[linux-lvm] exposing snapshot block device

[Tomas Dalebjörk]

yes, 
we need the snapshot data, as it is provisioned from the backup target and can’t be changed

we will definitely look into thin snapshots later, but want to first making sure that we can reanimate the cow device as a device and associate this with an empty origin

we want if possible be able to associate this cow to a new empty vg/lv using new vgname/lvname if possible 

after all, it is just an virutal volume

Sent from my iPhone

> On 7 Sep 2020, at 21:56, Tomas Dalebjörk <tomas.dalebjork at gmail.com> wrote:
> 
> 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
>>