n vg0
>> # lvconvert —splitsnapshot vg0/lvsnap
>> # vgreduce vg0 /dev/loop1
> 
> 
> Hi
> 
> Here you would need to use 'vgsplit' rather - otherwise you
> loose the mapping for whatever was living on /dev/loop1
> 
>> # vgcreate vg1 /dev/loop2
>> # lvcreate -n lv0 -l 1 vg1
>> # vgextend vg1 /dev/loop1
> 
> And  'vgmerge'
> 
> 
>> # lvconvert -s vg1/lvsnap vg1/lv0
>> not sure if the steps are correct?
> 
> 
> I hope you realize the content of vg1/lv0 must be exactly same
> as vg0/lv0.
> 
> As snapshot COW volume contains only 'diff chunks' - so if you
> would attach snapshot to 'different' lv - you would get only mess.
> 
> 
> Zdenek
>