So are you actually trying to access not a 'normal' LV - but an LV
under snapshot ?
No, /dev/vg/lv20 is a normal logical volume. But /dev/vg/lv15 has been
under snapshot before.
Now, I created a snapshot manually (before. it was created by an
automatic backup mechanism) and saw what happened:
1. I created the snapshot with 'lvcreate -s -L 2G /dev/vg/lv15'
2. Afterwards, a symbolic link /dev/vg/lvol0 existed and pointed to
/dev/dm-21. Additionally, a link /dev/mapper/vg-lv15-real existed,
pointing to /dev/dm-19
3. I deleted the snapshot with lvremove
4. Afterwards, /dev/vg/lvol0 had disappeared. /dev/mapper/vg-lv15-real
was still present, and it did not point to /dev/dm-19 any more, but to
/dev/dm-18, the same as /dev/vg/lv20
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