[linux-lvm] how to convert a disk containing a snapshot to a snapshot lv?

[Tomas Dalebjörk]

Hi,

I am trying to understand how to convert a disk containing snapshot data.
This is how I tested this:
1. locate the snapshot testlv.211218.232255
root at debian10:/dev/mapper# lvs
  LV                   VG          Attr       LSize   Pool Origin Data%
 Meta%  Move Log Cpy%Sync Convert
  home                 debian10-vg -wi-ao----   1.00g
  root                 debian10-vg -wi-ao----  <3.81g
  swap_1               debian10-vg -wi-ao---- 976.00m
  testlv               debian10-vg owi-aos--- 100.00m
  testlv.211218.232255 debian10-vg swi-a-s--- 104.00m      testlv 1.44
root at debian10:/dev/mapper#

2. copy the lv - cow data to a file
# dd if=/dev/mapper/debian10--vg-testlv.211218.232255-cow of=/tmp/out
bs=1024

3. Setup a loopback disk for the file
# losetup -fP /tmp/out

4. Verify that disk exists
root at debian10:/dev/mapper# losetup -a
/dev/loop0: [65025]:39274 (/tmp/out)
root at debian10:/dev/mapper#

5. Try converting the disk using lvconvert command
# lvconvert -Zn -s debian10-vg/testlv /tmp/out
  "/tmp/out": Invalid path for Logical Volume.

6. Trying creating a softlink in /dev/mapper
# ln -s /tmp/out debian10--vg-loopback

7. verify link
root at debian10:/dev/mapper# ls -la
total 0
drwxr-xr-x  2 root root     240 Dec 19 17:40 .
drwxr-xr-x 18 root root    3820 Dec 19 17:22 ..
crw-------  1 root root 10, 236 Dec 10 19:10 control
lrwxrwxrwx  1 root root       7 Dec 10 19:10 debian10--vg-home -> ../dm-5
lrwxrwxrwx  1 root root       8 Dec 19 17:40 debian10--vg-loopback ->
/tmp/out
lrwxrwxrwx  1 root root       7 Dec 13 22:20 debian10--vg-root -> ../dm-1
lrwxrwxrwx  1 root root       7 Dec 10 19:10 debian10--vg-swap_1 -> ../dm-4
lrwxrwxrwx  1 root root       7 Dec 18 23:23 debian10--vg-testlv -> ../dm-0
lrwxrwxrwx  1 root root       7 Dec 18 23:23
debian10--vg-testlv.211218.232255 -> ../dm-6
lrwxrwxrwx  1 root root       7 Dec 18 23:23
debian10--vg-testlv.211218.232255-cow -> ../dm-3
lrwxrwxrwx  1 root root       7 Dec 18 23:23 debian10--vg-testlv-real ->
../dm-2
root at debian10:/dev/mapper#

8. retrying lvconvert command
root at debian10:/dev/mapper# lvconvert -Zn -s debian10-vg/testlv
/dev/mapper/debian10--vg-loopback
  Failed to find logical volume "debian10-vg/loopback"
root at debian10:/dev/mapper#

Are there more things to be considered, such as recreating pv data on disk?

Regards Tomas
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