[linux-lvm] exposing snapshot block device

[Tomas Dalebjörk]

hi

I tried to perform as suggested
# lvconvert —splitsnapshot vg/lv-snap
works fine
# lvconvert -s vg/lv vg/lv-snap
works fine too

but...
if I try to converting cow data directly from the meta device, than it doesn’t work
eg
# lvconvert -s vg/lv /dev/mycowdev
the tool doesn’t like the path
I tried to place a link in /dev/vg/mycowdev -> /dev/mycowdev
and retried the operations 
# lvconveet -s vg/lv /dev/vg/mycowdev
but this doesn’t work either

conclusion 
even though the cow device is an exact copy of the cow device that I have saved on /dev/mycowdev before the split, it wouldn’t work to use to convert back as a lvm snapshot 

not sure if I understand the tool correctly, or if there are other things needed to perform, such as creating virtual information about the lvm VGDA data on the first of this virtual volume named /dev/mycowdev 

let me know what more steps are needed

beat regards Tomas

Sent from my iPhone

> On 7 Nov 2019, at 18:29, Tomas Dalebjörk <tomas.dalebjork at gmail.com> wrote:
> 
> 
> Great, thanks! 
> 
> Den tors 7 nov. 2019 kl 17:54 skrev Mikulas Patocka <mpatocka at redhat.com>:
>> 
>> 
>> On Tue, 5 Nov 2019, Tomas Dalebjörk wrote:
>> 
>> > Thanks,
>> > 
>> > That really helped me to understand how the snapshot works.
>> > Last question:
>> > - lets say that block 100 which is 1MB in size is in the cow device, and a write happen that wants to something or all data on that region of block 100.
>> > Than I assume; based on what have been previously said here, that the block in the cow device will be overwritten with the new changes.
>> 
>> Yes, the block in the cow device will be overwritten.
>> 
>> Mikulas
>> 
>> > Regards Tomas
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