[linux-lvm] exposing snapshot block device

[Tomas Dalebjörk]

hi 
I tried all these steps
but when I associated the snapshot cow device back to an empty origin, and typed the lvs command
the data% output shows 0% instead of 37% ?
so it looks like that the lvconvert -s vg1/lvsnap vg1/lv0 looses the cow data?

perhaps ypu can guide me how this can be done?

btw, just to emulate s full copy, I executed the 
dd if=/dev/vg0/lv0 of=/dev/vg1/lv0 
before the lvconvert -s, to make sure the last data is there

and than I tried to mount the vg1/lv0 which worked fine
but the data was not at snapshot view
even mounting vg1/lvsnap works fine
but with wrong data

confused over how and why vgmerge should be used as vgsplit does the work?

regards Tomas

Sent from my iPhone

> On 7 Sep 2020, at 18:42, Zdenek Kabelac <zkabelac at 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
>