From mboxrd@z Thu Jan  1 00:00:00 1970
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From: Zdenek Kabelac <zkabelac@redhat.com>
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Date: Mon, 7 Sep 2020 18:42:08 +0200
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Subject: Re: [linux-lvm] exposing snapshot block device
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To: =?UTF-8?Q?Tomas_Dalebj=c3=b6rk?= <tomas.dalebjork@gmail.com>
Cc: LVM general discussion and development <linux-lvm@redhat.com>

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